The dermatomes is a word of greek origin which means ‘areas of skin’. Dermatomes are indeed specific theoretical areas upon the human body that are derived from the cells of a certain somite. The areas are spread out according to the dominant spinal nerve that happens to innervate it. The spinal cord has thirty one segments each with its own bilateral anterior and posterior pair of nerve roots that innervate the anatomical structures with motoric and sensory fibers. The clinical significance of a dermatome is that when a dermatological case presents itself, a rash for example, that is limited to a single dermatome, that outer clinical symptom may indicate the pathological involvement of its related spinal root.
A system of points on the body within the region of a dermatome has been devised so that those without direct access to a dermatome map have the ability to localise the pain and correctly diagnose the nerve that may be affected. Here is a list of the dermatome test points, organized by the spinal root that governs each dermatome, along with an approximation of the area that it covers. The list runs through the spinal nerves in a cranial to caudal direction.
The dermatome test points of the upper body are as follows:
- C2 - Occipital Protuberance
- C3 - Supraclavicular Fossa
- C4 - Acromioclavicular Joint
- C5 - Lateral Antecubital Fossa
- C6 - Thumb
- C7 - Middle Finger
- C8 - Little Finger
- T1 - Medial Antecubital Fossa
- T2 - Apex of the Axilla
The dermatome test points of the lower body are as follows:
- L1 - Upper Anterior Thigh
- L2 - Mid Anterior Thigh
- L3 - Medial Femoral Condyle
- L4 - Medial Malleolus
- L5 - Dorsum of the third Metacarpal Joint
- S1 - Lateral Heel
- S2 - Popliteal Fossa
- S3 - Ischial Tuberosity
- S5 - Perianal Area
There are three other dermatome test points that are governed not by the spinal roots but the trigeminal nerve (CN V). The three trigeminal divisions are responsible for the innervation of the face.
Herpes Zoster, or shingles is a viral infection that is limited to one side of the body usually within the dermatome of the nerve it has affected. A red, painful blistering rash forms due to the presence of the varicella zoster virus (VZV) which is the adult form of the virus which causes chickenpox. It sits latent upon the nerve ganglia and can arise in times of stress or when the immune system is low. The condition usually clears up within a month, although some patients can suffer for years due to nerve damage known as postherpetic neuralgia.