Dermatomes, myotomes and reflexes! What’s it all about?
What are dermatomes?
A dermatome is an area of skin that is supplied by a single spinal nerve. Maps like the image above do vary from text to text, but familiarity with dermatomes can be useful in localising lesions to a nerve root or spinal cord. If you believe a problem is due to a radiculopathy it is useful to know the patterns of dermatomes. Symptoms that may indicate a pathology that involves the related nerve root include somatic dysfunction of the spine or viral infection such as shingles.
Commonly injury nerve roots and the pattern of dermatome distribution
C5 – over the shoulder
C6 – thumb and forearm
C7 – Middle fingers
C8 – Small finger and forearm
L4 – thigh
L5 – Inside part of the calf, foot and the big toe
S1 – Outside part of the calf, foot and smaller toes
What is a myotome?
A group of muscles innervated (supplied) by a single nerve root, they are useful to help you determine if damage has occurred to the spinal cord, and the corresponding level that damage has occurred.
Most muscles receive innervation from more than one spinal nerve root, such as the biceps brachii, this is supplied by the C5, C6 and C7 nerve roots by the musculocutaneous nerve.
Here is a list of movements that are most associated with each myotome:
- C5 – The deltoid muscle (abduction of the arm at the shoulder).
- C6 – The biceps (flexion of the arm at the elbow).
- C7 – The triceps (extension of the arm at the elbow).
- C8 – The small muscles of the hand.
- L4 – The quadriceps (extension of the leg at the knee).
- L5 – The tibialis anterior (upward flexion of the foot at the ankle).
- S1 – The gastrocnemius muscle (downward flexion of the foot at the ankle).
If there is a suspected spinal cord lesion, the clinician can test myotome function. This can help determine if there is spinal cord damage, and where the damage is located.
Myotomes are tested in terms of power, and graded 1-5:
- 0 = total paralysis.
- 1 = palpable or visible contraction.
- 2 = active movement, full range of motion (ROM) with gravity eliminated.
- 3 = active movement, full ROM against gravity.
- 4 = active movement, full ROM against gravity and moderate resistance in a muscle specific position.
- 5 = (normal) active movement, full ROM against gravity and full resistance in a muscle specific position expected from an otherwise unimpaired person.
Some, but not all, of the nerve roots have a reflex. C5, C6 and C7 have reflexes. L4 and S1 have reflexes.
For example, when the C6 nerve is pinched, there is loss of the pronator reflex in the forearm. When the L5 nerve is pinched, there is no reflex loss. Not all nerves have a reflex which can be tested.
List of Reflexes of Commonly Injured Nerve Roots
C5 – Flexion at the elbow, biceps.
C6 – Flexion at the elbow, brachioradialis.
C7 – Extension at the elbow, triceps.
C8 – Finger flexion.
L4 – The knee reflex, quadriceps.
L5 – No reflex.
S1 – The ankle reflex, gastrocnemius.