Dermatomes: A dermatome is an area of skin which is chiefly supplied by a single spinal nerve. There are 8 cervical nerves (C1 denoting an anomaly with no dermatome), 12 thoracic nerves(T1-T12), 5 lumbar nerves(L1-L5) and 5 sacral nerves(S1-S5).

Each of these nerves relays sensation (including pain) from a particular region of skin to the brain. Symptoms that follow a dermatome may designate a pathology that involves the related nerve root.

Dermatomes that are Commonly Injured Nerve Roots–

C5 – The area over the shoulder.

C6 – The thumb and part of the forearm.

C7 – The middle finger.

C8 – The smallest fingers and part of the forearm.

L4 – The thigh.

L5 – The medial part of the calf and foot, the big toe.

S1 – The lateral part of the calf and foot, the smaller toes.

Cervical Dermatomes

• C2 – At least one cm lateral to the occipital protuberance at the base of the skull.

• C3 – In the supraclavicular fossa, at the midclavicular line.

• C4 – Over the acromioclavicular joint.

Lumbar Dermatomes

• L1 – Midway within the key sensory points for T12 and L2.

• L2 – On the medial of the anterior thigh, at the midpoint of a line joining the midpoint of the inguinal ligament and the medial epicondyle of the femur.

• L3 – Medial epicondyle of the femur.

Thoracic Dermatomes

• T1 – On the ulnar side of the antecubital fossa, just proximally to the medial epicondyle of the humerus.

• T2 – At the apex of the axilla.

• T3 – Junction of the midclavicular line and the third intercostal space.

• T4 – Junction of the midclavicular line and the fourth intercostal area, positioned at the level of the nipples.

• T5 – Junction of the midclavicular line and the fifth intercostal space, horizontally located midway between the level of the nipples and the level of the xiphoid process.

• T6 – Junction of the midclavicular line and the horizontal level of the xiphoid process.

• T7 – Junction of the midclavicular line and the horizontal level at one quarter the distance between the level of the xiphoid process and the level of the umbilicus.

• T8 – Junction of the midclavicular line and the horizontal level at one half the distance within the level of the xiphoid process and the level of the umbilicus.

• T9 – Junction of the midclavicular line and the horizontal level at three-quarters of the distance between the level of the xiphoid process and the level of the umbilicus.

• T10 – Junction of the line of the mid clavicle, at the horizontal level of the umbilicus.

• T11 – Junction of the line of the mid clavicle, at the horizontal level midway between the level of the umbilicus and the inguinal ligament.

• T12 – Junction of the line of the mid clavicle and the midpoint of the inguinal ligament.

Sacral Dermatomes

• S2 – At the midpoint of the popliteal fossa.

• S3 – Over the tuberosity of the ischium or infra-gluteal fold

• S4 and S5 – In the perianal area, less than one cm lateral to the mucocutaneous zone

Clinical significance of Dermatomes: Assessing Spinal Cord Lesions

Following a traumatic injury that may affect the spinal cord, the clinician can test dermatomes to establish the presence and the extent of a spinal cord injury.

Firstly, the physician uses cotton wool to test for light touch sensation, which corresponds to the different dermatomes. Then, the physician uses a small pin to examine for responsiveness to pain.

Myotomes

A myotome is the group of muscles which have a single spinal nerve innervates. Likewise, a dermatome is an area of skin that a single nerve innervates.



Myotomes upper limb Nerve Roots-

C2-Neck flexion

C3-Neck side flexion

C4-Shoulder elevation

C5 – The deltoid muscle (abduction of the arm in the shoulder joint).

C6 – The biceps (flexion of the arm in the elbow joint).

C7 – The triceps (extension of the arm in the elbow joint).

C8 – The small muscles of the hand.

T1- Finger Abduction (ulnar nerve)

T1-Abductor pollis brevis (median nerve)

Myotomes lower limb Nerve Roots-