CHAPTER 31 Myotomes

What is a myotome?

A myotome is defined as the group of muscles supplied by one spinal nerve root level. There are 31 pairs of spinal nerves (S2.13) each contributing to the innervations of many muscles (e.g. C₅ innervates parts of supraspinatus, infraspinatus, deltoid and biceps). The muscles supplied by a single nerve root level are generally involved in a common muscle action/s and it is this muscle action that is assessed. The list below identifies a simplified version of the actions associated with each spinal nerve root level (Grieve 2004).

C2 – neck flexion

C3 – neck extension (see Fig. 31.1)

C4 – neck side flexion (left and right)

C5 – shoulder abduction (see Fig. 31.2)

C6 – elbow flexion or forearm supination

C7 – elbow extension or wrist flexion

C8 – thumb extension or adduction and ulna deviation

T1 – finger abduction/adduction

L2 – hip flexion or adduction

L3 – knee extension (see Fig. 31.3)

L4 – dorsi flexion at the ankle

L5 – big toe extension or ankle eversion

S1 – plantar flexion of the ankle or knee flexion.

Figure 31.1 Testing for myotome C3.

Figure 31.2 Testing for myotome C5.

Figure 31.3 Testing for myotome L3.

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Why do I need to assess myotomes?

Assessing a myotome gives information related to nerve integrity, in other words, whether the nerve pathway from the spinal cord to the muscle is intact. The assessment evaluates the strength of the muscle contraction, however it should be remembered that a weak muscle could be the result of a lesion anywhere along the nerve pathway but also of the muscle itself. Knowledge of the muscles supplied by the spinal nerve root (myotome) and the peripheral nerve allows the therapist to differentiate between lesions of each (Petty 2006). A complete lesion of the peripheral nerve will lead to complete paralysis of the muscles innervated by that nerve. Therefore, weakness will be evident immediately on testing and muscle atrophy will occur over time. However, the presentation of a lesion to a single nerve root will be more difficult to recognize because the muscle itself will still be innervated by other unaffected root levels within the peripheral nerve. For example, the biceps muscle is supplied by the musculocutaneous nerve (C_5/6/7). Therefore, any lesion affecting C₆ nerve root will present as minor weakness because sufficient motor units can be recruited via the remaining root levels C_5/7. However, if the therapist provides resistance over a period of time (5–10 seconds) the weakness may become evident.

Caution

A motor loss related to a single myotome may be indicative of a lesion at the spinal nerve root but must be confirmed by a similar finding for that particular root level for dermatomes (S3.24) and reflexes (S3.22).

In terms of neurologically impaired patients, the clinical presentation of any motor loss only requires assessment using myotomes when there is involvement of either the spinal cord specifically or the peripheral nervous system, e.g. spinal cord injury (SCI) and Guillain–Barré syndrome (GBS). As neither of these pathologies affects the spinal nerve root in isolation, myotome testing is less clinically useful as a diagnostic tool, however it is a very useful way of mapping the motor loss. The map produced gives the therapist a highly relevant outcome measure, by which the extent and level of motor loss can first be estimated and then re-evaluated. This is especially important in recovering conditions, such as GBS and in SCI, where a rising level of motor loss may reflect a serious deterioration of the injury.

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How do I assess a myotome?

Patient

For testing the upper limb, the posture of sitting should be acceptable. To complete the lower limb assessment sitting, supine or prone may be necessary.

Therapist

1 Identify a muscle action from the list above.

2 Resistance is applied by the therapist so that the muscles being assessed contract isometrically. Therefore no movement should occur at the joint which may confound the findings.

3 Where possible the test should be performed in middle range of the action (Figs 31.1-31.3).

4 The resistance should be brought on slowly to allow the patient to build up to the level of resistance offered by the therapist.

5 The amount of force applied should be appropriate to the size of the muscle being tested, especially for neck movements.

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6 The isometric contraction should be held for approximately 5–10 seconds.

7 Where possible, the left and right sides should be compared simultaneously (Fig. 31.2).

Is there weakness evident in the action? Yes, there may be a deficit within the peripheral nervous system. If other spinal root levels appear intact the deficit may be localized to the root level being tested and should be confirmed using evidence from dermatome and reflex testing for the same level. However, if the distribution of loss is more generalized, this is unlikely to be the case.

Is the strength of contraction the same on both sides? No, this could indicate a unilateral deficit.

Recording

The therapist should note all the muscle actions that present with weakness. A simple statement of what was assessed and a list of abnormal findings is sufficient. In the case of GBS and SCI, the loss may be extensive but detailed analysis is crucial and may be easier to record on a body chart.

Example

All myotomes assessed:

Upper limbs: Nothing abnormal detected (NAD) and left side = right side

Lower limbs: Left L3 presents with weakness. Nil else of note.

Analysis

The findings of weakness related to a single myotome should be analysed along with any abnormal findings from dermatome testing and reflexes. The findings from all three assessments can be used to identify specifically the level and extent of a lesion or if relevant to differentiate a spinal nerve root deficit.

Outcome measure

Research and clinically

Myotome testing is viewed as an outcome measure to be used as an ongoing tool for evaluation.

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References

Grieve GP. Grieve’s modern manual therapy: the vertebral column, ed 3. Edinburgh: Churchill Livingstone; 2004.

Petty NJ. Neuromusculoskeletal examination and assessment: a handbook for therapists, ed 3. Edinburgh: Churchill Livingstone/Elsevier; 2006.