Chapter 18 Strabismus

INTRODUCTION 736

Definitions 736

Anatomy of extraocular muscles 736

Ocular movements 739

Sensory considerations 741

AMBLYOPIA 745

CLINICAL EVALUATION 746

History 746

Visual acuity 747

Tests for stereopsis 748

Tests for binocular fusion in infants without manifest squint 750

Tests for sensory anomalies 750

Measurement of deviation 755

Motility tests 758

Investigation of diplopia 759

Refraction and fundoscopy 763

HETEROPHORIA 765

VERGENCE ABNORMALITIES 765

ESOTROPIA 765

Early-onset esotropia 766

Accommodative esotropia 768

Microtropia 770

Other esotropias 770

EXOTROPIA 771

Constant (early onset) exotropia 771

Intermittent exotropia 771

Sensory exotropia 772

Consecutive exotropia 772

SPECIAL SYNDROMES 772

Duane retraction syndrome 772

Brown syndrome 773

Monocular elevator deficit 774

Möbius syndrome 774

Congenital fibrosis of the extraocular muscles 774

Strabismus fixus 774

ALPHABET PATTERNS 774

‘V’ pattern 776

‘A’ pattern 777

SURGERY 778

Weakening procedures 778

Strengthening procedures 779

Treatment of paretic strabismus 779

Adjustable sutures 780

Botulinum toxin chemodenervation 780

Page 736

Introduction

Definitions

1 Visual axis (line of vision) passes from the fovea, through the nodal point of the eye to the point of fixation (object of regard). In normal binocular single vision (BSV) the two visual axes intersect at the point of fixation, with the images from the two eyes being aligned by the fusion reflex and combined by binocular responsive cells in the visual cortex to give BSV.

2 Orthophoria implies perfect ocular alignment in the absence of any stimulus for fusion (uncommon).

3 Heterophoria (‘phoria’) implies a tendency of the eyes to deviate when fusion is blocked (latent squint).

• Slight phoria is present in most normal individuals and is overcome by the fusion reflex. The phoria can be either a small inward imbalance (esophoria) or an outward imbalance (exophoria).

• When fusion is insufficient to control the imbalance, the phoria is described as decompensating and is often associated with symptoms of binocular discomfort (asthenopia) or double vision (diplopia).

4 Heterotropia (‘tropia’) implies a manifest deviation in which the visual axes do not intersect at the point of fixation.

• The images from the two eyes are misaligned so that either double vision is present or, more commonly in children, the image from the deviating eye is suppressed at cortical level.

• A childhood squint may occur because of failure of the normal development of binocular fusion mechanisms or as a result of oculomotor imbalance secondary to a difference in refraction between the two eyes (anisometropia).

• Failure of fusion, for example secondary to poor vision in one eye, may cause heterotropia in adulthood, or a squint may develop because of weakness or mechanical restriction of the extraocular muscles or damage to their nerve supply.

• Horizontal deviation of the eyes (latent or manifest) is the most common form of strabismus.

• Upward displacement of one eye relative to the other is termed a hypertropia and a latent upward imbalance a hyperphoria.

• Downward displacement is termed a hypotropia and a latent imbalance a hypophoria.

5 Anatomical axis is a line passing from the posterior pole through the centre of the cornea. Because the fovea is usually slightly temporal to the anatomical centre of the posterior pole of the eye, the visual axis does not usually correspond to the anatomical axis of the eye.

6 Angle kappa is the angle subtended by the visual and anatomical axes and is usually about 5° (Fig. 18.1).

• The angle is positive (normal) when the fovea is temporal to the centre of the posterior pole resulting in a nasal displacement of the corneal reflex, and negative when the converse applies.

• A large angle kappa may give the appearance of a squint when none is present (pseudosquint) and is seen most commonly as a pseudoexotropia following displacement of the macula in retinopathy of prematurity, where the angle may significantly exceed +5^°.

Fig. 18.1 Angle kappa

Anatomy of extraocular muscles

Principles

The lateral and medial orbital walls are at an angle of 45° with each other (Fig. 18.2A). The orbital axis therefore forms an angle of 22.5° with both lateral and medial walls. For the sake of simplicity this angle is usually regarded as being 23°.

• When the eye is looking straight ahead at a fixed point on the horizon with the head erect (primary position of gaze), the visual axis forms an angle of 23° with the orbital axis (Fig. 18.2B).

• The actions of the extraocular muscles depend on the position of the globe at the time of muscle contraction.

1 Primary action of a muscle is its major effect when the eye is in the primary position.

2 Subsidiary actions are the additional effects which depend on the position of the eye.

3 Listing plane is an imaginary coronal plane passing through the centre of rotation of the globe. The globe rotates on the axes of Fick, which intersect in the listing plane (Fig. 18.3).

• The globe rotates left and right on the vertical Z axis.

• The globe moves up and down on the horizontal X axis.

• Torsional movements (wheel rotations) occur on the Y (sagittal) axis which traverses the globe from front to back (similar to the anatomical axis of the eye).

• Intorsion occurs when the superior limbus rotates nasally, and extorsion on temporal rotation.

Fig. 18.2 Anatomy of the extraocular muscles

Fig. 18.3 The listing plane and axes of Fick

Page 737

Horizontal rectus muscles

When the eye is in the primary position, the horizontal recti are purely horizontal movers on the vertical Z axis and have only primary actions.

1 Medial rectus originates at the annulus of Zinn at the orbital apex and inserts 5.5 mm behind the nasal limbus. Its sole action in the primary position is adduction.

2 Lateral rectus originates at the annulus of Zinn and inserts 6.9 mm behind the temporal limbus. Its sole action in the primary position is abduction.

Vertical rectus muscles

The vertical recti run in line with the orbital axis and are inserted in front of the equator. They therefore form an angle of 23° with the visual axis (see Fig. 18.2C).

1 Superior rectus originates from the upper part of the annulus of Zinn and inserts 7.7 mm behind the superior limbus.

• The primary action is elevation (Fig. 18.4A); secondary actions are adduction and intorsion.

• When the globe is abducted 23°, the visual and orbital axes coincide (Fig. 18.4B). In this position it has no subsidiary actions and can only act as an elevator. This is therefore the optimal position of the globe for testing the function of the superior rectus muscle.

Page 738

• If the globe were adducted 67°, the angle between the visual and orbital axes would be 90° (Fig. 18.4C). In this position the superior rectus could only act as an intortor.

2 Inferior rectus originates at the lower part of the annulus of Zinn and inserts 6.5 mm behind the inferior limbus.

• The primary action is depression; secondary actions are adduction and extorsion.

• When the globe is abducted 23°, the inferior rectus acts purely as a depressor. As for superior rectus, this is the optimal position of the globe for testing the function of the inferior rectus muscle.

• If the globe were adducted 67°, the inferior rectus could only act as an extortor.

Fig. 18.4 Actions of the right superior rectus muscle

Spiral of Tillaux

The spiral of Tillaux is an imaginary line joining the insertions of the four recti and is an important anatomical landmark when performing surgery. The insertions are located progressively further away from the limbus in a spiral pattern; the medial rectus insertion is closest (5.5 mm) followed by the inferior rectus (6.5 mm), lateral rectus (6.9 mm) and superior rectus (7.7 mm; Fig. 18.5).

Fig. 18.5 Spiral of Tillaux

Oblique muscles

The obliques are inserted behind the equator and form an angle of 51° with the visual axis (see Fig. 18.2D).

1 Superior oblique originates superomedial to the optic foramen. It passes forwards through the trochlea at the angle between the superior and medial walls and is then reflected backwards and laterally to insert in the posterior upper temporal quadrant of the globe (Fig. 18.6).

• The primary action is intorsion (Fig. 18.7A); secondary actions are depression and abduction.

• The anterior fibres of the superior oblique tendon are primarily responsible for intorsion and the posterior fibres for depression, allowing separate surgical manipulation of these two actions (see below).

• When the globe is adducted 51°, the visual axis coincides with the line of pull of the muscle (Fig. 18.7B). In this position it can only act as a depressor. This is, therefore, the best position of the globe for testing the action of the superior oblique muscle. Thus, although the superior oblique has an abducting action in primary position, the main effect of superior oblique weakness is seen as failure of depression in adduction.

• When the eye is abducted 39°, the visual axis and the superior oblique make an angle of 90° with each other (Fig. 18.7C). In this position the superior oblique can only cause intorsion.

Page 739

2 Inferior oblique originates from a small depression just behind the orbital rim lateral to the lacrimal sac. It passes backwards and laterally to insert in the posterior lower temporal quadrant of the globe close to the macula.

• The primary action is extorsion; secondary actions are elevation and abduction.

• When the globe is adducted 51°, the inferior oblique acts only as an elevator.

• When the eye is abducted 39°, its main action is extorsion.

Fig. 18.6 Insertion of the superior oblique tendon

Fig. 18.7 Actions of the right superior oblique muscle

Muscle pulleys

The four rectus muscles pass through condensations of connective tissue and smooth muscle just posterior to the equator. These condensations act as pulleys and minimize upward and downward movements of the bellies of the medial and lateral rectus muscles during upgaze and downgaze, and horizontal movements of the superior and inferior rectus bellies in left and right gaze.

• Pulleys are the effective origins of the rectus muscles and play an important role in the coordination of eye movements by reducing the effect of horizontal movements on vertical muscle actions and vice versa.

• Displacement of the pulleys can be one cause of abnormalities of eye movements such as ‘V’ and ‘A’ patterns (see below).

Nerve supply

1 Lateral rectus is supplied by the 6th cranial nerve (abducent nerve – abducting muscle).

2 Superior oblique is supplied by the 4th cranial nerve (trochlear nerve – muscle associated with the trochlea).

3 Other muscles together with the levator muscle of the upper lid and the ciliary and sphincter pupillae muscles are supplied by the third (oculomotor) nerve.

Ocular movements

Ductions

Ductions are monocular movements around the axes of Fick. They consist of adduction, abduction, elevation, depression, intorsion and extorsion. They are tested by occluding the fellow eye and asking the patient to follow a target in each direction of gaze.

Versions

Versions are binocular, simultaneous, conjugate movements in the same direction (Fig. 18.8, top).

• Dextroversion and laevoversion (gaze right and gaze left), elevation (upgaze) and depression (downgaze). These four movements bring the globe into the secondary positions of gaze by rotation around either the vertical (Z) or the horizontal (X) axis of Fick.

• Dextroelevation and dextrodepression (gaze up and right; gaze down and right) and laevoelevation and laevodepression (gaze up and left; gaze down and left). These four oblique movements bring the eyes into the tertiary positions of gaze by rotation around oblique axes lying in the Listing plane, equivalent to simultaneous movement about both the horizontal and vertical axes.

Page 740

• Torsional movements to maintain upright images occur on tilting of the head; these are known as the righting reflexes. On head tilt to the right the superior limbi of the two eyes rotate to the left, causing intorsion of the right globe and extorsion of the left.

Fig. 18.8 Binocular movements

Vergences

Vergences are binocular, simultaneous, disjugate or disjunctive movements (in opposite directions) (Fig. 18.8, bottom). Convergence is simultaneous adduction (inward turning); divergence is outwards movement from a convergent position. Convergence may be voluntary or reflex.

Reflex convergence has four components:

1 Tonic convergence, which implies inherent innervational tone to the medial recti.

2 Proximal convergence is induced by psychological awareness of a near object.

3 Fusional convergence is an optomotor reflex which maintains binocular single vision (BSV) by ensuring that similar images are projected onto corresponding retinal areas of each eye. It is initiated by bitemporal retinal image disparity.

4 Accommodative convergence is induced by the act of accommodation as part of the synkinetic-near reflex.

• Each dioptre of accommodation is accompanied by a constant increment in accommodative convergence, giving the ‘accommodative convergence to accommodation’ (AC/A) ratio.

• This is the amount of convergence in prism dioptres (Δ) per dioptre (D) change in accommodation.

• The normal value is 3–5 Δ. This means that 1 D of accommodation is associated with 3–5 Δ of accommodative convergence. Abnormalities of the AC/A ratio play an important role in the aetiology of strabismus.

Changes in accommodation, convergence and pupil size which occur in concert with a change in the distance of viewing are known as the ‘near triad’.

Positions of gaze

1 Six cardinal positions of gaze are identified in which one muscle in each eye is principally responsible for moving the eye into that position as follows:

• Dextroversion (right lateral rectus and left medial rectus).

• Laevoversion (left lateral rectus and right medial rectus).

• Dextroelevation (right superior rectus and left inferior oblique).

• Laevoelevation (left superior rectus and right inferior oblique).

• Dextrodepression (right inferior rectus and left superior oblique).

• Laevodepression (left inferior rectus and right superior oblique).

2 Nine diagnostic positions of gaze are those in which deviations are measured. They consist of the six cardinal positions, the primary position, elevation and depression (Fig. 18.9).

Page 741

Fig. 18.9 Diagnostic positions of gaze

Laws of ocular motility

1 Agonist–antagonist pairs are muscles of the same eye that move the eye in opposite directions. The agonist is the primary muscle moving the eye in a given direction. The antagonist acts in the opposite direction to the agonist. For example the right lateral rectus is the antagonist to the right medial rectus.

2 Synergists are muscles of the same eye that move the eye in the same direction. For example, the right superior rectus and right inferior oblique act synergistically in elevation.

3 Yoke muscles (contralateral synergists) are pairs of muscles, one in each eye, that produce conjugate ocular movements. For example, the yoke muscle of the left superior oblique is the right inferior rectus.

