What are myotomes? How are spinal nerves distributed? What is the purpose of myotome testing? How is myotome testing performed? What is the clinical relevance of myotome mapping?

Defining Myotomes

A myotome is a group of muscles that is innervated by a single spinal nerve root. Myotomes are an essential part of the somatic nervous system, which is a component of the peripheral nervous system. The differentiation of somites into dermatomes, sclerotomes, and myotomes is a key part of embryonic development.

Spinal Nerves and Myotomes

There are 31 spinal nerves, with each vertebra having its own spinal nerve. These nerves are categorized based on the vertebra that houses them – 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. Of these 31 nerves, 16 have specific myotomes that control voluntary muscle movement.

Most muscles in the limbs receive innervation from more than one spinal nerve root, meaning they are comprised of multiple myotomes. For example, the biceps brachii muscle, which flexes the elbow, is innervated by the musculocutaneous nerve, which originates from the C5, C6, and C7 nerve roots.

Myotome Distribution

The specific movements associated with each myotome are as follows:

• Upper Extremity:
  • C5 – Shoulder abduction
  • C6 – Elbow flexion, wrist extension
  • C7 – Elbow extension, wrist flexion
  • C8 – Thumb extension, wrist ulnar deviation
  • T1 – Finger abduction
• Chest Wall and Abdominal Muscles:
  • T2 – L1
• Lower Extremity:
  • L2 – Hip flexion
  • L3 – Knee extension
  • L4 – Ankle dorsiflexion
  • L5 – Big toe extension
  • S1 – Ankle plantarflexion
  • S2 – Knee flexion

Purpose of Myotome Testing

Myotome testing, in the form of isometric resisted muscle testing, provides information about the level in the spine where a potential lesion may be present. It allows healthcare providers to assess muscle weakness in specific muscle groups, which can indicate a problem with the corresponding spinal nerve root or intervertebral disc herniation pressing on the nerve.

Myotome Testing Procedure

To test myotomes, the healthcare provider will ask the patient to perform a specific movement and hold an isometric contraction against resistance for a count of 5. The provider will then compare the strength of the movement between the two sides. Some examples of myotome testing include:

• C5 – Shoulder Abduction: Ask the patient to raise both arms to the side simultaneously as strongly as they can, while the provider provides resistance.
• C6 – Elbow Flexion and Wrist Extension: Test the strength of elbow flexion by having the patient flex their hand up to their shoulder against resistance at the wrist. Test wrist extension by having the patient extend their wrist against resistance.
• C7 – Elbow Extension and Wrist Flexion: Ask the patient to extend their forearm against resistance, starting from a fully flexed position.
• C8 – Finger Flexion: Have the patient grip the provider’s fingers tightly and resist the provider’s attempt to remove them.
• C8 and T1 – Finger Abduction/Adduction and Thumb Opposition: Instruct the patient to fan out their fingers and resist the provider’s attempt to compress them back in. Test thumb opposition by having the patient touch the tip of their thumb to the tip of their fingers.

Clinical Relevance

Myotome testing is an essential part of the neurological examination, particularly when evaluating for radiculopathy (nerve root compression). The results can help identify the specific spinal nerve root level involved and guide further diagnostic testing and treatment. Recognizing patterns of muscle weakness can be a valuable clinical tool for healthcare providers.

Conclusion

Myotomes are an important concept in the peripheral nervous system, as they represent the specific muscles innervated by individual spinal nerve roots. Thorough testing of myotomes can provide valuable information about the location of potential spinal cord or nerve root pathologies. Understanding the distribution and clinical significance of myotomes is a crucial part of a comprehensive neurological examination.