A dermatome is an area of skin on one side of the body that receives nerve signals from one spinal nerve. Dermatomes are zones of sensory information, or feeling, carried by the nerve branches in that area.
Through investigation of the effects of spinal injuries, researchers have mapped out all the dermatomes in the body. Doctors use their knowledge of the dermatome map (and of myotomes) to examine the areas or zones for pain and dysfunction. The map helps them locate the specific nerve or nerves that give rise to your pain.
The edges between dermatomes are fuzzy. In other words, there is overlapping coverage by more than one spinal nerve near the boundary of each dermatome zone. This overlap prevents doctors from testing just one dermatome during a physical exam. To correctly identify the source of pain and diagnose your condition, your doctor has to test all relevant dermatomes. Tests include those for reflexes, loss of feeling and muscle strength.