The taste receptors

The chemical receptors (chemoreceptors) that generate the nervous impulses that result in the sense of taste are called the taste buds. Taste buds are scattered in the oral cavity, with most concentrated over the surface of the tongue (Fig 32.1) and a few found on the soft palate and the inner surface of the cheeks. These chemoreceptors respond to substances present in food and generate nerve impulses that are transmitted to the brain for interpretation. The upper surface of the tongue is covered with small projections (papillae), some of which contain a taste bud. Each taste bud consists of sensory and supporting cells situated in the epithelium and opening into the surface through a small gustatory pore.

Figure 32.1 Olfactory structures

(Thibodeau & Patton 2010:301)

Between the cells of the taste bud lie the endings of afferent nerve fibres derived from several cranial nerves. Taste buds on the anterior two-thirds of the tongue connect with fibres of the facial (seventh cranial) nerve. Taste buds on the posterior one-third of the tongue are associated with the fibres of the glossopharyngeal (ninth cranial) nerve, while pharyngeal taste buds send impulses to the brain via the vagus (tenth cranial) nerve.

These taste buds can differentiate among sweet, salty, sour and bitter stimuli. The tip of the tongue is the most sensitive to sweet and salty substances; the edges of the tongue are most sensitive to bitter substances. Substances must be in solution (saliva) so that they can reach every opening in a taste bud and stimulate the nerve ending. Molecules pass into solution on the surface of the tongue and combine with the surface membranes of the receptor cells. Transmitter substances are released, which evoke action potentials on the sensory nerve fibres. Fibres from the seventh, ninth and tenth cranial nerves carry the taste impulses via the brainstem to an area of the cerebral cortex where the taste is experienced.

The sense of taste is intricately linked with the sense of smell, and the sense of taste depends on stimulation of the olfactory receptors. Both senses have a protective function; for example, in detecting substances that may be harmful. Interruption of the transmission of taste stimuli to the brain may cause taste abnormalities. Taste abnormalities may result from trauma, infection, vitamin or mineral deficiencies, neurological or oral disorders and the effects of drugs. Because tastes are most accurately perceived in a fluid medium, mouth dryness may interfere with taste. Alterations in taste may include ageusia (a complete loss of taste), hypogeusia (a partial loss of taste), dysgeusia (a distorted sense of taste) and cacogeusia (an unpleasant taste).

The smell receptors

The chemoreceptors responsible for the sense of smell are located in a small area of epithelial tissue in the upper part of the nasal cavity (Fig 32.2). These chemoreceptors are known as the olfactory receptors. They respond to airborne chemicals and generate impulses that are transmitted to the brain for interpretation.

Figure 32.2 The location and structure of the olfactory receptors in the nose

When these receptors are stimulated by chemicals they transmit impulses along the olfactory nerve to the brain. These receptors adapt quickly when exposed to an unchanging stimulus, which means that the client can become accustomed to an odour when constantly exposed to it.

Permanent alterations in the sense usually result when the olfactory neuroepithelium or part of the olfactory nerve is destroyed. Permanent or temporary loss can occur from inhaling irritants such as acid fumes that paralyse nasal cilia. Conditions such as ageing, Parkinson’s disease and Alzheimer’s disease may alter the sense of smell. Alterations in smell include anosmia (a total loss of sense of smell), hyposmia (an impaired sense of smell) and parosmia (an abnormal sense of smell).

There is a close relationship between the sense of smell and the sense of taste, and therefore they are not always distinguishable. The sensations of smell and taste play an important part in stimulating the secretions of digestive juices.

The nurse plays an important role in promoting the sense of taste and stimulating the sense of smell when caring for a client with altered sensory function. Client education regarding good oral hygiene practices will enhance taste perception. Discuss with the client which foods are the most taste appealing. If taste perception is improved, food intake and appetite will also improve. Taste perception is increased when food is seasoned. The client’s sense of smell can be stimulated by pleasant aromas such as flowers, perfumes and food.

The outer layer

The outer fibrous layer consists of the sclera and the cornea, which is a transparent structure continuous with the sclera. About five-sixths of the outer surface is made up of a tough white opaque fibrous layer (sclera), while the cornea comprises the anterior one-sixth. Both the sclera and the cornea consist of layers of collagen fibres. In the cornea the fibres are regular in size and arrangement, which accounts for its transparency. The cornea functions as a refracting and protective layer through which light rays pass en route to the brain.

The middle layer

The middle pigmented vascular layer is composed of three structures: the choroid, the iris and the ciliary body. The choroid is a layer of tissue that lies between the sclera and the retina. It is composed largely of blood vessels, and contains highly pigmented cells that absorb light and prevent it from being reflected within the eyeball.

The iris is a pigmented circular membrane between the cornea and the lens, and gives the eye its colour. Sphincter and dilator muscles within the iris regulate the central aperture (the pupil). By either dilation or constriction of the pupil, the amount of light entering the eye is regulated. The pupil appears black because light rays entering the eye are absorbed by the choroid and are not reflected.

The ciliary body is a thickened area of the choroid, lying anterior to the choroid extending to the root of the iris. A circular array of fibres stretches from the ciliary body to the lens to hold it in place. The ciliary body controls focusing of the lens and contains glands that secrete aqueous humour.