4 Sherrington law of reciprocal innervation (inhibition) states that increased innervation to an extraocular muscle (e.g. right medial rectus) is accompanied by a reciprocal decrease in innervation to its antagonist (e.g. right lateral rectus; Fig. 18.10). This means that when the medial rectus contracts the lateral rectus automatically relaxes and vice versa. The Sherrington law applies to both versions and vergences.

5 Hering law of equal innervation states that during any conjugate eye movement, equal and simultaneous innervation flows to the yoke muscles (Fig. 18.11).

• In the case of a paretic squint, the amount of innervation to both eyes is symmetrical, and always determined by the fixating eye, so that the angle of deviation will vary according to which eye is used for fixation.

• For example if, in the case of a left lateral rectus palsy, the right normal eye is used for fixation, there will be an inward deviation of the left eye due to the unopposed action of the antagonist of the paretic left lateral rectus (left medial rectus). The amount of misalignment of the two eyes in this situation is called the primary deviation (Fig. 18.12, left).

• If the paretic left eye is now used for fixation, additional innervation will flow to the left lateral rectus, in order to establish this. However, according to Hering law, an equal amount of innervation will also flow to the right medial rectus (yoke muscle). This will result in an overaction of the right medial rectus and an excessive amount of adduction of the right eye.

• The amount of misalignment between the two eyes in this situation is called the secondary deviation (Fig. 18.12, right). In a paretic squint, the secondary deviation exceeds the primary deviation.

6 Muscle sequelae are the effects of the interactions described by these laws. They are of prime importance in diagnosing ocular motility disorders and in particular in distinguishing a recently acquired from a longstanding palsy (see clinical evaluation). The full pattern of changes takes time to develop and can be summarized as follows:

• Primary underaction (e.g. left superior oblique).

• Secondary overaction of the contralateral synergist or yoke muscle (right IR; Hering law).

• Secondary overaction and later contracture of the unopposed ipsilateral antagonist (left IO; Sherrington law).

• Secondary inhibition of the contralateral antagonist (right SR; Hering and Sherrington laws).

Fig. 18.10 Sherrington law of reciprocal innervation

Fig. 18.11 Hering law of equal innervation of yoke muscles

Fig. 18.12 Primary and secondary deviations in paretic strabismus

Sensory considerations

Basic aspects

1 Normal binocular single vision (BSV) involves the simultaneous use of both eyes with bifoveal fixation, so that each eye contributes to a common single perception of the object of regard. This represents the highest form of binocular cooperation. Conditions necessary for normal BSV are:

• Normal routing of visual pathways with overlapping visual fields.

Page 742

• Binocularly driven neurones in the visual cortex.

• Normal retinal (retinocortical) correspondence (NRC) resulting in cyclopean viewing.

• Accurate neuromuscular development and coordination, so that the visual axes are directed at, and maintain fixation on, the object of regard.

• Approximately equal image clarity and size for both eyes.

• BSV is based on NRC which requires first an understanding of uniocular visual direction and projection.

2 Visual direction is the projection of a given retinal element in a specific direction in subjective space.

a Principal visual direction is the direction in external space interpreted as the line of sight. This is normally the visual direction of the fovea and is associated with a sense of direct viewing.

b Secondary visual directions are the projecting directions of extrafoveal points with respect to the principal direction of the fovea, associated with indirect (eccentric) viewing.

3 Projection is the subjective interpretation of the position of an object in space on the basis of stimulated retinal elements.

• If a red object stimulates the right fovea (F), and a black object which lies in the nasal field stimulates a temporal retinal element (T), the red object will be interpreted by the brain as having originated from the straight ahead position and the black object will be interpreted as having originated in the nasal field (Fig. 18.13A). Similarly, nasal retinal elements project into the temporal field, upper retinal elements into the lower field and vice versa.

Page 743

• With both eyes open, the red fixation object is now stimulating both foveae, which are corresponding retinal points. The black object is now not only stimulating the temporal retinal elements in the right eye but also the nasal elements of the left eye. The right eye therefore projects the object into its nasal field and the left eye projects the object into its temporal field.

• Because both of these retinal elements are corresponding points, they will both project the object into the same position in space (the left side) and there will be no double vision.

4 Retinomotor values

• The image of an object in the peripheral visual field falls on an extrafoveal element. To establish fixation on this object a saccadic version of accurate amplitude is required.

• Each extrafoveal retinal element therefore has a retinomotor value proportional to its distance from the fovea, which guides the amplitude of saccadic movements required to ‘look at it’.

• Retinomotor value, zero at the fovea, increases progressively towards the retinal periphery.

5 Corresponding ‘points’ are areas on each retina that share the same subjective visual direction (for example, the foveae share the primary visual direction).

• Points on the nasal retina of one eye have corresponding points on the temporal retina of the other eye and vice versa. For example, an object producing images on the right nasal retina and the left temporal retina will be projected into the right side of visual space. This is the basis of normal retinal correspondence.

• This retinotopic organization is reflected back along the visual pathways, each eye maintaining separate images until the visual pathways converge onto binocularly responsive neurones in the primary visual cortex.

6 The horopter is an imaginary plane in external space, relative to both the observer’s eyes for a given fixation target, all points on which stimulate corresponding retinal elements and are therefore seen singly and in the same plane (Fig. 18.13B). This plane passes through the intersection of the visual axes and therefore includes the point of fixation in BSV.

7 Panum fusional space (‘volume’) is a zone in front of and behind the horopter in which objects stimulate slightly non-corresponding retinal points (retinal disparity).

• Objects within the limits of the fusional space are seen singly and the disparity information is used to produce a perception of binocular depth (stereopsis). Objects in front of and behind Panum space appear double.

• This is the basis of physiological diplopia. Panum space is shallow at fixation (6 seconds of arc) and deeper towards the periphery (30–40 seconds of arc at 15° from the fovea).

• The retinal areas stimulated by images falling within Panum fusional space are termed Panum fusional areas.

• Therefore objects on the horopter are seen singly and in one plane. Objects in Panum fusional areas are seen singly and stereoscopically. Objects outside Panum fusional areas appear double.

• Physiological diplopia is usually accompanied by physiological suppression.

8 BSV is characterized by the ability to fuse the images from the two eyes and to perceive binocular depth:

a Sensory fusion involves the integration by the visual areas of the cerebral cortex of two similar images, one from each eye, into one image. It may be central, which integrates the image falling on the foveae, or peripheral, which integrates parts of the image falling outside the foveae. It is possible to maintain fusion with a central visual deficit in one eye, but peripheral fusion is essential to BSV and may be affected in patients with advanced field changes in glaucoma and pituitary lesions.

b Motor fusion involves the maintenance of motor alignment of the eyes to sustain bifoveal fixation. It is driven by retinal image disparity, which stimulates fusional vergences.

9 Fusional vergence involves disjugate eye movements to overcome retinal image disparity. Fusional vergence amplitudes can be measured with prisms or on the synoptophore. Normal values are:

• Convergence: about 15–20 Δ for distance and 25 Δ for near.

• Divergence: about 6–10 Δ for distance and 12–14 Δ for near.

• Vertical: 2–3 Δ.

• Cyclovergence: about 2–3°.

Fig. 18.13 Principles of projection

Fusional convergence helps to control an exophoria whereas fusional divergence helps to control an esophoria. The fusional vergence mechanism may be decreased by fatigue or illness, converting a phoria to a tropia. The amplitude of fusional vergence mechanisms can be improved by orthoptic exercises, particularly in the case of near fusional convergence for the relief of convergence insufficiency.

10 Stereopsis is the perception of depth. It arises when objects behind and in front of the point of fixation (but within Panum fusional space) stimulate horizontally disparate retinal elements simultaneously. The fusion of these disparate images results in a single visual impression perceived in depth. A solid object is seen stereoscopically (in 3D) because each eye sees a slightly different aspect of the object.

11 Sensory perceptions. At the onset of a squint two sensory perceptions arise based on the normal projection of the retinal areas stimulated; confusion and pathological diplopia may result. These require simultaneous visual perception i.e. the ability to perceive images from both eyes simultaneously. Young children readily suppress diplopia but it is persistent and usually troublesome in strabismus in older children and adults, when it arises after the sensitive period for binocularity (see below).

a Confusion is the simultaneous appreciation of two superimposed but dissimilar images caused by stimulation of corresponding retinal points (usually the foveae) by images of different objects (Fig. 18.14).

b Pathological diplopia is the simultaneous appreciation of two images of the same object in different positions and results from images of the same object falling on non-corresponding retinal points.

• In esotropia the diplopia is homonymous (uncrossed – Fig. 18.15A).

Page 744

• In exotropia the diplopia is heteronymous (crossed – Fig. 18.15B).

Fig. 18.14 Confusion

Fig. 18.15 Diplopia. (A) Homonymous (uncrossed) diplopia in right esotropia with normal retinal correspondence; (B) heteronymous (crossed) diplopia in right exotropia with normal retinal correspondence

Sensory adaptations to strabismus

The ocular sensory system in children has the ability to adapt to anomalous states (confusion and diplopia) by two mechanisms: (a) suppression and (b) abnormal retinal correspondence (ARC). These occur because of the plasticity of the developing visual system in children under the age of 6–8 years. Occasional adults who develop sudden-onset strabismus are able to ignore the second image after a time and therefore do not complain of diplopia.

Suppression

Suppression involves active inhibition by the visual cortex of the image from one eye when both eyes are open. Stimuli for suppression include diplopia, confusion and a blurred image from one eye resulting from astigmatism/anisometropia. Clinically, suppression may be:

1 Central or peripheral. In central suppression the image from the fovea of the deviating eye is inhibited to avoid confusion. Diplopia, on the other hand, is eradicated by the process of peripheral suppression, in which the image from the peripheral retina of the deviating eye is inhibited.

2 Monocular or alternating. Suppression is monocular when the image from the dominant eye always predominates over the image from the deviating (or more ametropic) eye, so that the image from the latter is constantly suppressed. This type of suppression leads to amblyopia. When suppression alternates (switches from one eye to the other) amblyopia is less likely to develop.

3 Facultative or obligatory. Facultative suppression occurs only when the eyes are misaligned. Obligatory suppression is present at all times, irrespective of whether the eyes are deviated or straight. Examples of faculative suppression include intermittent exotropia and Duane syndrome.

Abnormal retinal correspondence

Abnormal retinal correspondence (ARC) is a condition in which non-corresponding retinal elements acquire a common subjective visual direction, i.e. fusion occurs in the presence of a small angle manifest squint.

Page 745

• The fovea of the fixating eye is thus paired with a non-foveal element of the deviated eye.

• ARC is a positive sensory adaptation to strabismus (as opposed to negative adaptation by suppression), which allows some anomalous binocular vision in the presence of a heterotropia.

• Binocular responses in ARC are never as good as in normal bifoveal BSV. ARC is most frequently present in small angle esotropia (microtropia) associated with anisometropia.

Microtropia

Microtropia is a small angle (<10 Δ) squint in which stereopsis is present but reduced, and there is relative amblyopia of the more ametropic eye. Microtropia has two forms.

1 In microtropia with identity the point used for monocular fixation by the squinting eye corresponds with the fovea of the straight eye under binocular viewing conditions. Therefore on cover test there is no movement of the squinting eye when it takes up monocular fixation.

2 In microtropia without identity the monocular fixation point of the squinting eye does not correspond with the fovea of the straight eye in binocular viewing. There is therefore a small movement of the deviating eye when it takes up monocular fixation on cover testing. ARC is less common in accommodative esotropia because of the variability of the angle of deviation, or in large angle deviations because the separation of the images is too great.

Consequences of strabismus

• The fovea of the squinting eye is suppressed to avoid confusion.

• Diplopia will occur, since corresponding retinal elements receive different images.

• To avoid diplopia, the patient will develop either peripheral suppression of the squinting eye or ARC.

• If constant unilateral suppression occurs this will subsequently lead to strabismic amblyopia.

Motor adaptation to strabismus

Motor adaptation involves the adoption of an abnormal head posture (AHP) and occurs primarily in children with congenitally abnormal eye movements who use the AHP to maintain BSV. In these children loss of an AHP may indicate loss of binocular function and the need for surgical intervention. These patients may present in adult life with symptoms of decompensation, often unaware of their AHP. Acquired paretic strabismus in adults may be consciously controlled by an AHP provided the deviation is neither too large nor too variable with gaze (incomitance). The AHP eliminates diplopia and helps to centralize the binocular visual field. The patient will turn the head into the direction of the field of action of the weak muscle, so that the eyes are then automatically turned the opposite direction and as far as possible away from its field of action (i.e. the head will turn where the eye cannot). An AHP is analyzed in terms of the following three components:

• Face turn to right or left.

• Head tilt to right or left.

• Chin elevation or depression.

1 A face turn will be adopted to control a purely horizontal deviation. For example, if the left lateral rectus is paralyzed, diplopia will occur in left gaze; the face will be turned to the left which deviates the eyes to the right away from the field of action of the weak muscle and area of diplopia. A face turn may also be adopted in a paresis of a vertically acting muscle to avoid the side where the vertical deviation is greatest (e.g. in a right superior oblique weakness the face is turned to the left).

2 A head tilt is adopted to compensate for torsional and/or vertical diplopia. In a right superior oblique weakness, the right eye is relatively elevated and the head is tilted to the left (Fig. 18.16), towards the hypotropic eye; this reduces the vertical separation of the diplopic images and permits fusion to be regained. If there is a significant torsional component preventing fusion, tilting the head in the same left direction will reduce this by invoking the righting reflexes (placing the extorted right eye in a position which requires extorsion).

