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-
- L2 -Hip Flexion
- L3,4- Knee Extension
- L4 – The quadriceps (extension of the leg in the knee joint).
- L5 – The tibialis anterior (upward flexion of the foot in the ankle joint).
- S1 – The gastrocnemius muscle (downward flexion of the foot in the ankle joint).