The innermost layer

The innermost lays is a thin structure called the retina. The retina lines the inner wall of the posterior portion of the eyeball and is comprised of a number of layers. The two main layers are the pigmented layer (the outermost layer of the retina, lying next to the choroid, and whose cells contain melanin) and the rod and cone layer, which lies next to the pigmented layer and is highly sensitive to light.

Rods and cones (specialised nerve endings) are distributed as a tightly packed mass throughout the retina, except at a point where the ganglionic fibres converge to form the optic nerve. This area, which is about 1.5 mm in diameter, is termed the optic disc. Since it possesses no photosensitive cells it is also known as the ‘blind spot’. The prime function of rods and cones is to absorb light. Rods can be stimulated by dim light and allow perception of shapes and movement in dim light. Rods also provide far peripheral vision. Cones are specialised for fine visual discrimination of colour perception. There are three types of cones: one type responds mostly to blue light, another to red light and the third type responds to green light.

In the centre of the retina is an oval yellowish area, the macula lutea, and in the centre of the macula lutea is a small depression, the fovea centralis. Because the photosensitive cells are more exposed to light here than over the rest of the retina, visual acuity is at its highest here.

Extraocular structure—accessory structures

Extraocular, or accessory, structures of the eye are portions of the eye outside the eyeball. These structures consist of the eyebrows, eyelids, eyelashes and lacrimal apparatus. The eyeball is anchored into position by several structures, including the extraocular muscles, the conjunctiva and the eyelids. The eyeball is surrounded and protected by a bony orbit, the eyebrow ridge and some fatty tissue. The eyeball is lubricated by the lacrimal glands.

The extraocular muscles bring about rotational movements of the eyeball. The muscles arise from the orbit and consist of four rectus muscles, which are attached to the sclera, and two oblique muscles. The oblique muscles are arranged so that for part of their length they lie around the circumference of the eyeball. The eyebrows and eyelashes are short coarse hairs that shade the eyes and protect the eyes from dust and sweat. The eyelids consist of connective tissue covered by skin and lined with mucous membrane. The lining is reflected over the eyeballs, and is called the conjunctiva. The eyelids protect the eye from foreign bodies and excessive light, as well as distributing tears by blinking.

The lacrimal apparatus consists of the lacrimal gland, which is situated over the eye at the upper outer corner and secretes tears, which constantly wash over the conjunctiva (Fig 32.4). Tears leave the gland via several small ducts, and pass over the front of the eye eventually draining into the lacrimal sac, which is the expanded end of the nasolacrimal duct. Tears are secreted on to the anterior surface of the eyeball and are spread over it by the blinking movements of the eyelids. An antimicrobial substance in tears protects the eyes against microorganisms. Excess tears drain down the nasal cavity. The lacrimal glands are stimulated in response to chemical and mechanical irritants, thus producing tears to wash away irritants. Tears may also be produced as a result of emotion such as sadness or happiness.

Figure 32.4 The lacrimal apparatus of the eye

The refractive media

The refractive media are the transparent parts of the eye, and have the ability to bend light rays at the surfaces of two transparent media. The refractive media are the cornea, the lens, the aqueous humour and the vitreous humour. The cornea functions as a refracting and protective layer through which light rays pass en route to the brain.

The lens is a transparent, biconvex encapsulated structure suspended from the ciliary body posterior to the iris. The lens is an elastic structure and this allows its shape to change when the eye is focused. The function of the lens is to refract (bend) light rays and to focus them on the retina.

Aqueous humour is a clear watery fluid that fills the cavities around the lens. These cavities are called the anterior and posterior chambers. Aqueous humour is derived from the plasma in the capillaries of the ciliary body, and passes into the posterior chamber. It then passes forwards through the pupil to the anterior chamber, where it is absorbed into the ciliary veins. The rate of secretion and re-absorption is balanced so that intraocular pressure is regulated. Aqueous humour serves as a refractory medium and provides nutrients to the lens and cornea.

Vitreous humour is a clear jelly-like substance that fills the intraocular space from the posterior lens to the retina. Because it does not regenerate, any significant loss of vitreous humour—for example, as a result of injury to the eye—may distort other ocular structures. Vitreous humour helps to maintain the shape of the eyeball, helps keep the retina in position and helps with the refraction of light rays.

Refraction

Refraction, or bending of light rays, occurs as the light waves pass through the cornea, aqueous humour, the lens and the vitreous humour. At the lens the light is bent so that it converges at a single point on the retina.

Accommodation

Accommodation is the process whereby the curvature of the lens is altered to that the eye is able to focus light from objects at different distances. As an object moves closer to the eye the curvature of the lens increases so that the image remains in focus on the retina.

Regulation of pupil size

The diameter of the pupil influences image formation, and the muscles of the iris respond to changes in light intensity. An increase in light intensity initiates constriction of the pupil, whereas a decrease in light intensity causes dilation of the pupil. Adjustment of pupil size therefore regulates the amount of light entering the eyes. Too much light may damage the retina, and too little fails to stimulate it.

Convergence

Convergence is the medial movements of the two eyeballs so that they are both directed towards the object being viewed. Convergence allows light rays to fall and stimulate two identical spots on the retinas, resulting in the perception of a single image.

