Dermatomes

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Dermatomes

A dermatome is an area of skin that is mainly supplied by a single spinal nerve. There are 8 cervical nerves (note C1 has with no dermatome), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these spinal nerves relay sensation from a particular region of the skin to the brain.

Dysfunction or damage to a spinal nerve can trigger symptoms in the corresponding dermatome. Nerves damage or dysfunction may result from infection, compression, or traumatic injury.

The nerves from the

• C2 to C4 supply the skin of the neck.

• C5 to T1 nerves supply the arms. 

• T2 to L2 nerves supply the chest and abdomen. 

• L3 to S1 nerves supply the skin of the legs. 

• S1 to S4 nerves go to the groin.

Nerve Roots Dermatomes

The dermatome corresponding with the coccygeal nerves is located on the buttocks, in the area directly around the coccyx.

Nerve Root	Dermatomes
Cervical
C2	Temple, forehead, occiput
C3	Entire neck, posterior cheek, temporal area, prolongation forward under mandible
C4	Shoulder area, clavicular area, upper scapular area
C5	Deltoid area, anterior aspect of entire arm to base of thumb
C6	Anterior arm, radial side of hand to thumb and index finger
C7	Lateral arm and forearm to index, long, and ring fingers
C8	Medial arm and forearm to long, ring, and little fingers
Thoracic
T1	Medial side of forearm to base of little finger
T2	Medial side of upper arm to medial elbow, pectoral and midscapular areas
T3-T12	T3–T6, upper thorax; T5–T7, costal margin; T8–T12, abdomen and lumbar region
Lumbar
L1	Back, over trochanter and groin
L2	Back, front of thigh to knee
L3	Back, upper buttock, anterior thigh and knee, medial lower leg
L4	Medial buttock, latera thigh, medial leg, dorsum of foot, big toe
L5	Buttock, posterior and lateral thigh, lateral aspect of leg, dorsum of foot, medial half of sole, first, second, and third toes
Sacral
S1	Buttock, thigh, and leg posterior
S2	Same as S1
S3	Groin, medial thigh to knee
S4	Perineum, genitals, lower sacrum

Purpose

Testing of dermatomes is part of the neurological examination. They are primarily used to determine whether the sensory loss on a limb corresponds to a single spinal segment, implying the lesion is of that nerve root (i.e., radiculopathy), and to assign the neurologic “level” to a spinal cord lesion.

Technique

Dermatome Testing is done ideally with a pin and cotton wool. Ask the patient to close their eyes and give the therapist feedback regarding the various stimuli. Testing should be done on specific dermatomes and should be compared to bilaterally.

• Pinprick test (pain sensation) - Gently touches the skin with the pin ask the patient whether it feels sharp or blunt

• Light touch test (light touch sensation) - Dab a piece of cotton wool on an area of skin

During the review of systems, asking the patient to carefully describe the pattern or distribution of sensory symptoms (e.g., tingling, numbness, diminished, or absent sensation) provides the therapist with preliminary information to help guide the examination and to assist in identifying the dermatome(s) and nerve(s) involved.

Tactile dermatomes are larger than pain dermatomes. When only one or two segments are affected, testing for pain sensibility is a more sensitive method of examination than testing for abnormal touch.

Peripheral nerve injuries generally present sensory impairments that parallel the distribution of the involved nerve and correspond to its pattern of innervation.For example,

1. If a patient presents with complaints of numbness on the ulnar half of the ring finger, the little finger, and the ulnar side of the hand, the therapist would be alerted to carefully address ulnar nerve (C8 and T1) integrity during the sensory examination.
2. Complaints of sensory disturbances on the palmar surface of the thumb and the palmar and distal dorsal aspects of the index, middle, and the radial half of the ring finger would be indicative of median nerve (C6–8 and T1) involvement.
   
   There exist some discrepancies among published dermatome maps based on the methodologies used to identify skin segment innervation. In a clinical commentary, Downs and Laporte discuss the history of dermatome mapping, including the variations in methodologies employed, and the inconsistencies in the dermatome maps used in education and practice.[[Laporte C. Conflicting dermatome maps: educational and clinical implications. journal of orthopaedic & sports physical therapy. 2011 Jun;41(6):427-34.]]

Associated Health Conditions

Symptoms that occur within a dermatome sometimes indicate damage or disruption to the dermatome’s corresponding nerve. The location of these symptoms can help with diagnosis of certain underlying medical conditions. eg

• Shingles/Herpes Zoster
• Spinal Cord Injury
• Pinched Nerve/Nerve entrapment