3 Chin elevation or depression may be used to compensate for weakness of an elevator or depressor muscle or to minimize the horizontal deviation when an A or V pattern is present.

Fig. 18.16 Compensatory head posture in a right 4th nerve palsy

Amblyopia

Classification

Amblyopia is the unilateral, or rarely bilateral, decrease in best-corrected visual acuity caused by form vision deprivation and/or abnormal binocular interaction, for which there is no identifiable pathology of the eye or visual pathway.

Page 746

1 Strabismic amblyopia results from abnormal binocular interaction where there is continued monocular suppression of the deviating eye.

2 Anisometropic amblyopia is caused by a difference in refractive error between the eyes and may result from a difference of as little as 1.0 D sphere. The more ametropic eye receives a blurred image, in a mild form of visual deprivation. It is frequently associated with microstrabismus and may coexist with strabismic amblyopia.

3 Stimulus deprivation amblyopia results from vision deprivation. It may be unilateral or bilateral and is caused by opacities in the media (e.g. cataract) or ptosis which covers the pupil.

4 Bilateral ametropic amblyopia results high symmetrical refractive errors, usually hypermetropia.

5 Meridional amblyopia results from image blur in one meridian. It can be unilateral or bilateral and is caused by uncorrected astigmatism (usually >1 D) persisting beyond the period of emmetropization in early childhood.

Diagnosis

In the absence of an organic lesion, a difference in best corrected visual acuity of two Snellen lines or more (or >1 log unit) is indicative of amblyopia. Visual acuity in amblyopia is usually better when reading single letters than letters in a row. This ‘crowding’ phenomenon occurs to a certain extent in normal individuals but is more marked in amblyopes and must be taken into account when testing preverbal children.

Treatment

It is essential to examine the fundi to diagnose any visible organic disease prior to commencing treatment for amblyopia. Organic disease and amblyopia may coexist and a trial of patching may still be indicated in the presence of organic disease. If acuity does not respond to treatment, investigations such as electrophysiology or imaging should be reconsidered. The sensitive period during which acuity of an amblyopic eye can be improved is usually up to 7–8 years in strabismic amblyopia and may be longer (into teens) for anisometropic amblyopia where good binocular function is present.

1 Occlusion of the normal eye, to encourage use of the amblyopic eye, is the most effective treatment. The regimen, full-time or part-time, depends on the age of the patient and the density of amblyopia.

• The younger the patient the more rapid the likely improvement although the greater the risk of inducing amblyopia in the normal eye. It is therefore very important to monitor visual acuity regularly in both eyes during treatment.

• The better the visual acuity at the start of occlusion, the shorter the duration required, although there is wide variation between patients.

• If there has been no improvement after 6 months of effective occlusion, further treatment is unlikely to be fruitful.

• Poor compliance is the single greatest barrier to improvement and must be monitored. Amblyopia treatment benefits from time spent at the outset on communication of the rationale and the difficulties involved.

2 Penalization, in which vision in the normal eye is blurred with atropine, is an alternative method. It is best in the treatment of relatively mild amblyopia (6/24 or better) in association with hypermetropia. Conventional occlusion is likely to produce a quicker response than atropine which is generally used when compliance with occlusion is poor.

Clinical evaluation

History

1 Age of onset.

• The earlier the onset, the more likely the need for surgical correction.

• The later the onset, the greater the likelihood of an accommodative component (mostly arising between 18–36 months).

• The longer the duration of squint in early childhood the greater the risk of amblyopia, unless fixation is freely alternating. Inspection of previous photographs may be useful for the documentation of strabismus or an AHP.

2 Symptoms may indicate decompensation of a pre-existent heterophoria or more significantly a recently acquired (usually paretic) condition. In the former, the patient usually complains of discomfort, blurring and possibly diplopia of indeterminate onset and duration compared to the acquired condition with the sudden onset of diplopia.

• The type of diplopia (horizontal, cyclovertical) should be established, the direction of gaze in which it predominates and whether any BSV is retained.

• In adults it is very important to determine exactly what problems the squint is causing as a basis for decisions about treatment.

• It is not unusual for patients to present with spurious symptoms which mask embarrassment over a cosmetically noticeable squint.

3 Variability is significant because intermittent strabismus indicates some degree of binocularity. An equally alternating deviation suggests symmetrical visual acuity in both eyes.

4 General health or developmental problems may be significant (e.g. children with cerebral palsy have an increased incidence of strabismus). In older patients poor health and stress may cause decompensation, and in acquired paresis patients may report associations or causal factors (trauma, neurological disease, diabetes etc).

5 Birth history, including period of gestation, birth weight and any problems in utero, with delivery or in the neonatal period.

6 Family history is important because strabismus is frequently familial, although there is no definitive inheritance pattern. It is also important to know what therapy was necessary in other family members.

Page 747

7 Previous ocular history including refractive prescription and compliance with spectacles or occlusion, previous surgery or prisms is important to future treatment options and prognosis.

Visual acuity

Testing in preverbal children

The evaluation can be separated into the qualitative assessment of visual behaviour and the quantitative assessment of visual acuity using preferential looking tests. Assessment of visual behaviour is achieved as follows:

1 Fixation and following may be assessed using bright attention-grabbing targets (a face is often best). This method indicates whether the infant is visually alert and is of particular value in a child suspected of being blind.

2 Comparison between the behaviour of the two eyes may reveal a unilateral preference. Occlusion of one eye, if strongly objected to by the child, indicates poorer acuity in the other eye. However, it is possible to have good visual attention with each eye but unequal visual acuity and all risk factors for amblyopia must be considered in the interpretation of results.

3 Fixation behaviour can be used to establish unilateral preference if a manifest squint is present.

a Fixation is promoted in the squinting eye by occluding the dominant eye while the child fixates a target of interest (preferably incorporating a light).

b Fixation is then graded as central or non-central and steady or unsteady (the corneal reflection can be observed).

c The other eye is then uncovered and the ability to maintain fixation is observed.

d If fixation immediately returns to the uncovered eye, then visual acuity is probably impaired.

e If fixation is maintained through a blink, then visual acuity is probably good.

f If the patient alternates fixation, then the two eyes have equal vision.

4 The 10 Δ test is similar and can be used regardless of whether a manifest squint is present. It involves the promotion of diplopia using a 10 Δ vertical prism. Alternation between the diplopic targets suggests equal visual acuity.

5 Rotation test is a gross qualitative test of the ability of an infant to fixate with both eyes open. The test is performed as follows:

a The examiner holds the child facing him and rotates briskly through 360°.

b If vision is normal, the eyes will deviate in the direction of rotation under the influence of the vestibulo-ocular response. The eyes flick back to the primary position to produce a rotational nystagmus.

c When rotation stops, nystagmus is briefly observed in the opposite direction for 1–2 seconds and should then cease due to suppression of post-rotary nystagmus by fixation.

d If vision is severely impaired, the post-rotation nystagmus does not stop as quickly when rotation ceases because the vestibulo-ocular response is not blocked by visual feedback.

6 Preferential looking tests can be used from early infancy and are based on the fact that infants prefer to look at a pattern rather than a homogeneous stimulus. The infant is exposed to a stimulus and the examiner observes the eyes for fixation movements, without themselves knowing the stimulus position.

• Tests which are in common use include the Teller or Keeler acuity cards, which consist of black stripes (gratings) of varying widths, and Cardiff acuity cards, which consist of familiar pictures with variable outline width (Fig. 18.17).

• Low frequency (coarse) gratings or pictures with a wider outline are seen more easily than high frequency gratings or thin outline pictures, and an assessment of resolution (not recognition) visual acuity is made accordingly.

• Since grating acuity often exceeds Snellen acuity in amblyopia, Teller cards may overestimate visual acuity. These methods may not be reliable if a proper forced-choice staircase protocol is not followed during testing and neither method has high sensitivity to the presence of amblyopia. The results must be considered in combination with risk factors for amblyopia.

7 Pattern visual evoked potentials (VEP) give a representation of spatial acuity but are more commonly used in the diagnosis of optic neuropathy.

Fig. 18.17 Cardiff acuity cards

Testing in verbal children

Page 748

All of the tests described below are performed at 3 or 4 metres at which it is easier to obtain compliance than at 6 metres, with little or no clinical detriment. It is important to note that amblyopia can only be accurately diagnosed using a crowded test requiring target recognition and that logMAR tests (logarithm of the minimal angle of resolution) provide the best measure against which improvement with amblyopia therapy can be assessed. These are readily available in formats suited to normal children from 2 years onwards.

1 At age 2 years, most children will have sufficient language skills to undertake a picture naming test such as the crowded Kay pictures (Fig. 18.18A).

2 At age 3 years, most children will be able to undertake the matching of letter optotypes as in the Keeler logMAR (Fig. 18.18B) or Sonksen crowded tests. If a crowded letter test proves too difficult it is preferable to perform the crowded Kay pictures than to use single optotype letters.

3 Older children may continue with the crowded letter tests, naming or matching them; LogMAR tests are in common usage and are preferable to Snellen for all children at risk of amblyopia.

Fig. 18.18 (A) Kay pictures; (B) Keeler logMAR crowded test

(Courtesy of E Dawson)

Tests for stereopsis

Stereopsis is measured in seconds of arc (1° = 60 minutes of arc; 1 minute = 60 seconds). It is useful to remember that normal spatial visual acuity is 1 minute and normal stereoacuity is 60 seconds (which equals 1 minute). The lower the value the better the acuity. Various tests are employed using different test principles. Random dot tests (e.g. TNO, Frisby) provide the most definitive evidence of high grade BSV. Where this is weak and/or based on ARC, contour-based tests (e.g. Titmus) may give more reliable evidence of stereopsis.

TNO

The TNO random dot test consists of seven plates of randomly distributed paired red and green dots which are viewed with complementary red-green spectacles.

• Within each plate the dots of one colour forming the target shape (squares, crosses etc.) are displaced horizontally in relation to their paired dots of the other colour so that they have a different retinal disparity from those outside the target.

• Some control shapes are visible even without red-green spectacles (Fig. 18.19A) while the test targets are only visible to an individual with stereopsis, while wearing red-green spectacles (Fig. 18.19B).

• The first three plates are used to establish the presence of stereoscopic vision and subsequent plates to quantify it.

• Because there are no monocular clues, the TNO test provides a truer positive measurement of stereopsis than the Titmus test, but can give false negative errors when fusion is poor.

• The disparities measured range from 480 to 15 seconds of arc tested at 40 cm. Most children are able to do this (and the Frisby test) from about 4 years of age.

Fig. 18.19 TNO test. (A) Control shape; (B) control and test targets

Frisby

The Frisby stereotest consists of three transparent plastic plates of varying thickness.

• On the surface of each plate are printed four squares of small randomly-distributed shapes (Fig. 18.20). One of the squares contains a ‘hidden’ circle, in which the random shapes are printed on the reverse of the plate. The patient is required to identify this hidden circle.

• The test does not require special spectacles because the disparity is created by the thickness of the plate and can be varied by increasing or decreasing the working distance, which must be accurately measured.

• The disparities measured range from 600 to 15 seconds of arc. It is important not to allow the subject to tilt the plate or move their head during testing as this will provide monocular clues.

• A simple screening test with a choice of one stereoscopic picture from a plate of two provides an easy preferential looking test for the presence of stereopsis in very young patients.

Fig. 18.20 Frisby test

Lang

The Lang stereotest does not require special spectacles; the targets are seen alternately by each eye through the built-in cylindrical lens elements.

Page 749

• Displacement of the dots creates disparity and the patient is asked to name or point to a simple shape, such as a star, on the card (Fig. 18.21).

• The Lang test can often be used to assess stereopsis in very young children and babies, who may reach out to touch the pictures.

• The examiner can also observe the child’s eye movements from picture to picture on the card. However, the cards must be held exactly parallel to the plane of the face for the effect to be seen and the Frisby screening test may be superior simply for demonstrating stereopsis (e.g. to confirm BSV in infants with suspected squint).

• The degree of disparity is quite gross, ranging 200–1200 seconds of arc at 40 cm.

Fig. 18.21 Lang test

Titmus

The Titmus test consists of a three-dimensional Polaroid vectograph consisting of two plates in the form of a booklet viewed through Polaroid spectacles. On the right is a large fly, and on the left is a series of circles and animals (Fig. 18.22). The test is performed at a distance of 40 cm.

1 Fly is a test of gross stereopsis (3000 seconds of arc), and is especially useful for young children.

• The fly should appear to stand out from the page and the child is encouraged to pick up the tip of one of its wings between finger and thumb. In the absence of gross stereopsis the fly will appear as an ordinary flat photograph.

• If the book is inverted, the targets will appear to be behind the plane of the page. If the patient states that the fly’s wings are still ‘popping out’, then they are not appreciating true stereoscopic vision.

2 Circles comprise a graded series which tests fine depth perception. Each of the nine squares contains four circles.

• One of the circles in each square has a degree of disparity and will appear forward of the plane of reference in the presence of normal stereopsis. The disparities measured range from 800 to 40 seconds of arc.

• If a patient perceives the circle to be shifted to the side, then they are not appreciating stereoscopic vision, but are using monocular clues instead.

Page 750

3 The animals are similar to the circles test but consist of three rows of animals, one of which will appear forward of the plane of reference. The degree of disparity ranges from 400 to 100 seconds of arc.

Fig. 18.22 Titmus test

Frisby–Davis distance stereotest

This consists of a large cube with an open front through which four small objects are visible. Testing is usually performed at 6 metres. The patient has to decide which of four objects within the box is closest to them.