After an image has been formed on the retina by the processes of refraction, accommodation, regulation of pupil size and convergence, light impulses are converted into nerve impulses by the rods and cones. The light breaks down the photosensitive chemical in either rods or cones, which stimulates electrical impulses to the brain for interpretation.

Nerve impulses travel from the retina along the optic nerve. The optic nerve emerges from the back of the eyeball and passes to the optic chiasma, an area at the base of the brain.

The fibres then form the left and right optic tracts, which continue to the visual areas of the brain in the occipital lobes. Here, further processing occurs so that the image is given meaning.

The external ear

The external ear consists of the auricle (pinna) and the external auditory canal. The auricle consists of a piece of elastic cartilage with a number of associated ligaments and muscles, and has a small amount of adipose tissue in the earlobe. The skin of the auricle is covered with fine hairs and is continuous with the skin lining the external auditory canal. The external auditory canal is an S-shaped canal about 2.5 cm long. It extends from the auricle to the tympanic membrane (eardrum). The skin lining the canal contains glands that produce cerumen (wax) that protects the lining from damage and keeps dust particles away from the eardrum. The tympanic membrane lies between the external ear and the middle ear. It is a semi-transparent sheet consisting of three layers. The outermost layer consists of hairless skin, the middle layer is connective tissue and the innermost layer is continuous with the mucous membrane lining of the ear.

The middle ear

The middle ear, or tympanic cavity, is a small chamber within the temporal bone. Three small bones, or ossicles, lying in the middle ear transmit sound from the tympanic membrane to the oval window. Because of their shapes, the bones are named the malleus (mallot), the incus (the anvil) and the stapes (the stirrup). The malleus is attached to the tympanic membrane; the stapes is attached to the oval window; and the incus articulates with the other two ossicles. The three ossicles are attached to the wall of the middle ear by ligaments. The auditory tube (or eustachian tube) connects the middle ear with air from the nasopharynx. It is about 3–6 cm long and is lined with mucous membrane. Normally the tube is flattened and closed at the pharyngeal orifice, but swallowing or yawning opens it briefly.

The inner ear

The inner ear consists of a bony cavity within the temporal bone known as the osseous labyrinth. It is lined with periosteum and filled with a fluid called perilymph. The three divisions of the osseous labyrinth are the cochlea, the vestibule and the semicircular canals. The cochlea is a spiralling bony cavity, which resembles the shell of a snail. It houses the organ of Corti, the receptor organ of hearing. The vestibule lies between the cochlea and the semicircular canals. It contains structures responsible for maintaining equilibrium during movement of the head. The semicircular canals consist of three canals (in each ear), arranged at right angles to each other. They are filled with endolymph (the fluid inside the membranous labyrinth at the ear) and contain specialised nerve endings that are stimulated by the movement of the endolymph. The semicircular canals assist the body to adjust to changes of direction. The movement of fluid in this area can cause the client to experience a feeling of dizziness.

The membranous labyrinth is a membrane consisting of three layers, and lies within the osseous labyrinth. In the areas where it is not attached to the osseous labyrinth it is surrounded by perilymph. The fluid contained within the membranous labyrinth is known as endolymph. Part of the membranous labyrinth is concerned with hearing and part is concerned with the position of the head in space. The ear is responsible for hearing and maintenance of equilibrium.

Sensory deprivation

Sensory deprivation is thought of as a decrease or lack of meaningful stimuli. Because of reduced stimulation, a person becomes more acutely aware of the remaining stimuli and often perceives these in a distorted manner. The individual often experiences alterations in perception, cognition and emotion (Clinical Interest Box 32.1).

Sensory overload

Sensory overload generally occurs when a person is unable to process or manage the amount or intensity of sensory stimuli. Factors that can contribute to sensory overload are:

Sensory overload can prevent the brain from ignoring or responding to specific stimuli. The clinical signs of sensory overload are listed in Clinical Interest Box 32.2.

Sensory deficits

A sensory deficit is impaired reception, perception, or both, of one or more of the senses. Impaired hearing and sight are sensory deficits. When only one sense is affected, other senses may become more acute to compensate for the loss. However, sudden loss of eyesight can result in disorientation. When there is a gradual loss of sensory function, people often develop behaviours to compensate for the loss; for example, a person with gradual hearing loss in the right ear may unconsciously turn the left ear towards the speaker. Clients with sensory deficits are at risk of both sensory deprivation and sensory overload (Berman et al 2012).

Factors affecting sensory function

A range of factors affect the amount and quality of sensory stimulation, including a person’s developmental stage, culture, level of stress, medications, illness and lifestyle. These are outlined in Clinical Interest Boxes 32.3 and 32.4; an interview is required to assess a client’s sensory-perceptual functioning.

Stye

A hordeolum (stye) is a localised staphylococcal infection of a sebaceous gland of the eyelid. This gland is the base of a hair follicle or eyelash. Manifestations include localised inflammation, swelling and pain.

Blepharitis

Blepharitis is inflammation of the eyelid margins. It may be caused by bacterial infection or an allergic reaction to smoke, dust or chemical. Seborrhoea, a disorder of the sebaceous gland, may also cause blepharitis. Manifestations include itching, burning, redness of the eyelid margins, chronic conjunctivitis and yellow purulent discharge crusts on the lashes.

