Myotome Testing

Myotome Testing gives information related to nerve integrity, in other words, whether the nerve pathway from the spinal cord to the muscle is intact.

The Myotome Testing 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 examiner to differentiate between lesions of each.

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 (C5/6/7). Therefore, any lesion affecting C6 nerve root will present as minor weakness because sufficient motor units can be recruited via the remaining root levels C5/7. However, if the examiner provides resistance over a period of time (5–10 seconds) the weakness may become evident.

See Also: Myotomes of the Upper Limb

How to do Myotome Testing?

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

Identify a muscle action from the list above. Resistance is applied by the examiner so that the muscles being assessed contract isometrically. Therefore no movement should occur at the joint which may confound the findings. Where possible the test should be performed in middle range of the action. The resistance should be brought on slowly to allow the patient to build up to the level of resistance offered by the examiner . The amount of force applied should be appropriate to the size of the muscle being tested, especially for neck movements.

The isometric contraction should be held for approximately 5–10 seconds. Where possible, the left and right sides should be compared simultaneously.

• 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.

The examiner 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.

The findings of weakness related to a single myotome should be analyzed 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.

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).

See Also: Complex Regional Pain Syndrome

In terms of neurologically impaired patients, the clinical presentation of any motor loss only requires assessment using myotome testing 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 examiner 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.

References & More

1. Grieve GP: Grieve’s modern manual therapy: the vertebral column, ed 3, Edinburgh, 2004, Churchill Livingstone
2. Petty NJ: Neuromusculoskeletal examination and assessment: a handbook for therapists, ed 3, Edinburgh, 2006, Churchill Livingstone/ Elsevier.
3. Orthopedic Physical Assessment by David J. Magee, 7th Edition.
4. Sonoo M. Recent advances in neuroanatomy: the myotome update. J Neurol Neurosurg Psychiatry. 2023 Aug;94(8):643-648. doi: 10.1136/jnnp-2022-329696. Epub 2023 Jan 18. PMID: 36653172. Pubmed
5. Hong CG, Nam WD. Reliability and Diagnostic Accuracy of Standard Dermatomes and Myotomes for Determining the Pathologic Level in Surgically Verified Patients With Cervical Radiculopathy. Neurospine. 2022 Dec;19(4):1006-1012. doi: 10.14245/ns.2244194.097. Epub 2022 Dec 31. PMID: 36597659; PMCID: PMC9816603. Pubmed