Tests for binocular fusion in infants without manifest squint

Base-out prism

Base-out prism is a quick and easy method for detecting fusion in children. The test is performed by placing a 20 Δ base-out prism in front of one eye (in this case the right – Fig. 18.23). This displaces the retinal image temporally with resultant diplopia. The examiner observes corrective eye movements as follows:

a There will be a shift of the right eye to the left to resume fixation (right adduction) with a corresponding shift of the left eye to the left (left abduction) in accordance with Hering Law (Fig. 18.23B).

b The left eye will then make a corrective refixational saccade to the right (left re-adduction) (Fig. 18.23C).

c On removal of the prism both eyes move to the right (Fig. 18.23D).

d The left eye then makes an outward fusional movement (Fig. 18.23E).

Fig. 18.23 Base-out prism test

Most children with good BSV should be able to overcome a 20 Δ prism from the age of 6 months; if not weaker prisms (16 Δ or 12 Δ) may be tried but the response is harder to observe.

Binocular convergence

Simple convergence to an interesting target can be demonstrated from 3 to 4 months. Both eyes should follow the approaching target symmetrically ‘to the nose’. Over-convergence in the infant may indicate an incipient esotropia. Divergence may reflect a tendency to divergence or simply lack of interest in the target.

Tests for sensory anomalies

Worth four-dot

This is a dissociation test which can be used with both distance and near fixation and differentiates between BSV, ARC and suppression. Results can only be interpreted if the presence or absence of a manifest squint is known at time of testing.

1 Procedure

a The patient wears a green lens in front of the right eye, which filters out all colours except green, and a red lens in front of the left eye which will filter out all colours except red (Fig. 18.24A).

b The patient then views a box with four lights: one red, two green and one white.

Page 751

2 Results (Fig. 18.24B)

• If BSV is present all four lights are seen.

• If all four lights are seen in the presence of a manifest deviation, harmonious ARC (see ‘synoptophore’ below) is present.

• If two red lights are seen, right suppression is present.

• If three green lights are seen, left suppression is present.

• If two red and three green lights are seen, diplopia is present.

• If the green and red lights alternate, alternating suppression is present.

Fig. 18.24 Worth four-dot test. (A) Red-green glasses; (B) possible results

Bagolini striated glasses

This is a test for detecting BSV, ARC or suppression. Each lens has fine striations which convert a point source of light into a line, as with the Maddox rod (see below).

1 Procedure

a The two lenses are placed at 45° and 135° in front of each eye and the patient fixates a small light source (Fig. 18.25A).

b Each eye perceives an oblique line of light, perpendicular to that perceived by the fellow eye (Fig. 18.25B).

Page 752

c Dissimilar images are thus presented to each eye under binocular viewing conditions.

2 Results (Fig. 18.25C) cannot be interpreted correctly unless it is known whether or not strabismus is present:

• If the two streaks intersect at their centres in the form of an oblique cross (an ‘X’), the patient has BSV if the eyes are straight, or harmonious ARC in the presence of manifest strabismus.

• If the two lines are seen but they do not form a cross, diplopia is present.

• If only one streak is seen, there is no simultaneous perception and suppression is present.

• In theory, if a small gap is seen in one of the streaks, a central suppression scotoma (as found in microtropia) is present. In practice this is often difficult to demonstrate and the patient describes a cross. The scotoma can be confirmed with the 4 Δ prism test (see below).

Fig. 18.25 Bagolini test

(A) Striated glasses; (B) appearance of a point of light through Bagolini lenses; (C) possible results

4 Δ prism test

This test differentiates bifoveal fixation (normal BSV) from a central suppression scotoma (CSS) in microtropia and employs the principle described in the 20 Δ test (Hering law and convergence) to overcome diplopia.

1 In bifoveal fixation the response is as follows:

a The prism is placed base-out in front of the right eye with deviation of the image temporally and movement of both eyes to the left (Fig. 18.26A).

Page 753

b The left eye converges to fuse the images (Fig. 18.26B).

2 In left microtropia with CSS the response is as follows:

a The patient fixates a distance target with both eyes open and a 4 Δ prism is placed base-out in front of the left eye with suspected CSS.

b The image is moved temporally in the left eye but falls within the CSS and no movement of either eye is observed (Fig. 18.27A).

c The prism is then moved to the right eye which adducts to maintain fixation; the left eye similarly moves to the left (Hering), but the second image falls within the CSS and no refixation movement is seen (Fig. 18.27B).

Fig. 18.26 4 Δ prism test in bifoveal fixation. (A) Shift of both eyes away from the prism base; (B) fusional refixation movement of the left eye

Fig. 18.27 4 Δ prism test in left microtropia with a central suppression scotoma. (A) No movement of either eye; (B) both eyes move to the left but there is absence of re-fixation

Synoptophore

The synoptophore compensates for the angle of squint and allows stimuli to be presented to both eyes simultaneously (Fig. 18.28A). It can thus be used to investigate the potential for binocular function in the presence of a manifest squint and is of particular value in testing young children (from age 3 years), who generally find it enjoyable. It can also detect suppression and ARC.

• The instrument consists of two cylindrical tubes with a mirrored right-angled bend and a +6.50 D lens in each eyepiece (Fig. 18.28B, top). This optically sets the testing distance as equivalent to about 6 metres.

Page 754

• Pictures are inserted in a slide carrier situated at the outer end of each tube. The two tubes are supported on columns which enable the pictures to be moved in relation to each other, and any adjustments are indicated on a scale.

• The synoptophore can measure horizontal, vertical and torsional misalignments simultaneously and is valuable in determining surgical approach by assessing the different contributions in the cardinal positions of gaze.

Fig. 18.28 (A) Synoptophore; (B) optical principles and grading of binocular vision

Grades of binocular vision

Binocular vision is graded on the synoptophore as follows (Fig. 18.28B, bottom):

1 First grade (simultaneous perception, SP) is tested by introducing two dissimilar but not mutually antagonistic pictures, such as a bird and a cage.

• The subject is then asked to put the bird into the cage by moving the arm of the synoptophore.

• If the two pictures cannot be seen simultaneously, then suppression is present.

• Some retinal ‘rivalry’ will occur although one picture is smaller than the other, so that while the small one is seen foveally, the larger one is seen parafoveally (and is thus placed in front of the deviating eye).

• Larger macular and paramacular slides are used if foveal slides cannot be superimposed.

2 Second grade (fusion). If simultaneous perception slides can be superimposed then the test proceeds to the second grade which is the ability of the two eyes to produce a composite picture (sensory fusion) from two similar pictures, each of which is incomplete in one small different detail.

• The classic example is two rabbits, one lacking a tail and the other lacking a bunch of flowers. If fusion is present, one rabbit complete with tail and flowers will be seen.

• The range of fusion (motor fusion) is then tested by moving the arms of the synoptophore so that the eyes have to converge and diverge in order to maintain fusion.

• The presence of simple fusion without any range is of little value in everyday life.

3 Third grade (stereopsis) is the ability to obtain an impression of depth by the superimposition of two pictures of the same object which have been taken from slightly different angles. The classic example is the bucket which is appreciated in three dimensions.

Detection of abnormal retinal correspondence

ARC can be detected on the synoptophore as follows:

a The subjective angle of deviation is that at which the SP slides are superimposed. The examiner determines the objective angle of the deviation by presenting each fovea alternately with a target by extinguishing one or other light and moving the slide in front of the deviating eye until no movement of the eyes is seen.

b If the subjective and objective angles coincide then retinal correspondence is normal.

c If the objective and subjective angles are different, ARC is present. The difference in degrees between the subjective and objective angles is the angle of anomaly. ARC is said to be harmonious when the objective angle equals the angle of anomaly and inharmonious when it exceeds the angle of anomaly. It is only in harmonious ARC that binocular responses can be demonstrated; the inharmonious form may represent a lesser adaptation or an artefact of testing.

Page 755

Measurement of deviation

Hirschberg test

The Hirschberg test gives a rough objective estimate of the angle of a manifest strabismus and is especially useful in young or uncooperative patients or when fixation in the deviating eye is poor.

• A pen-torch is shone into the eyes from arm’s length and the patient asked to fixate the light. The corneal reflection of the light will be (more or less) centred in the pupil of the fixating eye, but will be decentred in a squinting eye, in the direction opposite to that of the deviation.

• The distance of the corneal light reflection from the centre of the pupil is noted; each mm of deviation is approximately equal to 7° (one degree ≈ 2 prism dioptres).

• For example, if the reflex is situated at the temporal border of the pupil (assuming a pupillary diameter of 4 mm), the angle is about 15° (Fig. 18.29A); if it is at the limbus, the angle is about 45° (Fig. 18.29B and C). This test is also useful in detecting pseudostrabismus, which may be caused by the following conditions:

1 Epicanthic folds may simulate an esotropia (Fig. 18.30A).

2 Abnormal interpupillary distance; if short may simulate an esotropia and if wide an exotropia (Fig. 18.30B).

3 Angle kappa is the angle between the visual and anatomical (pupillary) axes (see Fig. 18.1).

• Normally, the fovea is situated temporal to the anatomical centre of the posterior pole. The eyes are therefore slightly abducted to achieve bifoveal fixation and a light shone onto the cornea will therefore cause a reflex just nasal to the centre of the cornea in both eyes (Fig. 18.31A). This is termed a positive angle kappa.

• A large positive angle kappa may simulate an exotropia (Fig. 18.31B).

• A negative angle kappa occurs when the fovea is situated nasal to the posterior pole (high myopia and ectopic fovea). In this situation, the corneal reflex is situated temporally to the centre of the cornea and it may simulate an esotropia (Fig. 18.31C).

Fig. 18.29 Hirschberg test. (A) The right corneal reflex is near the temporal border of the pupil indicating an angle of about 15°; (B) the left corneal reflex is at the limbus indicating an angle of about 45°; (C) right corneal reflex is at the limbus in a divergent squint

(Courtesy of J Yanguela – fig. A)

Fig. 18.30 Pseudostrabismus. (A) Prominent epicanthic folds simulating esotropia; (B) wide interpupillary distance simulating exotropia

Fig. 18.31 Angle kappa (A) Normal; (B) negative simulates an exotropia; (C) positive simulates an esotropia

Krimsky and prism reflection tests

Corneal reflex assessment can be combined with prisms to give a more accurate approximation of the angle in a manifest deviation.

1 Krimsky test involves placement of prisms in front of the fixating eye until the corneal light reflections are symmetrical (Fig. 18.32). This test reduces the problem of parallax and is more commonly used than the prism reflection test.

2 Prism reflection test involves the placement of prisms in front of the deviating eye until the corneal light reflections are symmetrical.

Fig. 18.32 Krimsky test

(Courtesy of K Nischal)

Cover–uncover test

The cover–uncover test consists of two parts:

Page 756

1 Cover test to detect a heterotropia. It is helpful to begin the near test using a light to observe the corneal reflections and to assess the fixation in the deviating eye. It should then be repeated for near using an accommodative target and for distance as follows:

a The patient fixates a straight-ahead target.

b If a right deviation is suspected, the examiner covers the fixing left eye and notes any movement of the right eye to take up fixation.

c No movement indicates orthotropia (Fig. 18.33A) or left heterotropia (Fig. 18.33B).

d Adduction of the right eye to take up fixation indicates right exotropia and abduction right esotropia (Fig. 18.33C).

e Downward movement indicates right hypertropia and upward movement right hypotropia.

f The test is repeated on the opposite eye.

2 Uncover test detects heterophoria. It should be performed both for near (using an accommodative target) and for distance as follows:

a The patient fixates a straight-ahead distant target.

b The examiner covers the right eye and after 2–3 seconds removes the cover.

c No movement indicates orthophoria (Fig. 18.34A); a keen observer will frequently detect a very slight latent deviation in most normal individuals, as very few people are truly orthophoric, particularly on near fixation.

d If the right eye had deviated while under cover, a re-fixation movement (recovery to BSV) is observed on being uncovered.

e Adduction (nasal recovery) of the right eye indicated exophoria (Fig. 18.34B) and abduction esophoria (Fig. 18.34C).

f Upward or downward movement indicates a vertical phoria.

g After the cover is removed, the examiner notes the speed and smoothness of recovery as evidence of the strength of motor fusion.

h The test is repeated for the opposite eye.

Page 757

Fig. 18.33 Possible results of the cover test

Fig. 18.34 Possible results of the uncover test

Most examiners perform the cover test and the uncover test sequentially, hence the term cover–uncover test.

Alternate cover test

The alternate cover test is a dissociation test which reveals the total deviation when fusion is suspended. It should be performed after the cover–uncover test.

a The right eye is covered for several seconds.

b The occluder is quickly shifted to the opposite eye for 2 seconds, then back and forth several times. After the cover is removed, the examiner notes the speed and smoothness of recovery as the eyes return to their pre-dissociated state.

c A patient with a well compensated heterophoria will have straight eyes before and after the test has been performed whereas a patient with poor control may decompensate to a manifest deviation.

Prism cover test

The prism cover test measures the angle of deviation on near or distance fixation and in any gaze position. It combines the alternate cover test with prisms and is performed as follows:

a The alternate cover test is first performed.

b Prisms of increasing strength are placed in front of one eye with the base opposite the direction of the deviation (i.e. point the apex of the prism in the direction of the deviation). For example, in a convergent strabismus the prism is held base-out, and in a right hypertropia, base down before the right eye.

c The alternate cover test is continuously performed (Fig. 18.35). As stronger prisms are brought in, the amplitude of the re-fixation movement gradually decreases.

