How do dermatomes help in the diagnosis of diseases?

Your body has a total of 31 spinal nerves. A network of nerve branches spread out from the spinal nerves, all the way to our fingers and toes (peripheral nervous system). The signals that they pick up are sent to the brain along the spinal nerves and the spinal cord.

This is how sensations such as touch, pain, heat or cold get from our skin to our brains. For example, if you bump your arm, the spinal nerve responsible for that area of skin (that dermatome) sends a signal to your brain along the spinal cord. The signal is then processed in your brain, and you feel pain.

Spinal nerves and their dermatomes are named according to the sections of the spine where they branch off from the spinal cord:

• C stands for the cervical spine (cervical = relating to the neck).
• T stands for the thoracic spine (thoracic = belonging to the rib cage).
• L stands for the lumbar spine (lumbar = belonging to the lower part of the back).
• S stands for the sacrum (bone at the bottom of the spine, above the tailbone).

So, for example, the spinal nerve that exits at the 5^th lumbar vertebra (spine bone) is called L5.

The corresponding dermatomes are given the same letters and numbers. The following illustration shows where the dermatomes are and which spinal nerve they are connected to:

For example, dermatome C6 includes the shoulder, the outside of the arm, and the thumb. The spinal nerve responsible for this dermatome exits the spine at the 6^th cervical vertebra.

Certain symptoms can often be linked to one or more nerves. A typical example is a slipped disc that causes pain, muscle weakness and numbness in one leg. That’s because the bulging disc presses on the root of one of the spinal nerves that supply the leg.

The most commonly affected nerves are the nerve roots at the 4^th or 5^th lumbar vertebra and the top nerve root of the sacrum. Depending on where the affected nerve is, it will typically cause certain symptoms:

• If the nerve root at the 4^th lumbar vertebra (L4) is affected, the pain typically spreads from the buttocks across the front of the thigh to the knee, and sometimes to the inner side of the lower leg or to the arch of the foot too. Unusual sensations usually occur around the knee.
• If the nerve root at the 5^th lumbar vertebra (L5) is affected, the pain tends to spread from the outer back of the thigh to the shin or the top of the foot. Unusual sensations mainly occur on the outer side of the lower leg or on the top of the foot.
• If the uppermost nerve root on the sacrum (S1) is affected, the pain usually radiates to the back of the leg. Here, unusual sensations are typically felt in the calf or on the outside of the foot. It might be more difficult to stand on your toes or lift your leg when standing.

So, the area of skin (dermatome) where the symptoms occur lets the doctor know which nerve root the slipped disc is likely pressing on. This information can be important when deciding whether to have surgery, for example. If a magnetic resonance imaging (MRI) scan shows that there's a slipped disc at the corresponding nerve, it’s more likely that surgery will help. But if the MRI image and the symptoms don’t match up, something else might be causing the symptoms.

Myotomes are groups of muscles that certain spinal nerves lead to. Like dermatomes, myotomes can also be used for diagnosis: If a certain group of muscles is weak or paralyzed, doctors can tell which spinal nerve might be damaged. For example, if someone isn’t able to lift their arm, there might be a problem with the C5 spinal nerve, which supplies the large, triangular shoulder muscle.