Conjunctivitis

Conjunctivitis is the inflammation of the conjunctiva and may be caused by excessive exposure to wind, sun, heat, cold, allergens or chemicals, or by bacterial, viral or fungal infection. Manifestations include itching, burning, excessive tearing and pain. Allergic conjunctivitis may cause considerable swelling.

Trachoma

Trachoma is a chronic form of kerato-conjunctivitis that causes permanent damage to the cornea which, if untreated, can result in blindness. The condition results from infection by Chlamydia trachomatis and is associated with poor personal and community hygiene and lack of available clean water. There is a high incidence of trachoma among Australian Indigenous people. Manifestations, which may not become apparent for years, are similar to those for severe conjunctivitis. There is corneal inflammation and scarring, due to the eyelids turning inwards, which causes the lashes to rub against the cornea. Severe corneal scarring may result in blindness.

Keratitis

Keratitis is inflammation of the cornea and is frequently caused by trauma or infection. It usually affects only one eye, and frequently presents as a secondary infection to an upper respiratory tract infection involving cold cores (herpes simplex infection). Manifestations include opacity of the cornea, irritation, excessive tearing, blurred vision, redness and photophobia.

Orbital cellulitis

Orbital cellulitis is an acute infection of the orbital tissues and eyelids that does not involve the eyeball. The condition is generally secondary to infection of nearby structures. If orbital cellulitis is not treated, the infection may spread to the sinuses and meninges. Manifestations include unilateral eyelid oedema, inflammation of the orbital tissues and eyelids, pain, impaired eye movement and purulent discharge from indurated areas.

Diabetic retinopathy

Diabetes mellitus may cause a condition known as diabetic retinopathy which causes small haemorrhages in retinal blood vessels to disrupt the oxygen supply to the photoreceptors. The eye responds by building new, but abnormal, vessels that block vision and which may cause detachment of the retina.

Glaucoma

Another common condition that can damage the retina is glaucoma. This is excessive intraocular pressure caused by abnormal accumulation of aqueous humour. As fluid pressure against the retina increases, blood flow to the retina slows. The reduced blood flow cases degeneration of the retina and thus a loss of vision. There are various forms of glaucoma: primary open-angle, primary closed-angle, secondary, congenital and absolute. Generally speaking, glaucoma progresses slowly and may or may not be symptomatic. It rarely affects clients under 40 years of age. The condition may be caused by over-production of aqueous humour or obstruction to the outflow of aqueous humour, both of which result in accumulation of fluid and a rise in intraocular pressure. Manifestations of primary open-angle glaucoma are gradual and progressive; the client is unable to perceive changes in colour and experiences blurred vision and persistent aching eyes. Manifestations of primary closed-angle glaucoma occur suddenly. The client experiences intense pain, loss of vision, nausea and vomiting. The affected eye appears red, and the pupil is fixed, dilated and unresponsive.

The manifestations of secondary glaucoma are similar to those of the primary form depending on the cause. Congenital glaucoma is a rare disorder generally associated with other abnormalities, and manifests as a large-diameter cornea. Absolute glaucoma is the end result of any uncontrolled glaucoma resulting in blindness. Clinical Interest Box 32.6 provides information on glaucoma prevention and preservation of sight.

Strabismus

Strabismus is a condition of eye deviation in which the eyes fail to look in the same direction at the same time. The eyes may have uncoordinated appearance and the person may experience diplopia. Congenital strabismus is an anomaly in which there is a defect in the position and fusion ability of the two eyes. The condition is usually easily observed and, in addition to crossed eyes, the client may squint, tilt the head or close one eye to improve vision.

Ptosis

Ptosis, drooping of the upper eyelid, may be unilateral or bilateral. Congenital ptosis results from the failure of the levator muscle in the upper eyelid to develop. Manifestations are usually very obvious; the lid appears smooth and flat, and the client may tilt the head back to compensate if the lid droops over the pupil.

Choroidal melanoma

A choroidal melanoma is a malignant tumour of the middle of the eyeball. The major symptom is visual distortion. There is generally no pain unless glaucoma develops.

Retinoblastoma

Retinoblastoma is a congenital hereditary neoplasm that develops from retinal germ cells. It is the most common malignancy of the eye in childhood. Manifestations include diminished vision, strabismus, retinal detachment and an abnormal papillary reflex. The tumour grows rapidly and may invade the brain and metastasise to distant sites.

Abrasions

Abrasions are superficial scratches on the eyelid, conjunctiva or cornea. Corneal abrasions may be caused by foreign bodies or over-wearing of contact lenses. Manifestations include excessive tearing, pain, a sensation of something in the eye and photophobia.

Lacerations

Lacerations may involve the eyelids or the eyeball. Lacerations of the eyeball may lead to intraocular infection, cataract formation of loss of vision. Manifestations of a lacerated eyeball include severe pain and shock.

Contusions

Contusions of the eyeball involve a bruising injury in which intraocular damage occurs. The injury may result in bleeding into the anterior chamber (hyphema) or into the vitreous of surrounding tissues. Contusions generally result from a severe blow to the eye, such as from a fist, golf or squash ball. Manifestations include impaired vision, pain, bruising of the tissues surrounding the eye and blood in the anterior chamber.

