What is a dermatome?

A dermatome is defined as the area of skin supplied by one spinal nerve root level. There are 31 pairs of spinal nerves leaving the spinal cord (S2.13) and each one supplies a different area of skin. Therefore the skin supplied by all the individual spinal nerve roots ultimately produces a dermatome map that represents the entire body (Fig. 24.1).

Why do I need to assess dermatomes?

Assessing a dermatome gives information related to nerve integrity, in other words whether the nerve pathway from the skin to the spinal cord is intact. Knowledge of the cutaneous supply of both the spinal nerve root (dermatome) and the peripheral nerve allows the therapist to differentiate between lesions of each (Petty 2006). For example, a sensory loss of the area over the deltoid muscle at the shoulder is identified as dermatome C₅. The same area of skin is also part of the sensory distribution of the radial nerve (posterior cutaneous nerve of the arm C_5/6/7/8 T₁). It is therefore likely, but not certain that the lesion is at the level of the spinal nerve root because a lesion of the peripheral nerve is likely to be more extensive. Accurate diagnosis is only possible with knowledge of anatomy and accurate assessment.

In terms of neurologically impaired patients, the clinical presentation of any sensory loss only