Page 758

d The end-point is approached when no movement is seen; to ensure the maximum angle is found, the prism strength is increased further until a movement is observed in the opposite direction (point of reversal) and then reduced again to find the neutral value; the angle of deviation then equals the strength of the prism.

Fig. 18.35 Prism cover test

Maddox wing

The Maddox wing dissociates the eyes for near fixation (1/3 m) and measures heterophoria. The instrument is constructed in such a way that the right eye sees only a white vertical arrow and a red horizontal arrow, whereas the left eye sees only horizontal and vertical rows of numbers (Fig. 18.36). Measurements are made as follows:

a The horizontal deviation is measured by asking the patient to which number the white arrow points.

b The vertical deviation is measured by asking the patient which number the red arrow intersects.

c The amount of cyclophoria is determined by asking the patient to move the red arrow so that it is parallel with the horizontal row of numbers.

Fig. 18.36 Maddox wing

Maddox rod

The Maddox rod consists of a series of fused cylindrical red glass rods which convert the appearance of a white spot of light into a red streak. The optical properties of the rods cause the streak of light to be at an angle of 90° with the long axis of the rods; when the glass rods are held horizontally, the streak will be vertical and vice versa. The test is performed as follows:

a The rod is placed in front of the right eye (Fig. 18.37A). This dissociates the two eyes because the red streak seen by the right eye cannot be fused with the unaltered white spot of light seen by the left eye (Fig. 18.37B).

b The amount of dissociation (Fig. 18.37C) is measured by the superimposition of the two images using prisms. The base of the prism is placed in the position opposite to the direction of the deviation.

c Both vertical and horizontal deviations can be measured in this way but the test cannot differentiate phoria from tropia.

Fig. 18.37 (A) Maddox rod test; (B) appearance of a point of light through Maddox rods; (C) possible results

Motility tests

Ocular movements

Examination of ocular movements involves assessment of smooth pursuit movements followed by that of saccadic movements.

1 Versions towards the eight eccentric positions of gaze are tested by asking the patient to follow a target, usually a pen or pen-torch (the latter offers the advantage of corneal light reflections to aid assessment). A quick cover test is performed in each position of gaze to confirm whether a phoria has become a tropia or the angle has increased and the patient is questioned regarding diplopia. Versions may also be elicited involuntarily in response to a noise or by the doll’s head manoeuvre in uncooperative patients.

2 Ductions are assessed if reduced ocular motility is noted in either or both eyes. A pen-torch should be used with careful attention to the position of the corneal reflexes. The fellow eye is occluded and the patient asked to follow the torch into various positions of gaze. A simple numeric system may be employed using 0 to denote full movement, and −1 to −4 to denote increasing degrees of underaction (Fig. 18.38).

Fig. 18.38 Grading of right lateral rectus underaction

Near point of convergence

The near point of convergence (NPC) is the nearest point on which the eyes can maintain binocular fixation. It can be measured with the RAF rule which rests on the patient’s cheeks (Fig. 18.39A). A target (Fig. 18.39B) is slowly moved along the rule towards the patient’s eyes until one eye loses fixation and drifts laterally (objective NPC). The subjective NPC is the point at which the patient reports diplopia. Normal NPC should be nearer than 10 cm without undue effort.

Fig. 18.39 (A) RAF rule; (B) convergence target

Near point of accommodation

The near point of accommodation (NPA) is the nearest point on which the eyes can maintain clear focus. It can also be measured with the RAF rule. The patient fixates a line of print, which is then slowly moved towards the patient until it becomes blurred. The distance at which this is first reported is read off the rule and denotes the NPA. The NPA recedes with age; when sufficiently far away to render reading difficult without optical correction, presbyopia is present. At the age of 20 years the NPA is 8 cm and by the age of 50 years it has receded to approximately 46 cm. The amplitude of accommodation can also be assessed using concave lenses in 0.5 DS steps whilst fixating the 6/6 Snellen line and reporting when the vision blurs.

Page 759

Fusional amplitudes

Fusional amplitudes measure the efficacy of vergence movements. They may be tested with prisms bars or the synoptophore. An increasingly strong prism is placed in front of one eye, which will then abduct or adduct (depending on whether the prism is base-in or base-out), in order to maintain bifoveal fixation. When a prism greater than the fusional amplitude is reached, diplopia is reported or one eye drifts the other way, indicating the limit of vergence ability.

Postoperative diplopia test

This simple test is mandatory prior to strabismus surgery in all non-binocular patients over 7–8 years of age to assess the risk of diplopia after surgery.

• Corrective prisms are placed in front of one eye (usually the deviating eye) and the patient asked to fixate a straight-ahead target with both eyes open. The prisms are slowly increased until the angle has been significantly overcorrected and the patient reports if diplopia occurs.

• If suppression persists throughout there is little risk of diplopia following surgery; in a consecutive exotropia of 35 Δ, diplopia may be reported from 30 Δ and persist as the prism correction mimics an esotropia.

• Diplopia may be intermittent or constant but in either case constitutes an indication to perform a diagnostic botulinum toxin test (see below).

• Diplopia is not restricted to patients with good visual acuity in the deviating eye.

• Intractable diplopia is a difficult condition to treat.

Investigation of diplopia

The Hess screen and the Lees screen are two similar tests that plot the dissociated ocular position as a function of extraocular muscle action and enable differentiation of paretic strabismus caused by neurological pathology from restrictive myopathy such as in thyroid eye disease or a blow-out fracture of the orbit, and recent onset paresis from long-standing. They also allow quantitative monitoring of progress in a range of conditions.

Page 760

Electronic Hess test

The screen contains a tangent pattern (2D projection of a spherical surface) printed onto a dark grey background. Red lights that can be individually illuminated by a control panel indicate the cardinal positions of gaze within a central field (15° from primary position) and a peripheral field (30°); each square represents 5° of ocular rotation.

a The patient is seated 50 cm from the screen and wears red-green goggles (red lens in front of the right eye) and holds a green pointer.

b The examiner illuminates each point in turn which is used as the point of fixation. This can now be seen only with the right eye, which therefore becomes the fixating eye.

c The patient is asked to superimpose their green light on the red light, so plotting the relative position of the left eye. All the points are plotted in turn.

d In orthophoria the two lights should be more or less superimposed in all nine positions of gaze.

e The goggles are then reversed (red filter in front of the left eye) and the procedure is repeated.

f The relative positions are marked by the examiner on a Hess chart and connected with straight lines.

Lees screen

The apparatus consists of two opalescent glass screens at right-angles to each other, bisected by a two-sided plane mirror which dissociates the two eyes (Fig. 18.40). Each screen has a tangent pattern marked onto the back surface which is revealed only when the screen is illuminated.

1 Procedure. The test is performed with each eye fixating in turn.

Page 761

a The patient faces the non-illuminated screen with his chin stabilized on a chin rest attached to the mirror support and fixates the dots in the mirror.

b The examiner indicates the dot required for the patient to plot.

c The patient positions the pointer on the non-illuminated screen at a position perceived to be on top of the dot indicated by the examiner.

d When all of the dots have been plotted on a Hess chart the patient is repositioned to face the other screen and the procedure is repeated. The results are charted as before.

2 Interpretation

a The two charts are compared (Fig. 18.41).

b The smaller chart indicates the eye with the paretic muscle (right eye).

c The larger chart indicates the eye with the overacting yoke muscle (left eye).

d The smaller chart will show its greatest restriction in the main direction of action of the paretic muscle (right lateral rectus).

e The larger chart will show its greatest expansion in the main direction of action of the yoke muscle (left medial rectus).

f The degree of disparity between the plotted point and the template in any position of gaze gives an estimate of the angle of deviation (each square = 5°).

Fig. 18.40 Lees screen

Fig. 18.41 Hess chart of a recent right lateral rectus palsy

Changes with time

Changes with time are very useful as a prognostic guide.

• For example, in right superior rectus palsy, the Hess chart will show underaction of the affected muscle with an overaction of its yoke muscle (left inferior oblique) (Fig. 18.42A). Because of the great incomitance of the two charts, the diagnosis is straightforward. If the paretic muscle recovers its function, both charts will revert to normal.

• Secondary contracture of the ipsilateral antagonist (right inferior rectus) will show up on the chart as an overaction which will lead to a secondary (inhibitional) palsy of the antagonist of the yoke muscle (left superior oblique), which will show up on the chart as an underaction (Fig. 18.42B). This could lead to the incorrect impression that the left superior oblique was the primary muscle at fault.

• With further passage of time, the two charts become more and more concomitant until it may be impossible to determine which was the primary paretic muscle (Fig. 18.42C).

Fig. 18.42 Hess chart showing changes with time of a right superior rectus palsy

Clinical examples

By analyzing the following examples familiarization is gained with ocular motor nerve palsies as discussed in Chapter 19.

1 Left 3rd nerve palsy (Fig. 18.43).

• The left chart is much smaller than the right.

• Left exotropia – note that the fixation spots in the inner charts of both eyes are deviated laterally. The deviation is greater on the right chart (when the left eye is fixating), indicating that secondary deviation exceeds the primary, typical of a paretic squint.

• Left chart shows underaction of all muscles except the lateral rectus.

• Right chart shows overaction of all muscles except the medial rectus and inferior rectus, the ‘yokes’ of the spared muscles.

• The primary angle of deviation (fixing right eye – FR) in the primary position is −20° and R/L 10°.

Page 762

• The secondary angle (fixing left eye – FL) is −28° and R/L 12°.

Fig. 18.43 Hess chart of a left 3rd nerve palsy

In inferior rectus palsy, the function of the superior oblique muscle can only be assessed by observing intorsion on attempted depression. This is best performed by observing a conjunctival landmark on the slit-lamp.

2 Recently acquired right 4th nerve palsy (Fig. 18.44).

• Right chart is smaller than the left.

Page 763

• Right chart shows underaction of the superior oblique and overaction of the inferior oblique.

• Left chart shows overaction of the inferior rectus and underaction (inhibitional palsy) of the superior rectus.

• The primary deviation (FL) is R/L 8°; the secondary deviation FR is R/L 17°.

3 Congenital right 4th nerve palsy (Fig. 18.45).

• No difference in overall chart size.

• Primary and secondary deviation R/L 4°.

• Right hypertropia – note that the fixation spot of the right inner chart is deviated upwards and the left is deviated downwards.

• Hypertropia increases on laevoversion and reduces on dextroversion.

• Right chart shows underaction of the superior oblique and overaction of the inferior oblique.

• Left chart shows overaction of the inferior rectus and underaction (inhibitional palsy) of the superior rectus.

4 Right 6th nerve palsy (Fig. 18.46).

• Right chart is smaller than the left.

• Right esotropia – note that the fixation spot of the right inner chart is deviated nasally.

• Right chart shows marked underaction of the lateral rectus and slight overaction of the medial rectus.

• Left chart shows marked overaction of the medial rectus.

• The primary angle FL is +15° and the secondary angle FR +20°.

• Inhibitional palsy of the left lateral rectus has not yet developed.

Fig. 18.44 Hess chart of a recently acquired right 4th nerve palsy

Fig. 18.45 Hess chart of a congenital right 4th nerve palsy

Fig. 18.46 Hess chart of a right 6th nerve palsy

Refraction and fundoscopy

It should be emphasized that dilated fundoscopy is mandatory in the context of strabismus, to exclude any underlying ocular pathology such as macular scarring, optic disc hypoplasia or retinoblastoma. Strabismus is often secondary to refractive error and hypermetropia (hyperopia), astigmatism, anisometropia and myopia may all be associated.

Cycloplegia

The commonest refractive error causing strabismus is hypermetropia. Accurate measurements of hypermetropia necessitate effective paralysis of the ciliary muscle (cycloplegia), in order to neutralize the effect of accommodation, which masks the true degree of this refractive error.

1 Cyclopentolate affords adequate cycloplegia in most children.

• The concentration employed is 0.5% under the age of 6 months and 1% thereafter. One drop, repeated after 5 minutes, usually results in maximal cycloplegia within 30 minutes, with recovery of accommodation within 2–3 hours and resolution of mydriasis within 24 hours.

• The adequacy of cycloplegia can be determined by comparing retinoscopy readings with the patient fixating for distance and then for near. If cycloplegia is adequate, there will be little or no difference.

Page 764

• If cycloplegia is incomplete there will be a difference between the two readings and it may be necessary to wait another 15 minutes and to instil another drop.

• Topical anaesthesia with an agent such as proxymetacaine prior to instillation of cyclopentolate is useful in preventing ocular irritation and reflex tearing, thus affording better retention of the cyclopentolate in the conjunctival sac and effective cycloplegia.

2 Atropine may be necessary in some children with either high hypermetropia or heavily pigmented irides, in whom cyclopentolate may be inadequate.

• Atropine may be used as drops or ointment. Drops are easier for an untrained person to instil, but there is less risk of overdose with ointment. The concentration is 0.5% under the age of 12 months and 1% thereafter. Maximal cycloplegia occurs at 3 hours; recovery of accommodation starts after about 3 days and is usually complete by 10 days.

• Atropine is instilled b.d. for 3 days before retinoscopy, but not on the day of examination. The parents should be warned to discontinue medication if there are signs of systemic toxicity, such as flushing, fever or restlessness, and to seek immediate medical attention.

