A dermatome is an area of skin that gets its sensation from a specific spinal nerve root. These nerves send signals for things like pressure, pain, temperature, and texture from your skin to the spinal cord and then the brain.
Spinal nerves exist in 31 pairs, one on each side of the spine. A nerve root is a bundle of nerve fibers that branches off from the spinal cord and passes through an opening between two vertebrae. Vertebrae are small bones that make up your backbone.
From there, the nerves travel throughout your torso, arms, and legs. They make up the peripheral nervous system. These nerves provide sensory (sensation-based) information, motor function (movement), and other functions.
While the body has 31 spinal nerves, you only have 30 dermatomes. That’s because the spinal nerve highest in your neck, called cervical 1 (C1), doesn’t provide sensory function.
Dermatomes are divided into groups based on where in the spine they originate:
Cervical nerves: There are eight pairs from your neck, numbered C1 through C8. Since C1 has no sensory function, you have seven cervical dermatomes.
Thoracic nerves: There are 12 pairs from the torso, numbered T1 through T12. You have 12 thoracic dermatomes.
Lumbar nerves: There are five pairs from the lower back, numbered L1 through L5. You have five lumbar dermatomes.
Sacral nerves: There are five pairs from the sacrum (part of the pelvis), numbered S1 through S5. You have five sacral dermatomes.
Coccygeal nerve: There is one pair from the coccyx (tailbone) area, so you have a coccygeal dermatome.
The lumbar, sacral, and coccygeal dermatomes are often grouped together and called the “lower extremity dermatomes.”
Anatomy of the Spinal Nerves
The exact location of dermatomes is different from person to person because the nerves may overlap in some areas. However, dermatome maps are a good guide to where they lie in most people.
On the torso, dermatomes are horizontal stripes stacked on top of each other. However, they’re a combination of angular and straight stripes on your arms and legs.
Cervical Nerves and Dermatomes
The seven dermatomes associated with cervical nerves are:
C2: Behind the ear and the base of the skull
C3: Back of the head and behind the neck
C4: Across the lower neck and top of the shoulders
C5: Across the collarbones and upper shoulders
C6: Shoulders, down the outside of the arm, and the thumb
C7: The upper back, down the back of the arm, index and middle fingers
C8: Upper back, down the inside of the arm, ring and pinky fingers
Thoracic Nerves and Dermatomes
The thoracic dermatomes travel downward from where their nerves emerge from the spinal column and wrap around the torso.
The first nine form relatively straight lines down the chest, while the lower ones dip down in the center. The 12 dermatomes associated with the thoracic nerves are:
T1: Across the upper back and chest, through the armpit, and down the front of the arm
T2: Across the upper back and chest, just below the armpit
T3: Across the upper back and chest, just above the nipples
T4: Across the upper back and chest, including the nipples
T5, T6, T7, Narrow bands across the mid-back and chest, starting below the nipples
T8, T9: Narrow bands across the mid-back and upper abdomen, ending just above the navel
T10: Mid-back and abdomen, including the navel
T11: Mid-back and abdomen just below the navel, dipping toward the pelvis in front
T12: Mid-back and lower abdomen, dipping more sharply toward the pelvis in front
Lumbar Nerves and Dermatomes
The lumbar dermatomes also travel downward from where the nerves leave the spine, and they continue their downward course as they come around the hips.
The five dermatomes associated with lumbar nerve roots are:
L1: Across the lower back, around the hips, then dipping down to the groin
L2, L3: Across the lower back, angling down across the front and inside of the thigh
L4: Across the lower back, angling down the front of the thigh, knee, and calf, and wrapping around the inside of the ankle
L5: Across the lower back, angling down the front and outside of the calf, the foot (top and bottom), and the first four toes
Sacral Nerves and Dermatomes
The sacral dermatomes associated with the five sacral nerves are:
S1: Across the lowest part of the back, then arching steeply and traveling down the back of the thigh, back and inside of the calf, and to the little toe
S2: The genitals in front, across the buttocks in the back, then arching sharply to run down the back of the thigh and calf
S3: An egg-shaped area of the buttocks in back, the genitals in front
S4: A smaller egg-shaped area of the buttocks
S5: A tiny dot in the buttocks
Coccygeal Nerve and Dermatome
The final dermatome and single spinal nerve in the coccygeal region affects an area of the buttocks and the tailbone area.
Why Dermatomes Matter
In clinical practice, dermatomes are important for helping doctors identify where nerve problems originate and diagnose certain conditions.
Shingles, also called herpes zoster, results from a reactivation of the varicella-zoster virus, which causes chickenpox. Once you’ve had chickenpox, the virus stays in your body forever, lying dormant in a collection of nerve fibers between vertebrae called the dorsal root ganglion.1
When this virus becomes active later in life, it causes shingles. It is a painful, itchy, or tingly rash. It can also cause headaches, sensitivity to bright light, and malaise for several days before the rash appears.
The rash is most common in the thoracic dermatomes. If it’s in one or two neighboring dermatomes, it’s called localized zoster. When it spreads across three or more of these sensory regions, it’s called disseminated zoster.1
Doctors suspect herpes zoster if they see a linear rash that follows the pattern of the dermatomes, especially if the person is known to have had chickenpox. To confirm the diagnosis, they‘ll generally test fluid from a blister or scrapings of cells to see if the virus is present.2
What's the Connection Between Herpes Zoster and Immunosuppression?
Disseminated zoster occurs almost exclusively in people who have a suppressed or compromised immune system. It is hard to distinguish from chickenpox.
Dermatomes are especially useful in determining the location of damage in spinal cord injuries. Doctors test one dermatome at a time during an exam.3
When they identify a particular dermatome as affected, it tells them where on the spinal cord the damage is. For example, if someone has impaired sensory function down the back of the calf and in the little toe, that points to the S1 dermatome as a likely site of injury.
Injury to Spinal Nerve Roots
A pinched nerve, also called radiculopathy, results from narrowing of the space where the nerve roots pass between the vertebrae. Because the nerve root is involved, radiculopathy can cause symptoms in a dermatomal pattern, including:4
Loss of reflexes
Tingling, a pins-and-needles feeling, and other abnormal nerve sensations
Symptoms may vary depending on which nerve root is affected. The specific changes and location of symptoms can tell doctors where the radiculopathy has occurred.5
What Strange Reflexes Say About Your Health
Dermatomes are areas on your skin attached to a specific nerve bundle. These nerve bundles provide sensation to dermatomes. Dermatomes can help your doctor identify which body part certain conditions affect, such as in the case of shingles, injury, and pinched nerves.