What Are Dermatomes, and Why Are They Important to Our Health?
A dermatome is the area of the skin of the human anatomy that is mainly supplied by branches of a single spinal sensory nerve root. These spinal sensory nerves enter the nerve root at the spinal cord, and their branches reach to the periphery of the body. The sensory nerves in the periphery of the body are a type of nerve that transmits signals from sensations (for example, pain symptoms, touch, temperature) to the spinal cord from specific areas of our anatomy.
To understand dermatomes, it is important to understand the anatomy of the spine. The spinal column is divided into 31 segments, each with a pair (right and left) of anterior and posterior nerve roots. The types of nerves in the anterior and posterior roots are different. Anterior nerve roots are responsible for motor signals to the body, and posterior nerve roots receive sensory signals like pain or other sensory symptoms. The anterior and posterior nerve roots combine on each side to form the spinal nerves as they exit the vertebral canal (the bones of the spine, or backbone).
The 31 spine segments on each side give rise to 31 spinal nerves, which are composed of 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal spinal nerves. There are specific dermatomes for each of these spinal nerves, except the first cervical spinal nerve. Dermatomes are used to represent the patterns of sensory nerves that cover various parts of the body, including the:
- Head and neck
- Upper extremities (arms, hands, torso etc.)
- Lower extremities (hip, leg, foot, buttocks, feet, etc.)
How Many Dermatomes Are There, and Where are They Located on the Body?
There are 30 dermatomes on the body. They are numbered according to the corresponding spinal vertebral level from which they emanate. For example, There are:
7 cervical dermatomes (C2-C8, as C1 nerve has no dermatome) - The "C" stands for cervical, which means having to do with any part of the neck, including the neck on which the head is perched, and the neck of the uterus.
- C2 - Head and neck
- C3 - Head and neck
- C4 - Neck
- C5 - Neck
- C6 - Thumb
- C7 - Middle finger
- C8 - Little finger
12 thoracic dermatomes (T1-T12) - "T" refers to the thoracic, or to the upper torso area of the body or chest (frontal and dorsal).
- T1- Inner forearm
- T2 - Upper inner arm
- T3 - Middle of the back (dorsal)
- T4 - Level of the nipples
- T5 - Halfway down from the level of the nipples to the xiphoid process
- T6 - Bottom of the foot
- T7 - Upper section between the xiphoid process and the belly button
- T8 - Halfway down from the level of the xiphoid process to the level of the belly button
- T9 - From the middle section of the xiphoid process to the belly button.
- T10 - Level of the belly button (umbilicus)
- T11 - Between the level of the belly button and the groin (inguinal ligament)
- T12 - The midpoint of the groin
5 lumbar dermatomes (L1-L5) that supply sensation from these spinal nerves in the lower limb (leg, foot, hip, etc.) - "L" refers to the five lumbar vertebrae, the disks below them, and the corresponding area of the lower back.
- L1 - The hip and groin area
- L2 - The inside of the thigh
- L3 - Knee
- L4 - The inside part of the ankle bone (medial malleolus)
- L5 - Bottom of the foot and toes 1-3
5 sacral dermatomes (S1-5) - "S" refers to the sacral or the sacrum, which are located below L5.
- S1 - Toes and 4 and 5, and the outside part of the ankle bone (lateral malleolus)
- S2 - The outer side (lateral portion) of the heel bone (calcaneous)
- S3 - The middle portion of the buttocks, perianal area, penis, and scrotum
- S4 - The skin over the perianal area (in addition to S5); perianal areas, and genitals
- S5 - The skin over the perianal region (along with S4) and the skin immediately at and next to the anus
1 coccygeal nerve that originates in the spinal cord and emerges at the level of the tail bone (coccyx).
The C1-C7 nerve roots emerge above their respective vertebrae; the C8 nerve root emerges between the C7 and T1 vertebrae, while the remaining nerve roots emerge below their respective vertebrae.
How Are Dermatomes Used by Medical Professionals?
Because the distribution pattern of the spinal nerve dermatomes is so defined, the dermatomes can be used to detect the area or location that causes the sensation of abnormalities along its location in the human body. For example:
- If a patient had sciatica (nerve pain from the irritation of the largest nerve in the body, the sciatica nerve) with signs and symptoms of low back pain and numbness and tingling on top of the right foot, the doctor could recognize this as a problem with the nerve that comes out below the right side of the fifth lumbar vertebra (L5) and disc. The most common cause of this particular medical situation would be a rightward herniation of the disc beneath the fifth lumbar vertebra (L5).
- If a patient is experiencing numbness and tingling down the right arm to the thumb (limb), index and middle finger (the C6,7,8 dermatome spinal nerves), the doctor must consider abnormalities that can affect the spinal nerves coming from the C6,7,8 nerve roots. This could be caused by abnormalities in the spinal cord or discs or vertebrae (C6-C8) or anywhere along the distribution of the nerve through the right upper limb (arm or hand).
The location of dermatomes on the skin can be used on a patient to precisely pinpoint the area that is the cause of many medical conditions and illness because their distribution is precisely located in the same areas in every human. For example:
- Shingles is an outbreak of a virus that inflamed the spinal nerve along a specific skin dermatome. It is relatively easily identifiable with signs and symptoms of a painful, red, blistering rash on the skin that tracks along a specific dermatome on one side of the body (right or left). Moreover, the intense itching and potentially chronic pain that a patient may experience with shingles is precisely located in the identical skin dermatome distribution.
Dermatomes also can be used by medical professionals to assess and diagnosis the exact level of spinal cord or nerve injury in in a patient with spinal cord trauma, such as from a motor vehicle accident or diving. They also are used by medical professionals in anesthesiology when epidural blocks are given to patients for pain. The doctor can determine when the block is effectively blocking the nerve by noted lack of sensation (for example, pain, numbness, and tingling) in the dermatome distribution of the particular level in the spine where the anesthetic was injected into the patient.