Change of refraction

Because refraction changes with age, it is important to check this in patients with strabismus at least every year and more frequently in younger children and if acuity is reduced. At birth most babies are hypermetropic. After the age of 2 years there may be an increase in hypermetropia and a decrease in astigmatism. Hypermetropia may continue to increase until the age of about 6 years, levelling off between the ages of 6 and 8 and subsequently.

When to prescribe

Most children are mildly hypermetropic (1 to 3 D). There is some evidence that fully correcting hypermetropia in a normal child may reduce physiological emmetropization.

1 Hypermetropia. In general up to 4 D of hypermetropia should not be corrected in a child without a squint unless they are having problems with near vision. With degrees of hypermetropia greater than this a two-thirds correction is usually given. However, in the presence of esotropia the full cycloplegic correction should be prescribed, even under the age of 2 years.

2 Astigmatism. A cylinder of 1.50 D or more should be prescribed, especially in cases of anisometropia after the age of 18 months.

3 Myopia. The necessity for correction depends on the age of the child. Under the age of 2 years, −5.00 D or more of myopia should be corrected; between the ages of 2 and 4 the amount is −3.00 D. Older children should have correction of even low myopia to allow clear distance vision.

4 Anisometropia. After the age of 3 the full difference in refraction between the eyes should be prescribed if it is more than 1D. If there is no squint then any associated hypermetropic correction may be equally reduced for each eye.

Page 765

Heterophoria

Heterophoria may present clinically with associated visual symptoms, particularly at times of stress or poor health, when the fusional amplitudes are insufficient to maintain alignment.

1 Signs. Both esophoria and exophoria can be classified by the distance at which the angle is greater: (respectively, convergence excess or weakness, divergence weakness or excess and mixed).

2 Treatment

• Orthoptic treatment is of most value in convergence weakness exophoria.

• Any significant refractive error should be appropriately corrected.

• Symptom relief may otherwise be obtained using temporary stick-on Fresnel prisms and may be subsequently incorporated into spectacles (maximum usually 10–12 Δ, split between the two eyes).

• Surgery may occasionally be required for larger deviations.

Vergence abnormalities

Convergence insufficiency

Convergence insufficiency (CI) typically affects individuals, such as students, with excessive near visual demand.

1 Signs. Reduced near point of convergence independent of any heterophoria.

2 Treatment involves orthoptic exercises aimed at normalizing the near point and maximizing fusional amplitudes. With good compliance, symptoms should be eliminated within a few weeks but if persistent can be treated with base-in prisms.

3 Accommodative insufficiency (AI) is occasionally also present. It may be idiopathic (primary) or post-viral and typically affects school-age children. The minimum reading correction to give clear vision is prescribed but is often difficult to discard.

Divergence insufficiency

Divergence paresis or paralysis is a rare condition typically associated with underlying neurological disease, such as intracranial space-occupying lesions, cerebrovascular accidents and head trauma. Presentation may be at any age and may be difficult to differentiate from 6th nerve palsy, but is primarily a concomitant esodeviation with reduced or absent divergence fusional amplitudes. It is difficult to treat; prisms are the best option.

Near reflex insufficiency

1 Paresis of the near reflex presents as dual convergence and accommodation insufficiency. Mydriasis may be seen on attempted near fixation. Treatment involves reading glasses, base-in prisms and possibly botulinum toxin (orthoptic exercises have no effect). It is difficult to eradicate.

2 Complete paralysis in which no convergence or accommodation can be initiated may be of functional origin, due to midbrain disease or may follow head trauma; recovery is possible.

Spasm of the near reflex

Spasm of the near reflex is a functional condition affecting patients of all ages (mainly females).

1 Signs

• Diplopia, blurred vision and headaches are accompanied by esotropia, pseudomyopia and miosis.

• Spasm may be triggered when testing ocular movements (Fig. 18.47A).

• Observation of miosis is the key to the diagnosis (Fig. 18.47B).

• Refraction with and without cycloplegia confirms the pseudomyopia, which must not be corrected optically.

2 Treatment involves reassurance and advising the patient to discontinue any activity that triggers the response. If persistent, atropine and a full reading correction are prescribed but it is difficult later to abandon treatment without recurrence. Patients usually manage to live a fairly normal life despite the signs and symptoms.

Fig. 18.47 (A) Spasm of the near reflex precipitated on testing ocular movements; (B) right esotropia and miosis

Esotropia

Page 766

Esotropia (manifest convergent squint) may be concomitant or incomitant. In a concomitant esotropia the variability of the angle of deviation is within 5 Δ in different horizontal gaze positions. In an incomitant deviation the angle differs in various positions of gaze as a result of abnormal innervation or restriction. This section deals only with concomitant esotropia. A classification is shown in Table 18.1. However, all squints are different and not all fit neatly into a classification. For example, a microtropia may occur with a number of the other categories. It is more important to understand the part played by binocular function, refractive error and accommodation in the pathophysiology of each individual squint and to tailor treatment accordingly.

Table 18.1 Classification of esotropia

1. Accommodative

a Refractive

• Fully accommodative

• Partially accommodative

b Non-refractive

• With convergence excess

• With accommodation weakness

c Mixed

2. Non-accommodative

• Essential infantile

• Microtropia

• Basic

• Convergence excess

• Convergence spasm

• Divergence insufficiency

• Divergence paralysis

• Sensory

• Consecutive

• Acute onset

• Cyclic

Early-onset esotropia

Up to the age of 4 months, infrequent episodes of convergence are normal but thereafter ocular misalignment is abnormal. Early-onset (congenital, essential, infantile) esotropia is an idiopathic esotropia developing within the first 6 months of life in an otherwise normal infant with no significant refractive error and no limitation of ocular movements.

Signs

• The angle is usually fairly large (>30 Δ) and stable.

• Fixation in most infants is alternating in the primary position (Fig. 18.48).

• There is cross-fixating in side gaze, so that the child uses the left eye in right gaze (Fig. 18.49A) and the right eye on left gaze (Fig. 18.49B). Such cross-fixation may give a false impression of bilateral abduction deficits, as in bilateral 6th nerve palsy.

• Abduction can usually be demonstrated, either by the doll’s head manoeuvre or by rotating the child.

• Should these fail, uniocular patching for a few hours will often unmask the ability of the other eye to abduct.

• Nystagmus is usually horizontal.

• Latent nystagmus (LN) is only seen when one eye is covered and the fast phase beats towards the side of the fixing eye. This means that the direction of the fast phase reverses according to which eye is covered.

• Manifest latent nystagmus (MLN) is the same except that nystagmus is present with both eyes open, but the amplitude increases when one is covered.

• The refractive error is usually normal for the age of the child (about +1 to +2 D).

• Asymmetry of optokinetic nystagmus is present.

• Inferior oblique overaction may be present initially or develop later (see Fig. 18.51).

• Dissociated vertical deviation (DVD) develops in 80% by the age of 3 years (see Fig. 18.52).

Fig. 18.48 Alternating fixation in early-onset esotropia. (A) Fixating right eye; (B) fixating left eye

(Courtesy of J Yangüela)

Fig. 18.49 Cross fixation in early-onset esotropia. (A) left fixation on right gaze; (B) right fixation on left gaze

(Courtesy of R Bates)

Initial treatment

Page 767

Early ocular alignment gives the best change of the child developing some degree of binocular function. Ideally, the eyes should be surgically aligned by the age of 12 months, and at the very latest by the age of 2 years, but only after amblyopia or significant refractive errors have been corrected.

• The initial procedure can be either recession of both medial recti or unilateral medial rectus recession with lateral rectus resection. Very large angles may require recessions of 6.5 mm or more. Associated significant inferior oblique overaction should also be addressed.

• An acceptable goal is alignment of the eyes to within 10 Δ.

• Associated with peripheral fusion and central suppression (Fig. 18.50). This small-angle residual strabismus is often stable, even though bifoveal fusion is not achieved.

Fig. 18.50 Early onset esotropia. (A) Before surgery; (B) after surgery

Subsequent treatment

1 Undercorrection may require further recession of the medial recti, resection of one or both lateral recti or surgery to the other eye, depending on the initial procedure.

2 Inferior oblique overaction may develop subsequently, most commonly at age 2 years (Fig. 18.51). The parents should therefore be warned that further surgery may be necessary despite an initially good result. Initially unilateral, it frequently becomes bilateral within 6 months. Inferior oblique weakening procedures include disinsertion, recession and myectomy.

Page 768

3 DVD may appear several years after the initial surgery, particularly in children with nystagmus. It is characterized by the following:

• Up-drift with excyclorotation of the eye when under cover (Fig. 18.52B) or spontaneously during periods of visual inattention.

• When the cover is removed the affected eye will move down without a corresponding down-drift of the other eye (Fig. 18.52C).

• Thus DVD does not obey the Hering law. Although it is usually bilateral, it may be asymmetrical.

• Surgical treatment is indicated when the condition is cosmetically unacceptable. Superior rectus recession with or without posterior fixation sutures (see later) or inferior oblique anterior transposition are useful for DVD, although full elimination is seldom possible.

4 Amblyopia subsequently develops in about 50% of cases as unilateral fixation preference commonly develops postoperatively.

5 An accommodative element should be suspected if the eyes are initially straight or almost straight after surgery and then start to reconverge. It is therefore important to perform repeated refractions on all children and to correct any new accommodative elements accordingly.

Fig. 18.51 Bilateral inferior oblique overaction. (A) Straight eyes in the primary position; (B) left inferior oblique overaction on right gaze; (C) right inferior oblique overaction on left gaze

Fig. 18.52 Dissociated vertical deviation. (A) Straight eyes in the primary position; (B) up-drift of left eye under cover; (C) up-drift of right eye under cover and down-drift of left eye

Differential diagnosis

1 Congenital bilateral 6th nerve palsy, which is rare and can be excluded as described above.

2 Secondary (sensory) esotropia due to organic eye disease.

3 Nystagmus blockage syndrome in which convergence dampens a horizontal nystagmus. Nystagmus can be elicited on abduction and the infant adopts a face turn to fixate in the adducted position.

4 Duane syndrome types I and III.

5 Möbius syndrome.

6 Strabismus fixus.

Accommodative esotropia

Near vision involves both accommodation and convergence. Accommodation is the process by which the eye focuses on a near target, by altering the curvature of the crystalline lens. Simultaneously, the eyes converge, in order to fixate bi-foveally on the target. Both accommodation and convergence are quantitatively related to the proximity of the target, and have a fairly constant relationship to each other (AC/A ratio) as described previously. Abnormalities of the AC/A ratio are an important cause of certain types of esotropia.

Refractive accommodative esotropia

In this type of accommodative esotropia, the AC/A ratio is normal and esotropia is a physiological response to excessive hypermetropia, usually between +2.00 and +7.00 D. The considerable degree of accommodation required to focus clearly on even a distant target is accompanied by a proportionate amount of convergence, which is beyond the patient’s fusional divergence amplitude. It cannot therefore be controlled, and a manifest convergent squint results. The magnitude of the deviation varies little (usually <10 Δ) between distance and near. The deviation typically presents at the age of 18 months–3 years (range 6 months–7 years).

1 Fully accommodative esotropia is characterized by hypermetropia with esotropia when the refractive error is uncorrected (Fig. 18.53A). The deviation is eliminated and BSV is present at all distances following optical correction of hypermetropia (Fig. 18.53B).

2 Partially accommodative esotropia is reduced, but not eliminated by full correction of hypermetropia (Fig. 18.54). Amblyopia is frequent as well as bilateral congenital superior oblique weakness. Most cases show suppression of the squinting eye although ARC may occur, but of lower grade than in microtropia.

Fig. 18.53 Fully accommodative esotropia. (A) Left esotropia without glasses; (B) straight eyes for near and distance with glasses

Fig. 18.54 Partially accommodative esotropia. (A) Right esotropia without glasses; (B) angle is reduced but not eliminated with glasses

Non-refractive accommodative esotropia

In this type of accommodative esotropia the AC/A ratio is high so that a unit increase of accommodation is accompanied by a disproportionately large increase in convergence. This occurs independently of refractive error, although hypermetropia frequently coexists. It can be subdivided into:

1 Convergence excess

• High AC/A ratio due to increased accommodative convergence (accommodation is normal, convergence is increased).

Page 769

• Normal near point of accommodation.

• Straight eyes with BSV for distance (Fig. 18.55A).

• Esotropia for near, usually with suppression (Fig. 18.55B).

• Straight eyes through bifocals (Fig. 18.55C).

2 Hypoaccommodative convergence excess

• High AC/A ratio due to decreased accommodation (accommodation is weak, necessitating increased effort, which produces over-convergence).

• Remote near point of accommodation.

• Straight eyes with BSV for distance.

• Esotropia for near, usually with suppression.

Fig. 18.55 Convergence excess esotropia. (A) Eyes straight for distance; (B) right esotropia for near; (C) eyes straight when looking through bifocals

Treatment

1 Correction of refractive error is the initial treatment.

• In children under the age of 6 years, the full cycloplegic refraction revealed on retinoscopy should be prescribed, with a deduction only for the working distance. In the fully accommodative refractive esotrope this will control the deviation for both near and distance.

• After the age of 8 years, refraction should be performed without cycloplegia and the maximal amount of ‘plus’ that can be tolerated (manifest hypermetropia) prescribed.

• For convergence excess esotropia bifocals may be prescribed to relieve accommodation (and thereby accommodative convergence), thus allowing the child to maintain bi-foveal fixation and ocular alignment at near (see Fig. 18.55C). The minimum ‘add’ required to achieve this is prescribed.

Page 770

• The most satisfactory form of bifocals is the executive type in which the intersection crosses the lower border of the pupil. The strength of the lower segment should be gradually reduced and eliminated by the early teenage years.

