Skin Dermatomes

A dermatome refers to a portion of skin in the body that gets its sensation from a single spinal nerve. When I examine people in the office, I will test a dermatome for sensation. I saw a patient last week who had hip osteoarthritis.

He had pain over his right thigh. When I examined him, he had a hypersensitivity over the L2,3 distribution. As it turns out, he has a herniated disc at that level in his lumbar spine that is the source of his pain. Here are 4 things to take away about a dermatome and what to know about your body’s innervation pattern.

1. Spinal cord segments. The spinal cord, as you can imagine, is highly specialized. It is divided into segments. Spinal nerve roots arise from the spinal cord in pairs of nerves. These nerves are numbered based upon where they exit. For instance, there are 8 cervical, 12 thoracic, 5 lumbar and 5 sacral segments. Each of these nerve pairs will give motor, sensation and in some cases reflexes to the body. It is in understanding this wiring that we can identify where a particular problem lies in the spine.

2. Dermatome and myotome. Just as a dermatome will give information about a segment of skin, a myotome is a map of motor power for a group of muscles. In addition to testing your skin at various levels, a motor examination will help confirm if a particular nerve root or nerve roots are affected. For instance, with a C5 radiculopathy, I will specifically test the deltoid muscle of the shoulder, and the skin over the outside or lateral aspect of the arm. In addition, I will test the biceps reflex which is a predominately supplied by the C5 nerve root.

3. A dermatome can explain referred pain. Because a dermatome is a roadmap of nerves it can also explain referred pain. Referred pain is a phenomenon where structures in the body can confuse nerve perception. An example is when someone has an inflamed gall bladder orcholecystitis. The gall bladder can inflame the diaphragm which is innervated by C3,4 and C5. The C3,4 dermatome innervate above the shoulder. Rarely, a patient will come to the office with right shoulder pain but the pain is coming from cholecystitis.

4. Virus can express themselves in a dermatome distribution. Viruses can hibernate in the sensory cell bodies. An example is varicella zoster virus. Patients can present with severe pain in the back or trunk and this can be a manifestation of a zoster attack. Diagnosis can be confusing because it can be misinterpreted as back or flank pain and not the beginning of shingles.

In medicine you often have to play detective. The dermatome map is one clue to a diagnosis and hopefully a cure. I have included a dermatome map here for you to look at further.