Foreign bodies

Foreign bodies may enter the conjunctiva or cornea, or they may penetrate and perforate the eyeball. Examples of foreign bodies that gain entry include eyelashes, dirt and small particles of metal, dust and glass. Manifestations include irritation, pain, excessive tearing and photophobia.

Burns

Burns to the eye may result from exposure to a chemical, radiant energy or high temperatures. The extent of damage to the eye depends on the duration of exposure and the causative agent. Manifestations include extreme pain, excessive tearing, photophobia, inflammation or destruction of tissue and visual impairment.

Subjective eye tests

A visual acuity test evaluates the client’s ability to distinguish the form and detail of an object. Visual acuity is measured by the use of a Snellen chart. The client is required to read letters on this chart from a distance of 6 metres. Generally, the smaller the symbol the client can identify the sharper their visual acuity.

Colour vision tests evaluate the client’s ability to recognise differences in colour. The most common colour vision tests use plates made up of patterns of dots of primary colours, superimposed on backgrounds of randomly mixed colours. A client with normal colour vision can identify the patterns.

Accommodation tests evaluate the ability of the eyes to adjust to the curvature of the lenses. The examiner may perform this test by questioning the client concerning their visual acuity, while placing trial lenses before their eyes. Alternatively the test can be performed using an ophthalmoscope.

Visual field tests evaluate the functions of the retina, optic nerve and optic pathways. The client’s peripheral and central visual fields are assessed when the examiner moves an object from outside the field into the field, on a radial line, until the client states that they can see the object. Tangent screen examinations detect visual field loss. In this examination, a screen is used and test objects from 1–50 mm in size are placed on the screen. Each eye is individually tested for visualisation of the objects.

Objective eye tests

Intraocular pressure is assessed using a tonometer. A topical anaesthetic is instilled and the examiner places the tonometer on the apex of the cornea to determine pressure in the eye. An alternative method involves the use of an ‘air-puff’ tonometer that does not contact the eye. Tonometry serves as a valuable screening test for early detection of glaucoma.

Ophthalmoscopic examination involves the use of an ophthalmoscope to view the interior structures of the eye. The ophthalmoscope examines the fundus, or interior aspect, of the eye, allowing magnified examination of the optic disc, retinal vessels, macula and retina.

Slit-lamp examination allows the examiner to visualise in detail the anterior segment of the eye. Before the examination, dilating eye drops (mydriatics) may be instilled. The client sits with their chin on a rest and their forehead against a bar attached to the lamp. Slit-lamp examination helps determine corneal abrasions, dermatitis and cataracts.

Fluorescein staining is a technique in which staining the eye’s surface with dye provides a better view of the anterior portion of the eye. The test is generally performed when the conjunctival or corneal abrasions are suspected. Surface defects absorb more dye than normal areas.

Special procedures for assessing the eye

Fluorescein angiography involves rapid-sequence photographs of the fundus after intravenous injection of a contrast medium. Fluorescein angiography records the appearance of blood vessels inside the eye.

A culture to determine the microorganism causing ocular infection may be obtained by passing a sterile swab over the conjunctival surface.

Computerised tomography (CT) of the orbit may be used to detect abnormalities such as intraocular foreign bodies or retinoblastoma. Contrast dye is injected and the eyeball scanned.

Ocular ultrasonography involves the transmission of high-frequency sound waves through the eye, and measurement of their reflection forms the ocular structures.

Vetrasonography can identify abnormalities that are undetectable through ophthalmoscopy, and may be used to locate intraocular foreign bodies.

Maintaining a safe environment

Any visual impairment or loss renders the client susceptible to injury; therefore the nurse has a responsibility to protect the client from environmental hazards. These hazards may include such things as low levels of lighting, poor colour contrast or badly positioned furniture. The nurse should describe the layout of the room to the client. If the client is ambulant, the nurse should encourage them to locate the various pieces of furniture so that they become familiar with their position in the room. Leave furniture and personal belongings in the same position in the room. Ensure that any doors are either fully closed or open, as a visually impaired client can easily by injured if a door is left ajar.

Nurses can foster independence in the hospitalised visually impaired client by encouraging self-sufficiency in completing activities of daily living, and improving self-esteem as the client attempts and masters activities.

Client education

Education involves assisting the client to adjust to a visual impairment. The nurse should initially describe to the client with the alteration to visual function what has occurred. The client who is experiencing visual loss that is either temporary or permanent needs assistance to adjust to the physical and psychosocial implications. Care of the client includes encouraging the client to express reactions and feelings to their visual loss. The visually impaired client may lose the ability to observe body language and the reading and writing components of communication. It is important for the client to be able to interact with people whom they encounter. Communication methods may also be taught to family members and significant others. Clinical Interest Box 32.7 provides information on preventive screening for children.

Key aspects related to assisting the visually impaired client

Table 32.2 Aids for the visually impaired

Local eye care

Various nursing procedures involving the eye should be performed in accordance with the policies of the healthcare institution. General principles that should be followed in all ophthalmic procedures include:

Application of eye pads

A light eye pad may be applied to prevent further injury after trauma to the eye or to avoid eye damage after administration of a local anaesthetic. Before the eye pad is applied the client is asked to close their eyelid firmly. The pad should be applied so that the eyelid cannot be opened. Pressure should not be applied, unless a medical officer has prescribed it. The eye pad is secured into position using hypoallergenic tape, placing the tape diagonally from forehead to cheek.

