Dermatome: definition and meaning
A dermatome is a segment of your body's surface supplied by the nerves that exit or enter at a specific spinal vertebra. Based on your symptoms, the doctor can identify at which vertebral level the problem should be sought.
For example, if you feel pain on the back of your upper arm and on your hand in the area of the index and middle fingers, that indicates a problem with the nerve that exits/returns at the level of the seventh cervical vertebra (C7). A herniated disc, calcification, fracture, displacement, etc., may be responsible here. Therefore they won't start by X‑raying your leg, or even your hand, but your neck.
If, for example, you cannot lift your foot (peroneal palsy), the problem lies at the fifth lumbar vertebra (L5). In such cases they will examine the lumbar spine, not the foot.
If you don't know the dermatomes and how the spinal nerves map your body in these bands, your pains can be misleading. One of the best examples is sciatica, where the pain deep in the buttock and on the back of the thigh can be unbearably severe. But during treatment they do not concentrate on the leg or buttock—because the pain is only referred there—but on the lumbosacral junction, since that area corresponds to the S1 vertebra's territory.
Knowledge of dermatomes is also very important when using spinal anesthesia (epidural anesthesia). Based on the dermatome map, the physician decides which nerve root must be blocked to achieve anesthesia in the surgical area.