• Bifocals are best suited to hypoaccommodative esotropia where the AC/A ratio is not overly excessive and there is a reasonable chance of discarding bifocal correction with time.

• At higher levels surgery is the better long-term option. The ultimate prognosis for complete withdrawal of spectacles is related to the magnitude of the AC/A ratio and to the degree of hypermetropia and associated astigmatism. Spectacles may be needed only for close work.

2 Surgery is aimed at restoring or improving BSV, or to improve the appearance of the squint and so the child’s social functioning.

• Surgery should only be considered if spectacles do not fully correct the deviation and after every attempt has been made to treat amblyopia.

• Bilateral medial rectus recessions are performed in patients in whom the deviation for near is greater than that for distance.

• If there is no significant difference between distance and near measurements, and equal vision in both eyes, some perform unilateral medial rectus recession combined with lateral rectus resection, whereas others prefer bilateral medial rectus recessions.

• In patients with residual amblyopia surgery is usually performed on the amblyopic eye.

• In partially accommodative esotropia surgery to improve appearance is best delayed until requested by the child to avoid early consecutive exotropia, and should aim to correct only the residual squint present with glasses worn.

• The usual first procedure for convergence excess esotropia is recession of both medial rectus muscles. This relies on fusion to prevent a distance exotropia; a few patients become divergent after surgery and need a further procedure.

• Medial rectus posterior fixation sutures (Faden operation) can also be used either as a first procedure, or in the case of under-correction following bimedial recessions.

Microtropia

Microtropia (monofixation syndrome), may be primary or follow surgery for a large deviation. It may occur in apparent isolation, but it is often associated with other conditions such as anisometropic amblyopia. Microtropia is more a description of binocular status than a specific diagnosis, for example a patient with fully accommodative esotropia may control to a microtropia rather than true bifoveal BSV with glasses. It is characterized by the following:

1 Very small angle manifest deviation measuring 8 Δ or less, which may or may not be detectable on cover testing.

2 Central suppression scotoma in the deviating eye.

3 ARC with reduced stereopsis and variable peripheral fusional amplitudes.

4 Anisometropia is often present, commonly with hypermetropia or hypermetropic astigmatism.

5 Symptoms are rare unless there is an associated decompensating heterophoria.

6 Treatment involves correction of refractive errors and occlusion for amblyopia as indicated. Most patients remain stable and symptom-free.

Others esotropias

Near esotropia (non-accommodative convergence excess)

1 Presentation is usually in older children and young adults.

2 Signs

• No significant refractive error.

• Orthophoria or small esophoria with BSV for distance.

• Esotropia for near but normal or low AC/A ratio.

• Normal near point of accommodation.

3 Treatment involves bilateral medial rectus recessions.

Distance esotropia

1 Presentation is in healthy young adults who are often myopic.

2 Signs

• Intermittent or constant esotropia for distance.

• Minimal or no deviation for near.

• Normal bilateral abduction.

• Fusional divergence amplitudes may be reduced.

• Absence of neurological disease.

3 Treatment is with prisms until spontaneous resolution or surgery in persistent cases.

Acute (late-onset) esotropia

1 Presentation is around 5–6 years of age.

2 Signs

• Sudden onset of diplopia and esotropia.

• Normal ocular motility without significant refractive error.

• Underlying 6th nerve palsy must be excluded.

3 Treatment is aimed at re-establishing BSV to prevent suppression, using prisms, botulinum toxin or surgery.

Secondary (sensory) esotropia

Secondary esotropia is caused by a unilateral reduction in visual acuity which interferes with or abolishes fusion; causes include cataract, optic atrophy or hypoplasia, macular scarring or retinoblastoma. Fundus examination under mydriasis is therefore essential in all children with strabismus.

Consecutive esotropia

Page 771

Consecutive esotropia follows surgical overcorrection of an exodeviation. If it occurs following surgery for an intermittent exotropia in a child it should not be allowed to persist for more than 6 weeks without further intervention.

Cyclic esotropia

Cyclic esotropia is a very rare condition characterized by alternating manifest esotropia with suppression and BSV, each typically lasting 24 hours. The condition may persist for months or years and the patient may eventually develop a constant esotropia requiring surgery. Earlier correction of the full manifest angle can be successfully performed during the intermittent phase.

High myopia esotropia

Patients with high myopia may have instability of the muscle pulleys that stabilize the superior rectus and lateral rectus muscles. This results in nasal displacement of the superior rectus and inferior displacement of the lateral rectus. The possibility of this condition should be considered in high myopes with acquired esotropia; MR scan is mandatory in making the diagnosis. Treatment involves plication of the superior and lateral recti with a non-absorbable suture.

Exotropia

Constant (early-onset) exotropia

1 Presentation is often at birth.

2 Signs

• Normal refraction.

• Large and constant angle.

• DVD may be present.

3 Neurological anomalies are frequently present, in contrast with infantile esotropia.

4 Treatment is mainly surgical and consists of lateral rectus recession and medial rectus resection.

5 Differential diagnosis is secondary exotropia which may conceal serious ocular pathology.

Intermittent exotropia

Diagnosis

1 Presentation is often at around 2 years with exophoria which breaks down to exotropia under conditions of visual inattention, bright light (resulting in reflex closure of the affected eye), fatigue or ill health.

2 Signs. The eyes are straight with BSV at times (Fig. 18.56A) and manifest with suppression at other times (Fig. 18.56B). Control of the squint varies with the distance of fixation and other factors such as concentration.

Fig. 18.56 Intermittent exotropia. (A) Eyes straight most of the time; (B) left exotropia under conditions of visual inattention or fatigue

(Courtesy of M Parulekar)

Classification

1 Distance exotropia, in which the angle of deviation is greater for distance than near and increases further beyond 6 metres. The two types are true and simulated.

a Simulated is associated with a high AC/A ratio or ‘tenacious proximal convergence’ (TPC). The deviations for near and distance are similar when the near angle is remeasured with the patient looking through +3.00 D lenses (high AC/A controlling exodeviation) or after a period of uniocular occlusion (TPC).

b True. The angle for near remains significantly less than that for distance with the above tests.

2 Non-specific exotropia in which control of the squint and the angle of deviation are the same for distance and near fixation.

3 Near exotropia in which the deviation is greater for near fixation. It tends to occur in older children and adults and may be associated with acquired myopia or presbyopia.

Treatment

1 Spectacle correction in myopic patients may, in some cases, control the deviation by stimulating accommodation, and with it, convergence. In some cases over-minus prescription may be useful.

2 Part-time occlusion of the deviating eye may improve control in some patients, and orthoptic exercises may be helpful for near exotropia.

Page 772

3 Surgery. Patients with effective and stable control of their intermittent exotropia are often just observed. Surgery is indicated if control is poor or is progressively deteriorating. Unilateral lateral rectus recession and medial rectus resection are generally preferred except in true distance exotropia when bilateral lateral rectus recessions are more usual. The exodeviation is rarely completely eliminated by surgery.

Sensory exotropia

Secondary (sensory) exotropia is the result of monocular or binocular visual impairment by acquired lesions, such as cataract (Fig. 18.57) or other media opacities. Treatment consists of correction of the visual deficit, if possible, followed by surgery if appropriate. A minority of patients develop intractable diplopia due to loss of fusion, even when good visual acuity is restored to both eyes and the eyes are realigned.

Fig. 18.57 Left sensory exotropia due to a mature cataract

Consecutive exotropia

Consecutive exotropia develops spontaneously in an amblyopic eye, or more frequently following surgical correction of an esodeviation. In early postoperative divergence muscle slippage must be considered. Most cases present in adult life with concerns about cosmesis and social function, and can be greatly helped by surgery. Careful evaluation of the risk of postoperative diplopia is required, although serious problems are uncommon. About 75% of patients are still well-aligned 10 years after surgery, although re-divergence may occur.

Special syndromes

Recent genetic and neuropathological studies have shown that a group of congenital neuromuscular disorders are the result of developmental errors in the innervation of ocular and facial muscles. These conditions are now referred to as congenital cranial dysinnervation disorders (CCDD) and include Duane syndrome, Möbius syndrome, congenital fibrosis of the extraocular muscles, Marcus Gunn jaw-winking syndrome (see Ch. 1), congenital ptosis and congenital facial palsy.

Duane retraction syndrome

In Duane retraction syndrome there is failure of innervation of the lateral rectus by the 6th nerve, with anomalous innervation of the lateral rectus by fibres from the 3rd nerve. The condition is often bilateral, although frequently involvement in one eye may be very subtle. Some children have associated congenital defects such as perceptive deafness and speech disorder.

Signs

There is usually BSV in the primary position, often with a face turn. The affected eye shows the following motility defects (Figs 18.58-18.60).

1 Restricted abduction, which may be complete or partial.

2 Restricted adduction which is usually partial and rarely complete.

Page 773

3 Retraction of the globe on adduction as a result of co-contraction of the medial and lateral recti with resultant narrowing of the palpebral fissure. The degree of globe retraction may vary from gross to almost imperceptible. On attempted abduction, the palpebral fissure opens and the globe assumes its normal position.

4 An up-shoot or down-shoot in adduction may be present. It has been suggested that this is a ‘bridle’ or ‘leash’ phenomenon, produced by a tight lateral rectus muscle which slips over or under the globe and produces an anomalous vertical movement of the eye. However, recent studies with MRI have shown that this is not always the case.

5 Deficiency of convergence in which the affected eye remains fixed in the primary position while the unaffected eye is converging.

Fig. 18.58 Duane syndrome Huber type III in an infant. (A) Straight eyes in the primary position; (B) limited left abduction with widening of the left palpebral fissure; (C) gross limitation of left adduction with narrowing of the left palpebral fissure

(Courtesy of K Nischal)

Fig. 18.59 Duane syndrome type I in a child. (A) Straight in the primary position; (B) gross limitation of left abduction with slight widening of the left palpebral fissure; (C) slightly limited left adduction

Fig. 18.60 Duane syndrome in an adult. (A) Straight in the primary position. (B) gross limitation of right abduction with slight widening of the right palpebral fissure and marked narrowing of the left palpebral fissure

Classification (Huber)

1 Type I (see Fig. 18.59), the most common, is characterized by:

• Limited or absent abduction.

• Normal or mildly limited adduction.

• In the primary position, straight or slight esotropia.

2 Type II, the least common, is characterized by:

• Limited adduction.

• Normal or mildly limited abduction.

• In primary position, straight or slight exotropia.

3 Type III (see Fig. 18.58), is characterized by:

• Limited adduction and abduction.

• In the primary position, straight or slight esotropia.

The underlying pathophysiology is similar in all three types, the differences being due to variation in the degree of anomaly in the innervation to the lateral and medial recti.

Treatment

The majority of patients with Duane syndrome do not need surgical intervention.

• Most young children maintain BSV by using an AHP to compensate for their lateral rectus weakness and surgery is only needed if there is evidence of loss of binocular function; this may be indicated by failure to continue to use an AHP.

• In adults or children over the age of about 8 years surgery can reduce a head posture which is cosmetically unacceptable or causing neck discomfort. Surgery may also be necessary for cosmetically unacceptable up-shoots, down-shoots or severe globe retraction.

• Amblyopia, when present, is usually the result of anisometropia rather than strabismus. Unilateral or bilateral muscle recession or transposition of the vertical recti are the procedures of choice. The lateral rectus of the involved side should not be resected, as this increases retraction.

Brown syndrome

Brown syndrome is a condition involving mechanical restriction. It is usually congenital but occasionally acquired:

Page 774

Classification

1 Congenital

• Idiopathic.

• ‘Congenital click syndrome’ where there is impaired movement of the superior oblique tendon through the trochlea.

2 Acquired

• Trauma to the trochlea or superior oblique tendon.

• Inflammation of the tendon, which may be caused by rheumatoid arthritis, pansinusitis or scleritis.

Diagnosis

A left Brown syndrome has the following characteristics:

1 Major signs

• Usually straight with BSV in the primary position (Fig. 18.61A).

• Limited left elevation in adduction (Fig. 18.61B).

• Limited left elevation on upgaze is common (Fig. 18.61C).

• Normal left elevation in abduction (Fig. 18.61D).

• Absence of left superior oblique overaction (Fig. 18.61E).

• Positive forced duction test on elevating the globe in adduction.

2 Variable signs

• Down-shoot in adduction.

• Hypotropia in primary position.

• AHP with chin elevation and ipsilateral head tilt (Fig. 18.61F).

Fig. 18.61 Left Brown syndrome. (A) Straight in the primary position; (B) limited left elevation in adduction; (C) limited left elevation on upgaze; (D) normal left elevation in abduction; (E) absence of left superior oblique overaction; (F) chin elevation and left head tilt

(Courtesy of K Nischal)

Treatment

1 Congenital cases do not usually require treatment as long as binocular function is maintained with an acceptable head posture. Spontaneous improvement is often seen towards the end of the first decade. Indications for treatment include significant primary position hypotropia, deteriorating control and/or an unacceptable head posture. The recommended procedure for congenital cases is lengthening of the superior oblique tendon.

2 Acquired cases may benefit from steroids, either orally or by injection near the trochlea, together with treatment of any underlying cause.

Monocular elevator deficit

Monocular elevator palsy, sometimes also referred to as double elevator palsy, is a rare sporadic condition. It is thought to be caused either by a tight or contracted inferior rectus muscle or a hypoplastic or ineffective superior rectus muscle.

1 Signs

• Profound inability to elevate one eye.