Cleansing the eyelids

It may be necessary to cleanse the eyelids before: applying a compress; removing or inserting an artificial eye; instilling drops or ointment; removing discharge from the margins of the eyelids. A suggested technique for eyelid cleansing is outlined in Procedural Guideline 32.1. The equipment required includes:

Eye irrigation

Irrigation is a technique performed to flush secretions, chemicals or foreign bodies from the conjunctival sac. Chemical injuries to the eye require flushing of the conjunctival sac with copious amounts of solution. It may be necessary to cleanse the eyelids before irrigation; for example, if there is excessive discharge or crusting. A suggested technique for eye irrigation is outlined in Procedural Guideline 32.2.

The equipment required includes:

Instillation of drops or ointment

Eye drops or ointment that may be prescribed include those that combat infection, dilate or constrict the pupil, act as a local anaesthetic, stain the cornea, reduce inflammation or reduce intraocular pressure. Eye drops or ointment are instilled only when they have been prescribed by a medical officer, and the nurse who is instilling them must follow the regulations and policies regarding the administration of medications. Key aspects regarding the use of ophthalmic medications include:

Eye prostheses

Two types of eye prosthesis are contact lenses and the artificial eye.

Contact lenses: a contact lens is a small plastic disc that is positioned on the cornea and held in place by surface tension. A client who has a refractive error, for example astigmatism, may wear contact lenses in place of glasses. A variety of lenses is available including hard, soft, gas permeable, extended wear, daily wear, coloured and disposable. It is important to know that all lenses require care and must be removed periodically to prevent corneal damage and eye infection. Client education is required regarding proper lens care; for example, daily wear lenses should be removed overnight for cleaning and disinfection. Only the recommended solutions should be used for cleaning, soaking and storing lenses. Wetting lenses with contact lens solution before insertion minimises discomfort. The hands should be washed and dried before insertion or removing lenses (see Procedural Guideline 32.3).

Artificial eyes: An artificial eye may be inserted after surgical removal of an eye. Clients with artificial eyes have had enucleation of an entire eyeball. An artificial eye may be removed from the socket, cleaned and replaced. Alternatively, the prosthesis may remain in the socket permanently. Most clients prefer to care for their own eyes but there may be times when assistance from the nurse is required. When a prosthesis is removed from the socket it is placed in warm normal saline for cleansing (see Procedural Guideline 32.4).

Eye surgery

Laser eye surgery is commonly performed to correct refractive errors such as myopia, hyperopia and astigmatism. A laser is used to permanently change the shape of the cornea, and in most cases the need to use corrective lenses is reduced or eliminated. Several surgical procedures are now available:

These procedures reshape the cornea using laser technology to remove a thin layer of epithelial cells or to shrink and reshape the cornea. Candidates for laser vision-correction should be in good health and must have adequate corneal thickness to ensure that the risk of perforation does not occur.

Following surgery, clients may experience a temporary loss of contrast sharpness (images do not appear crisp as with corrective lenses), over- or under-correction of visual acuity, dry eyes or temporarily decreased night vision with halos, glare and starbursts (LeMone et al 2011).

Corneal transplant: Once the cornea has become scarred and opaque, no treatment can restore clarity. The first successful corneal transplant was performed in 1906. Corneas are harvested from cadavers of uninfected adults who were under the age of 65 and who die as a result of acute trauma or illness. Corneal transplantation is usually an elective procedure, although emergency transplantation may be required for perforation of the cornea (LeMone et al 2011).

Congenital factors

Congenital alteration, which may be either hereditary or acquired, include structural malformations that can lead to hearing loss or deafness, disorders resulting from maternal infection during pregnancy or trauma incurred during pregnancy or delivery or before maturity.

Degenerative factors

Degenerative changes in the bones and joints of the ossicles can result in conductive hearing loss, while narrowing of blood vessels to the inner ear can result in tinnitus, vertigo or hearing loss. Gradual sensorineural hearing loss can result from a decrease in the number of hair cells and nerve fibres in the auditory system as a consequence of the ageing process.

Infectious factors

Microorganisms can cause external, middle or inner ear infections, all of which result in pain and varying degrees of conductive hearing loss.

Obstructive factors

Obstruction of the auditory system, which may result in conductive or sensorineural hearing loss, includes impacted cerumen, foreign bodies, oedema and neoplasms. An obstruction can impede the passage of sound waves causing an imbalance of pressure on either side of the tympanic membrane or it may disrupt the transmission of sound to the inner ear.

Neoplastic factors

Neoplasms, which may be benign or malignant, can affect the external middle or inner ear. The effects of a neoplasm include pain, discharge, vertigo, tinnitus and hearing loss.

Traumatic factors

Trauma to the external ear includes bruising, lacerations and frostbite. Trauma to the head can result in fractures of the temporal bone, dislocation of the ossicles, rupture of the tympanic membrane and damage to the auditory nerve. Trauma can lead to vertigo, tinnitus, nystagmus, pain, bleeding and hearing loss. Hearing loss can also result from sudden exposure to a loud explosive sound or from continued exposure to loud noise.

