A dermatome map is a representation of the zones on the skin that are supplied with sensation by individual spinal nerves. Each spinal nerve contains a sensory fiber and a motor fiber. The motor fiber is responsible for providing motion to a specific group of muscles, and the sensory fiber is responsible for providing sensation to a specific area of the skin (dermatome). By determining abnormalities in sensation in a specific dermatome, physicians can determine whether nervous transmission is normal within the corresponding spinal nerve.
Abnormal dermatome sensation can be indicative of nerve conduction issues resulting from bulging discs, prolapsed discs, sciatica, degenerative disc disease, or a number of other spinal abnormalities that cause neural compression.
Specifics of the dermatome map
The dermatome map is exactly what it sounds like: a map of the body that physicians have compiled over time that shows where each dermatome begins and ends, as well as which spinal nerve corresponds to each area. Physicians use their knowledge of this map to test for potential spinal nerve injuries.
Dermatome maps correspond to the levels of the spine. Listed from top to bottom, the spinal levels are called the cervical ©, thoracic (T), lumbar (L) and sacral (S) levels. Some examples of dermatomes and their corresponding spinal levels are:
- Middle and sides of calves – L4 and L5
- Little toes – S1
- Fronts of thighs – L2
- Inner and outer forearms – C6 and T1
- Shoulders – C4 and C5
- Thumbs and little fingers – C6, C7 and C8
Tests using the dermatome map
Since tests using the dermatome map are fairly simple, they are generally conducted as part of an initial examination to rule out nerve damage. The physician will want to make sure that sensation is the same on each side of a dermatome, as these areas tend to be symmetrical.
In most dermatome tests, the physician draws his or her fingers, a cotton swab, a paperclip, a pin or a similar instrument across the skin in the suspicious dermatome. He or she will ask the patient to respond to the sensation to make sure the stimulation is being felt. The same will be done on the other side of the dermatome to make sure the symmetrical sensation is intact.
If dermatome testing indicates a problem with a nerve root, imaging tests, such as CT scans, or MRIs, may be ordered to confirm the diagnosis.