What are Dermatomes?
We recently published a guide to understanding myotomes, which are key muscles supplied by a single nerve exiting from the spine. In this article, we will discuss dermatomes, which are different from myotomes in that dermatomes are specific areas of the skin that supplied by a single nerve root, as opposed to key muscles.
Myotome and dermatome tests are used in conjunction with each other to help differentiate between certain conditions and come up with a diagnosis. That said, while these tests are each trying to achieve the same goal, there are pretty obvious differences you will notice as a patient. We will explain these differences below, as well as more information about dermatome testing in general.
Spinal Nerve Roots and Dermatomes
All of the muscles in your limbs are supplied by nerves that come from your spine. The spine, also known as the vertebral column, is made of a linkage of vertebrae, which are the individual bones of the spine. When properly aligned, these vertebrae form a canal running down inside of the spine. Inside of this canal is the spinal cord, which is attached directly to the brain. At each vertebral level, the spinal cord sends out nerves to each side of the body. The very beginning part of a nerve that comes directly off the spinal cord is referred to as a nerve root.
This means that at each vertebral level, there are are two nerve roots, one going to the left side of the body, the other going to the right side of the body. These nerve roots then go on to branch off into many different directions, ultimately forming a network of nerves that supply the entire body. This network is pretty complicated, but nevertheless, we can trace individual nerves in the limb all the way back to the nerve roots in the spine.
With that, dermatomes are defined as specific areas of the skin supplied by a single nerve root. The cervical nerve roots, which are the ones that come from your neck, generally supply skin to the head, some of the face, neck, shoulders, arms, and hands. The thoracic nerve roots, which are the ones that are located in the torso, supply the skin of the chest and abdomen. The lumbar and sacral nerve roots, which are located in the lower back, go on to supply the skin around the hips, groin, legs, ankles, and feet.
If you're looking around online at different dermatome charts, you may notice some slight variation between charts. This is because there is some natural overlap between dermatomes, which makes it very difficult to define crisp precise boundaries for individual dermatomes. In addition to that, there always some anatomical variability between individuals, which can also make it difficult to define the exact boundaries.
Nevertheless, most of the charts do look very similar, so if you need to learn each individual dermatome for school, any chart will give you the basic info. The chart like the one above is pretty standard, but if you're in doubt, go by whatever textbook or resource is being used for your class. Another good way to distinguish dermatomes are by distinct differences, even if they are small. For example, the C5 and C6 dermatomes supply very similar areas of the skin on the arm, but C6 includes the thumb, while C5 does not. In this case, the thumb will be extremely important in differentiating between C5 and C6 on any patient.
Testing dermatomes is similar to testing myotomes in that we are systematically testing certain areas of the body supplied by a single nerve root. As you may have already guessed, as opposed to myotome testing which feels very similar to muscle strength testing, dermatome testing will focus on examining skin sensation.
So what can you expect from dermatome testing? First, it's really on the patient, and requires basically no physical effort. However, it does require focus. During these tests, the doctor or physiotherapists will run there hands along the specific dermatomal pattern on your skin, doing both sides at the same time. They will do this one dermatome at a time, asking you to report if you feel any differences between sides. The examiner should explain this to you ahead of time, as they will be contacting you directly on your skin.
Additionally, it is standard practice to ask the patient to close their eyes during this testing. This allows the patient to focus completely on what their feeling, as opposed to what they are seeing. Sometimes the differences are so subtle that they can go missed by the patient if they are watching movement. The tests are extremely quick, probably no longer than 30 seconds for both the arm and the legs, so you won't be tasked with this for long.
Upper Limb Dermatome Testing
Lower Limb Dermatome Testing
We discussed radicular pain in our article about myotomes, but given this is one of the biggest reasons we test both myotomes and dermatomes, we feel it's important to mention again.
Radicular pain is pain that results from inflammation, irritation, or damage to a nerve root , whereby the pain is referred down the limb to the most distal segments (e.g. below the knee and below the elbow). Often times, the pain will not be associated with any mechanism of injury, and it can be difficult for someone with radicular pain to provide an exact time point for the onset of symptoms or anything that could have caused the symptoms. However, there can sometimes be trauma associated with radicular pain, such as whiplash from a car accident.
Furthermore, radicular pain is really tricky to manage. It can often be quite severe, and it can be difficult to find anything to alleviate the pain. The most common easing factor that people will report is changing their back or neck posture, but it's rare that adjusting the limbs themselves will alleviate any pain. Additionally, the pain may intensify over the course of the day, especially in those who work at a desk all day.
The reason for these complaints and symptoms goes back to the spinal nerve roots. If there is inflammation to the nerve root, this can send symptoms downstream to arms and hands, or legs and feet, even if there is no actual damage to those downstream areas. One of the most reported symptoms is either dull achy pain, or stabbing shooting pain, but other symptoms can include numbness, tingling, weakness, or a feeling of heaviness of that area of the limb. By performing myotome and dermatome testing, one can determine if a nerve root is the problem, as opposed to an injury to the limb, and if so, pinpoint the exact nerve root causing the problem.
This is by no means an all-encompassing guide to understanding dermatomes, but we hope it gives you a good foundation for understanding what dermatomes are and why they matter. There are many different conditions that can lead to positive dermatome tests, but we will leave those for later articles that can be found in this Anatomy and Injury Info section. Until then, all the best with your rehabilitation, or with your studies if you're on the other end of it!