Pain

Earache (otalgia) is a common manifestation of many ear disorders. Middle ear pain associated with otitis media is often described as deep seated and throbbing. If the external ear is infected and inflamed, the client may experience pain and tenderness. Earache may also be referred from other parts of the body via various cervical or cranial nerves, as occurs with tonsillitis.

Discharge

Discharge from the ear (otorrhoea) is generally associated with infections of the middle ear. Purulent or haemo-purulent discharge may drain from the middle ear through a perforated tympanic membrane. Otitis externa may be accompanied by profuse discharge and irritation of the pinna.

Tinnitus

Tinnitus, which may be constant or intermittent, describes a noise in the inner ear. Noises such as ringing, buzzing, hissing or roaring are due to irritation of hair cells in the organ of Corti. Causes include pressure from cerumen on the tympanic membrane, pharyngotympanic tube obstruction, otitis media, otosclerosis, Ménière’s syndrome and tumours.

Vertigo

Vertigo is a sensation of spinning. The person may feel as if their head is spinning or as if the room is spinning around them. Causes of vertigo include trauma, tumours, inflammation, degeneration of the maculae in the inner ear, Ménière’s syndrome and pharyngotympanic tube obstruction.

Nystagmus

Nystagmus is involuntary rhythmic movements of the eyes. The movements may be horizontal, vertical, rotary or mixed. Jerking nystagmus, characterised by faster movements in one direction than the opposite direction, may be a manifestation of certain disorders of the inner ear, although it can be caused by other factors such as neurological disorders. Nystagmus may also be induced by introducing cold or warm water into the external auditory canal.

Hearing loss

Impaired hearing or deafness may be conductive, sensorineural or a combination of both these types.

Conductive hearing loss results from disturbances of the sound transmission mechanism of the external or middle ear, which prevents sound waves from reaching the inner ear. Conductive hearing loss or deafness can result from impacted cerumen, foreign body in the ear canal, middle ear infection, ruptured tympanic membrane, otosclerosis, pharyngotympanic tube obstruction, neoplasms or barotraumas (injury due to pressure).

Sensorineural (perceptive or nerve) hearing loss results from disturbances of the inner ear neural structures, or nerve pathways leading to the brainstem. Sensorineural hearing loss or deafness can result from hereditary or de novo genetic factors, acoustic trauma, infection, neoplasms such as acoustic neuroma, advanced otosclerosis, haemorrhage or thrombosis of the cochlear vessels and Ménière’s syndrome.

Mixed hearing loss combines aspects of both conductive and sensorineural hearing loss.

Congenital disorders

Congenital disorders may be caused by a genetic defect or may result from prenatal trauma or toxicity. Some of the disorders affect the cosmetic appearance only, while others result in varying degrees of hearing loss. Congenital disorders include partial or total absence of the external ear (auricle), protruding ears, fused or absent ossicles and malformation of the cochlea.

Degenerative disorders

Presbycusis is a slowly progressive bilateral hearing loss resulting from physiological changes of ageing. It results from a loss of hair cells in the organ of Corti and causes sensorineural hearing loss, usually of high-frequency tones. Manifestations include gradual progressive hearing loss, which may be accompanied by tinnitus and dizziness, depression or irritability.

Infectious disorders

Otitis externa, which may be acute or chronic, is inflammation of the external ear canal and auricle; the most common causative organism is Streptococcus pyogenes although Proteus vulgaris, Staphylococcus aureus, Candida albicans and Aspergillus niger may be responsible for infection. Predisposing factors include swimming in contaminated water, abrasion of the ear canal from inserting sharp or small objects to clean the ear, exposure to irritants and constant use of earphones or ear plugs. Manifestations include otalgia which is exacerbated on chewing or opening the mouth, lymphadenopathy, pyrexia, regional cellulitis, offensive smelling discharge, pruritus and partial hearing loss.

Otitis media, which may be acute or chronic, is inflammation of the middle ear. Causative organisms include Haemophilus influenzae, pneumococci and beta-haemolytic streptococci. Predisposing factors include the wider, shorter and more horizontal pharyngotympanic tube in infants and children; anatomical abnormalities such as cleft palate; and respiratory tract infection. Manifestations include severe otalgia, pyrexia, hearing loss, dizziness, nausea and vomiting. If the tympanic membrane ruptures, there is often purulent discharge from the ear canal.

Neoplastic disorders

An acoustic neuroma is a benign tumour arising from the eighth cranial nerve and occurring within the internal auditory canal. As the tumour grows the cochlear branch of the eighth cranial nerve becomes involved. Manifestations include tinnitus followed by an increasing high-frequency hearing loss. As the tumour increases in size, symptoms of other cranial nerve involvement occur, such as trigeminal pain and loss of taste in part of the tongue. Nystagmus and ataxia may occur.

Traumatic disorders

Abrasion: because of its vulnerable position, the external ear may sustain abrasion of the canal, burns or contusions of the pinna. Foreign bodies may be inserted into the canal and become lodged, or insects may fly into the ear, causing irritation.

Perforated eardrum: a perforated tympanic membrane may be caused by a variety of factors, such as the introduction of a sharp object to clean the canal, unskilled irrigation of the auditory canal, middle ear infection, trauma to the head or sudden exposure to loud noise. Most traumatic perforations heal spontaneously, but a perforated tympanic membrane results in scarring and renders the client susceptible to chronic ear infections and hearing loss. Manifestations include sudden severe pain inside the ear, loss of hearing, tinnitus and bleeding.

