Dermatome Definition with Back Pain Example

Dermatome map
Dermatome map. BSIP/UIGM/Universal Images Group/Getty Images

Dermatome Defined

A dermatome is the zone of skin supplied by the activity of one spinal or cranial nerve.

Dermatomes and Spinal Nerve Roots

Dermatomes receive signals from the sensory nerves in one spinal nerve root. A spinal nerve root is a mix of several types of nerves – sensory, motor and autonomic – that branch off from the spinal cord.  The nerve root lives in an archway called the  intervertebral foramen, which is a hole at the side of the spinal column made of parts of individual vertebrae as they stack on top of one another.  Past the foramen, the nerve root begins to branch out into individual nerves to reach and serve all areas of the body.

The Dermatome Map and What It Means

Dermatomes are zones of sensory input, that transport data about things you feel – cold, heat, pressure, light touch and pain, for example.  This is in contrast to myotomes, which are groups of muscles enervated by a single nerve root.

Each dermatome occupies long strip of skin that starts from the midline and extends, in some cases, all the way to the ends of an extremity.

Both dermatomes and myotomes have been graphed.  You or your doctor or physical therapist can use a dermatome chart to trace the area in which your symptoms present themselves back to the compromised nerve root.  In this way, the evaluating health provider can determine the spinal level that is likely the source of your symptoms. 

Dermatomes and Radiculopathy

Dermatomes are used as reference when diagnosing radiculopathy.  Radiculopathy causes symptoms – pain, numbness, weakness, electrical sensations such as shock, burning and/or pins and needles that go down one arm or leg.  These symptoms are caused by compression and/or irritation to a spinal nerve root.  To come up with a radiculopathy diagnosis, your health provider learns about the quality and location of your symptoms (by asking you), and tests certain positions and movements that are designed to elicit those symptoms (called provocation tests.)  She then correlates the information she gleans with the dermatomes that are mapped out on the chart.  (Caveat: Most doctors and physical therapists know the chart in their head.) The result is you’ll know where the pain and other symptoms are coming from (i.e. which level.)  This information obviously helps the members of your health team know where to address the treatment:  A surgeon needs to know where to operate, for example.

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 means that during a physical exam, your doctor has to test several dermatomes. Tests include those for reflexes, loss of feeling and muscle strength.

In case you were wondering, dermatome is pronounced: durm-uh-tohm

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