• The abnormality of upgaze persists across the horizontal plane, from abduction to adduction (Fig. 18.62).

• Orthophoria in the primary position in about one-third of cases.

• Chin elevation to obtain fusion in downgaze may be present.

2 Treatment involving a base-up prism over the involved eye or surgery should be considered when fusion in the primary position has been compromised, or chin elevation is required to maintain fusion.

Fig. 18.62 Right monoelevation deficit. (A) Defective elevation in abduction; (B) in upgaze; (C) and in adduction

Möbius syndrome

Möbius syndrome is a very rare congenital, sporadic condition.

1 Systemic features

• Bilateral facial palsy which is usually asymmetrical and often incomplete, giving rise to a mask-like facial expression and problems with lid closure (Fig. 18.63B).

• Paresis of the 10th and 12th cranial nerves; the latter results in atrophy of the tongue (Fig. 18.63C). Occasionly the 5th and 8th cranial nerves are affected.

• Mild mental handicap.

• Limb anomalies.

2 Ocular features

• Horizontal gaze palsy is present in 50% of cases.

• Bilateral 6th nerve palsy (Fig. 18.63A).

• Occasionally 3rd and 4th nerve palsy and ptosis.

Fig. 18.63 Möbius syndrome. (A) Esotropia due to bilateral 6th nerve palsy; (B) defective lid closure due to facial nerve palsy; (C) atrophic tongue due to hypoglossal nerve palsy

(Courtesy of K. Nischal)

Congenital fibrosis of the extraocular muscles

Congenital fibrosis of the extraocular muscles (CFOEM) is a rare non-progressive, usually AD, disorder characterized by bilateral ptosis and restrictive external ophthalmoplegia (Fig. 18.64).

• In the primary position each eye is fixed below the horizontal by about 10°.

• The hypotropic eye may be secondarily exotropic, esotropic or neutral.

• The degree of residual horizontal movement varies from full to absent.

• Vertical movements are always severely restricted with inability to elevate the eyes above the horizontal plane.

• Absence of binocular vision and amblyopia may be present in some cases.

Fig. 18.64 Congenital fibrosis of the extraocular muscles.

(A) Bilateral ptosis and divergent strabismus; (B) compensation for severe ptosis

(Courtesy of M Parulekar)

Strabismus fixus

Strabismus fixus is a very rare condition in which both eyes are fixed by fibrous tightening of the medial recti (convergent strabismus fixus – Fig. 18.65A), or the lateral recti (divergent strabismus fixus – Fig. 18.65B).

Fig. 18.65 Strabismus fixus. (A) Convergent; (B) divergent

Alphabet patterns

Page 775

Page 776

Page 777

‘V ‘or ‘A’ patterns may occur when the relative contributions of the superior rectus and inferior oblique to elevation, or of the inferior rectus and superior oblique to depression are abnormal, resulting in abnormal balance of their horizontal vectors in up- and downgaze. They can also be caused by anomalies in the position of the rectus muscle pulleys leading to abnormal lines of action of the muscles. They are assessed by measuring horizontal deviations in the primary position, upgaze and downgaze and may occur regardless of whether a deviation is concomitant or incomitant.

‘V’ pattern

A ‘V’ pattern is said to be significant when the difference between upgaze and downgaze is ≥15 Δ (allowing for a small physiological variation).

Causes

• Inferior oblique overaction associated with 4th nerve palsy.

• Superior oblique underaction with subsequent inferior oblique overaction, seen in infantile esotropia as well as other childhood esotropias. The eyes are often straight in upgaze with a marked esodeviation in downgaze.

• Superior rectus underaction.

• Brown syndrome.

• Craniofacial anomalies featuring shallow orbits and down-slanting palpebral fissures.

Treatment

By inferior oblique weakening or superior oblique strengthening when oblique dysfunction is present. Without oblique muscle dysfunction treatment is as follows:

1 ‘V’ pattern esotropia (Fig. 18.66A) can be treated by bilateral medial rectus recessions and downward transposition of the tendons.

2 ‘V’ pattern exotropia (Fig. 18.66B) can be treated by bilateral lateral rectus recessions and upward transposition of the tendons.

Fig. 18.66 ‘V’ pattern. (A) Esotropia; (B) exotropia

(Courtesy of Wilmer Institute)

‘A’ pattern

An ‘A’ pattern is considered significant if the difference between upgaze and downgaze is ≥10 Δ. In a binocular patient it may cause problems with reading.

Causes

• Primary superior oblique overaction is usually associated with exodeviation in the primary position of gaze.

• Inferior oblique underaction/palsy with subsequent superior oblique overaction.

• Inferior rectus underaction.

Treatment

Patients with oblique dysfunction are treated by superior oblique posterior tenotomy. Treatment of cases without oblique muscle dysfunction is as follows:

Page 778

1 ‘A’ pattern esotropia (Fig. 18.67A) is treated by bilateral medial rectus recessions and upward transposition of the tendons.

2 ‘A’ pattern exotropia (Fig. 18.67B) is treated by bilateral lateral rectus recessions and downward transposition of the tendons.

Fig. 18.67 ‘A’ pattern. (A) Esotropia; (B) exotropia

(Courtesy of Wilmer Institute)

Surgery

The most common aims of surgery on the extraocular muscles are to correct misalignment to improve appearance, and if possible to restore BSV. Surgery can also be used to reduce an abnormal head posture and to expand or centralize a field of BSV. However, the first step in the management of childhood strabismus involves correction of any significant refractive error and/or treatment of amblyopia. Once maximal visual potential is reached in both eyes, any residual deviation can be treated surgically. The three main types of procedure are: (a) weakening, which decreases the pull of a muscle, (b) strengthening, which enhances the pull of a muscle and (c) procedures that change the direction of muscle action.

Weakening procedures

The procedures for weakening the action of a muscle are: (a) recession, (b) disinsertion (or myectomy) and (c) posterior fixation suture.

Recession

Recession slackens a muscle by moving it away from its insertion. It can be performed on any muscle except the superior oblique.

1 Rectus muscle recession

a The muscle is exposed and two absorbable sutures are tied through the outer quarters of the tendon.

b The tendon is disinserted from the sclera, and the amount of recession is measured and marked on the sclera with callipers.

c The detached end of the muscle is sutured to the sclera at the measured distance behind its original insertion (Fig. 18.68).

2 Inferior oblique disinsertion or recession

a The muscle belly is exposed through an inferotemporal fornix incision.

b A squint hook is passed behind the posterior border of the muscle which must be clearly visualized. Care is taken to pick up the muscle without disrupting the Tenon’s capsule and fat posterior to it.

c An absorbable suture is passed through the anterior border of the muscle at its insertion and tied.

d The muscle is disinserted and the cut end sutured to the sclera 3 mm posterior and temporal to the temporal edge of the inferior rectus insertion (Fig. 18.69).

Fig. 18.68 Recession of a horizontal rectus muscle

Fig. 18.69 Recession of an inferior oblique muscle

Disinsertion

Page 779

Disinsertion involves detaching the muscle from its insertion without reattachment. It is most commonly used to weaken an overacting inferior oblique muscle when the technique is the same as for a recession except that the muscle is not sutured. Very occasionally, the procedure is performed on a severely contracted rectus muscle.

Posterior fixation suture

The principle of this (Faden) procedure is to suture the muscle belly to the sclera posteriorly so as to decrease the pull of the muscle in its field of action without affecting the eye in the primary position. The Faden procedure may be used on the medial rectus to reduce convergence in a convergence excess esotropia and on the superior rectus to treat DVD. When treating DVD, the superior rectus muscle may also be recessed. The belly of the muscle is then anchored to the sclera with a non-absorbable suture about 12 mm behind its insertion.

Strengthening procedures

1 Resection shortens a muscle to enhance its effective pull. It is suitable only for a rectus muscle and involves the following steps:

a The muscle is exposed and two absorbable sutures tied into the muscle at a measured distance behind its insertion.

b The muscle anterior to the sutures is excised and the cut end reattached to the original insertion (Fig. 18.70).

2 Tucking of a muscle or its tendon is usually reserved to enhance the action of the superior oblique muscle in congenital 4th nerve palsy.

3 Advancement of the muscle nearer to the limbus can be used to enhance the action of a previously recessed rectus muscle.

Fig. 18.70 Resection of a horizontal rectus muscle

Treatment of paretic strabismus

Lateral rectus palsy

Surgical intervention for 6th nerve palsy should be considered only when it is clear that spontaneous improvement will not occur. This is usually after at least 3 months have elapsed without improvement, typically at least 6 months from onset of the palsy. Treatment of partial and complete lateral rectus palsies is different.

1 Partial palsy (paresis) is treated by adjustable medial rectus recession and lateral rectus resection of the affected eye, aiming for a small exophoria in the primary position to maximize the field of BSV.

2 Complete palsy is treated by transposition of the superior and inferior recti to positions above and below the affected lateral rectus muscle (Fig. 18.71), coupled with an injection of botulinum toxin to the medial rectus (toxin transposition).

Fig. 18.71 Transposition of the superior and inferior rectus muscles in lateral rectus palsy

Three rectus muscles should not be detached from the globe at the same procedure because of the risk of anterior segment ischaemia.

Superior oblique palsy

Surgical intervention should be considered to improve troublesome diplopia or an abnormal head posture. The treatment of unilateral and bilateral palsies is different. General principles are as follows:

1 Unilateral

a Congenital cases can usually be treated either by inferior oblique weakening or by superior oblique tucking.

b Acquired

• A small hypertropia is treated by ipsilateral inferior oblique weakening.

• A moderate to large hypertropia may be treated by ipsilateral inferior oblique weakening which can be combined with, or followed by, ipsilateral superior rectus weakening and/or contralateral inferior rectus weakening if required. It should be noted that weakening the inferior oblique and superior rectus of the same eye may result in defective elevation.

2 Bilateral

Page 780

a Excyclotorsion should first be corrected by the Harada–Ito procedure which involves splitting and anterolateral transposition of the lateral half of the superior oblique tendon (Fig. 18.72).

b Any associated vertical deviation can be corrected either at the same procedure or subsequently.

Fig. 18.72 Harada–Ito procedure for superior oblique palsy

Adjustable sutures

Indications

The results of strabismus surgery can be improved by the use of adjustable suture techniques on the rectus muscles. These are particularly indicated when a precise outcome is essential and when the results with more conventional procedures are likely to be unpredictable; for example, acquired vertical deviations associated with thyroid myopathy or following a blow-out fracture of the floor of the orbit. Other indications include 6th nerve palsy, adult exotropia and re-operations in which scarring of surrounding tissues may make the final outcome unpredictable. The main contraindication is patients who are too young or unwilling to cooperate during postoperative suture adjustment.

Initial steps

a The muscle is exposed, sutures inserted and the tendon disinserted from the sclera as for a rectus muscle recession.

b The two ends of the suture are passed, close, together, through the stump of the insertion.

c A second suture is knotted and tied tightly around the muscle suture anterior to its emergence from the stump (Fig. 18.73A).

d One end of the suture is cut short and the two ends tied together to form a loop (Fig. 18.73B).

e The conjunctiva is left open.

Fig. 18.73 Adjustable suture technique

Postoperative adjustment

This is performed under topical anaesthesia, usually a few hours after surgery when the patient is fully awake.

a The accuracy of alignment is assessed.

b If ocular alignment is satisfactory the muscle suture is tied off and its long ends cut short.

c If more recession is required, the bow is pulled anteriorly along the muscle suture, thereby providing additional slack to the recessed muscle and enabling it to move posteriorly (Fig. 18.73C).

d If less recession is required, the muscle suture is pulled anteriorly and the knot tightened against the muscle stump (Fig. 18.73D).

e Alignment is retested and adjustment repeated as required.

f The conjunctiva is closed.

A similar technique can be used for rectus muscle resection.

Botulinum toxin chemodenervation

Temporary paralysis of an extraocular muscle can be created by an injection of botulinum toxin under topical anaesthesia and EMG control. The effect takes several days to develop, is usually maximal at 1–2 weeks following injection and has generally worn off by 3 months. Side-effects are uncommon, although about 5% of patients may develop some degree of temporary ptosis. The following are the main indications for chemodenervation:

Page 781

1 To determine the risk of postoperative diplopia. For example, in an adult with a consecutive left divergent squint and left suppression, straightening the eyes may make suppression less effective resulting in diplopia. If postoperative diplopia testing by correcting the angle with prisms is negative then the risk of double vision after surgery is very low. If testing is positive then the left lateral rectus muscle can be injected with toxin so that the eyes will either straighten or converge and the risk of diplopia can be assessed over several days while the eyes are straight. If diplopia does occur the patient is able to judge whether it is troublesome.

2 To assess the potential for BSV in a patient with a constant manifest squint by straightening the eyes temporarily. The deviation can then be corrected surgically if appropriate. A small proportion of patients maintain BSV long-term when the effects of the toxin have worn off.

3 In lateral rectus palsy botulinum toxin can be injected into the ipsilateral medial rectus to give symptomatic relief during recovery and to see whether there is any lateral rectus action when there is medial rectus contracture (Fig. 18.74A). The temporary paralysis of the muscle causes relaxation so that the horizontal forces on the globe are more balanced, thus allowing assessment of lateral rectus function (Fig. 18.74B).

4 Patients with a cosmetically poor deviation who have undergone multiple squint operations can be treated by repeated BT injections which may reduce in frequency with time.

Fig. 18.74 Principles of botulinum toxin chemodenervation in left 6th nerve palsy

Page 782