Barotrauma: barotrauma is injury caused by changes of air pressure to the wall of the pharyngotympanic tube and mucous membrane of the middle ear. It is caused by failure of the pharyngotympanic tube to open sufficiently, resulting in unequal pressure between the middle ear and the atmospheric pressure. It may occur during take-off or landing in planes, or during deep-sea diving. Manifestations include severe pain, decreased hearing, tinnitus and vertigo.

Noise-induced hearing loss: this condition develops gradually as a result of continued exposure to environmental or industrial noise. Acoustic trauma is sudden hearing loss from brief exposure to a high-intensity sound at close range, such as gunshots. Manifestations of noise trauma include hearing loss, tinnitus and a feeling of fullness within the ear.

Ototoxicity: ototoxicity is a toxic reaction to medications or chemicals that can damage the auditory and/or vestibular system. Auditory damage may be permanent, whereas vestibular damage may be reversed after the drug therapy is discontinued. A variety of substances may result in ototoxic reactions, including diuretics, alcohol, lead, carbon monoxide, aspirin and antibiotics. Manifestations include bilateral hearing loss, tinnitus, vertigo and ataxia.

Disorders of multiple causes

Otosclerosis is a disorder in which the spongy temporal bone becomes a dense sclerotic mass, eventually immobilising the stapedial footplate and causing a conductive hearing loss. Although no definite cause has been identified, heredity is a significant factor. Females have a higher incidence of the disorder than males. Manifestations are slowly progressive and include bilateral conductive hearing loss and mild tinnitus.

Ménière’s syndrome is a disorder of the inner ear that results from over-production or decreased absorption of endolymph. The resulting dilation of the membranous labyrinth can progress to herniation and rupture. Manifestations are exacerbation and remissions of three symptoms: vertigo, tinnitus and sensorineural hearing loss. During an acute episode the client may experience nausea and vomiting, sweating, giddiness, nystagmus and ataxia.

Auditory tests

Otoscopic examination is the direct visualisation of the external auditory canal and the tympanic membrane through an otoscope (auroscope). Otoscopy is performed to detect foreign bodies or cerumen in the external canal, or to detect external and middle ear pathology. Tuning forks detect hearing loss and provide information as to the type of loss:

Audiometry: pure tone audiometry is the testing of each ear separately for air conduction via earphones, and for bone conduction via a vibrator placed on the mastoid bone. The faintest point at which the individual hears the tone is called the hearing threshold level. Comparison of air and bone thresholds can suggest a conductive, sensorineural or mixed-type hearing loss.

Speech audiometry tests the client’s ability to understand and discriminate sounds. Two-syllable words are presented through earphones to measure how loud speech must be before it is heard.

Speech discrimination testing measures the client’s ability to distinguish phonetic elements of speech and thus understand what is heard. The examiner presents one-syllable words through the ear phones.

Impedance audiometry evaluates middle ear function by measuring the flow of sound energy into the ear, and the resistance to that flow. The objective is to determine the resistance (impedance) of flexibility (compliance) of the tympanic membrane.

Vestibular tests

Electronystagmography (ENG) evaluates vestibular function by measuring the effect of the semicircular canals on the ocular muscles. Electrodes are positioned at precise points on the face and eye, with movements in response to specific stimuli recorded on a graph.

Caloric tests compare the nystagmus produced by warm and cold stimulation in each ear. Each ear is irrigated with water to stimulate mild vertigo and nystagmus. No response indicates that the labyrinth is non-functional.

Other tests used to diagnose ear disorders include cultures of secretions to detect the source of an infection, and radiological examination of the temporal bone.

Maintaining a safe environment

Any hearing impairment renders the client susceptible to accidents and injury. Any environment can heighten or reduce sensory stimulation. The nurse must assess the client’s environment in the healthcare setting as well as assisting the client and the family to identify hazards in the home environment, and recommend appropriate adaptations. Clients with vestibular disorders are at risk of losing their balance and falling. Clients with proprioceptive problems may lose balance easily. It is important to ensure that bathrooms have a non-skid surface in the shower and bath, grab bars are installed and that the nurse provides the family with education regarding supervision of the client when ambulant, to minimise the risk of falls.

Client education

The client who is experiencing hearing loss that is either temporary or permanent needs assistance to adjust to the physical and psychosocial implications. Communication is a key component of assisting the client with impaired hearing. Initially the client may withdraw by avoiding communication or socialisation with others to cope with the hearing loss. This can lead to social isolation and low self-esteem. The nurse should encourage the client to discuss feelings of fear and frustration with their family members. Clinical Interest Box 32.8 provides an overview of hearing aid care and client education.

Key aspects related to assisting the hearing-impaired client

Table 32.3 Aids available to hearing-impaired clients

Local ear care

Various nursing procedures involving the ear should be performed in accordance with the policies of the healthcare facility. The general principles should include:

Instillation of ear drops

Ear drops that may be prescribed include those that combat infection and those that soften wax. Ear drops are instilled only when they have been prescribed by a medical officer, and the nurse must follow the regulations and policies regarding the administration of medications. Key aspects regarding the use of otic medications are the same as those for using ophthalmic medications.