Where is That Pain coming From?

Many people with neck and back problems are often disturbed by the fact that they have pain but the physician tells them that the herniated disc on the MRI is not causing the pain they are feeling. How does he know?

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A dermatome refers to an area of skin that is innervated by a single spinal nerve. These areas of skin carry sensation, including pain, from that area, to the brain. When a patient complains of pain in a certain part of their body the physician will first look at the radiological studies to see if there is any type of disruption in the spine in the area that correlates with the dermatome. For example, if the patient has pain in his low back that goes down the back of the leg to the bottom of the foot, the physician will look at the L5/S1 area of the spine as that is the nerve that usually serves that area. If the pain goes across the front of the thigh, the problem is usually higher up in the spine (see picture) such as L2 or L3.

The same works with neck problems. If the pain, numbness or tingling go into the thumb on the right, the disruption is probably at the C6 nerve on the right. The physician will test the sensory pathways that enervate a certain dermatome in several different ways. You will have a sharp object used as a “pinprick” test and something softer such as a cotton swab used to check “light touch”. These are all compared to other parts of the body to see if there are changes in sensation. This should be a part of any neuro/ spine exam.

This sounds simple but, because we are dealing with the human body, it is not. Much variability exists between dermatome maps in standard anatomy and medical guideline textbooks. A review of 14 different dermatome maps by Lee et al showed variations within each individual map. Each individual will also have their own variation but the above map will give you some idea of what to expect.

Most areas of the skin are innervated by 2 or more spinal nerve roots, which may be the reason for variability between individuals. We also frequently see people who are “wired differently”. They show signs that the nerves expected in one area can be one level above or below the expected level. This is why some people with a C5 spinal cord injury can breathe on their own and some need to be on a ventilator.

So if you see the doctor for your herniated disc at L4/5 and your pain is in your groin area, the chances are, fixing the L4/5 level will do nothing to relieve your pain.

Last modified: December 11, 2020