What is a dermatome?
Your nervous system is made up of your nerves, spinal cord, and brain.
Nerve information—pain, heat, or cold, for instance—travels from your hands, arms, legs, feet, trunk, or head to your spinal cord and then to your brain. When doctors are trying to diagnose a nerve problem, they find it useful to think of what nerve or part of the spinal cord may be involved in the problem. If your leg doesn't work as it should, for instance, your doctor may look at the nerve or nerves that send nerve information from your leg. On the other hand, the doctor might decide your leg problem is linked to a certain part of your spinal cord. When that's the case, the area affected may involve a dermatome – a spinal nerve tied to that area of your leg.
What is a dermatome map?
The illustration below is called a dermatome map. It shows a series of relatively straight lines that extend from the spine around the trunk or out along the arms or legs. The areas between the lines are dermatomes—areas where nerves in various parts of the body connect to the spinal cord. The actual location of the lines vary from person to person.
If your doctor thinks you might have a problem in your spine, he or she may look for "dermatome" patterns of loss of function, especially loss of sensation. If your doctor thinks you might have a problem in one or more nerves after they leave the spine, then he or she may look for a pattern of problems that correspond to where that nerve goes and what it does: Is it involved in sensation or does it govern motor function?