The term “dermatome” is a combination of two Ancient Greek words; “derma” meaning “skin”, and “tome”, a “cutting” or “thin segment”. It is an area of skin which is innervated by a single dorsal root of the spinal nerve. As dorsal roots are organised in segments, dermatomes are as well.
The dermatomes of neighbouring dorsal roots often overlap as the sensory fibers have many rostrocaudal branches in the spinal cord. This overlap means that if a single spinal nerve is affected, there is likely still innervation to that section of skin. For a dermatome to be completely numb, usually three neighbouring dorsal roots need to be affected.
|Development||Develop from somites|
Head, face and neck - trigeminal nerve (CN V) and spinal nerves C2-C4
Upper limbs - spinal nerves C5-T2
Thorax and abdomen - spinal nerves T3-T12
Lower limbs and genitalia - spinal nerves L1-S5
|Clinical rotations||Herpes Zoster (shingles), radicular pain|
This article will discuss the development, distributions, and function of dermatomes.
The rostrocaudal organisation of the spinal cord happens early in intrauterine development. Mesodermal tissue is divided into cubed-shaped, bilaterally paired segments called somites. There are 38-40 segments; 8 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 sacral segments, and 8-10 coccygeal segments. Each somite will differentiate into three regions:
- myotome – which forms the skeletal muscle of the neck, trunk and limbs
- sclerotome – which forms the ribs and vertebrae
- dermatome – which forms connective tissue and dermis of the skin
Each somite has a corresponding spinal nerve, and by extension, a corresponding dorsal root of a spinal nerve that innervates a dermatome. Sensory information from the skin on the face is relayed via the trigeminal nerve (CN V). C1 is the only spinal nerve that does not have an associated dermatome as in many individuals, there is no dorsal root for spinal nerve C1.
Dermatomes are evenly spaced horizontally along the thorax and abdomen, and then they run in a different longitudinal pattern on the upper and lower limbs as you can see on a dermatome map. This difference is because of the way that the limbs bud and rotate during early embryonic development. To understand this distribution, imagine someone standing up straight and then leaning over to touch the floor with their fingertips. In this position, the dermatomes are aligned in the way they were prior to the rotation of limbs.
From anterior to posterior, dermatomes tend to dip more inferiorly than staying horizontal. As with all anatomy, there is natural variation among individuals. Natural variation can include intrathecal intersegmental anastomoses of dorsal spinal rootlets; an occurrence in which the sensory neurons of a dorsal nerve enter the spinal cord at a different level.
Dermatome maps show the sensory distribution of each dermatome across the body. Clinicians can use test touch with a dermatome map as a way to localise lesions, damage, injury to specific spinal nerves, and to determine the extent of the injury, for example, if a patient is experiencing numbness in only one area. However, because of the overlapping segmentation of dermatomes, it is unlikely that numbness would occur if only one dorsal root is affected. Since the areas of skin are typically innervated by at least two spinal nerves, at least two neighbouring dorsal roots would need to be affected for numbness to occur.
Face, Head, and Neck
Dermatomes of the head, face, and neck include the trigeminal nerve (CN V), and spinal nerves C2-C4.
- Ophthalmic branch of the trigeminal nerve (CN V1) - forehead and nose
- Maxillary branch of the trigeminal nerve (CN V2) - skin over the maxilla (upper jaw)
- Mandibular branch of the trigeminal nerve (CN V3) - skin over the mandible (lower jaw), including the area of skin in front of the ear
- C2 - superior aspect of posterior head
- C3 - anterior neck and posterior aspect of upper neck and posterior head
- C4 - anterior skin superficial to supraclavicular fossa and posterior lower neck
*C1 - if this did exist, it would cover a small area of the posterior neck by the external occipital protuberance.
Dermatomes of the upper limbs are innervated by spinal nerves C5-T2. Here, the organisation of dermatomes is complex because of how the upper limbs bud in embryonic development.
- C5 - anterior skin below the clavicles spreading over the lateral aspect of the upper limb, posterior skin around the base of the neck
- C6 - shoulders and longitudinally down the middle posterior aspect of the upper limb, radial side of the hand, thumb
- C7 - hand, middle finger
- C8 - ulnar side of the hand, ring finger, and little finger
- T1 - level of the infraclavicular fossa, extending to the medial aspect of the forearm
- T2 - anterior and posteriorly extends at the level of the upper axilla and medial and upper aspect of the arm
Thorax and Abdomen
The dermatomes of the thorax and abdomen are T3-T12. Anteriorly, each is quite evenly spaced, with T1-T9 being near horizontal lines, and T10-T12 having the lower borders dip inferiorly. Posteriorly, each is evenly spaced and oriented as infero-lateral sloped lines from the spine.
- T3 - anterior and posteriorly extends at the level of the lower axilla
- T4 - anteriorly at the level of the nipple
- T5 - anteriorly at the level just inferior to the nipple
- T6 - anteriorly at the level of the xiphoid process
- T7-T9 - evenly distributed anteriorly between T6 and T10 dermatomes
- T10 - anteriorly at the level of the umbilicus
- T11 - evenly distributed anteriorly between T10 and T12 dermatomes
- T12 - anteriorly just superior to the pelvic girdle
Lower Limbs and Genitalia
The dermatomes of the lower limbs are distributed in spiral arrangements with segments L1-S5. This is because of how the limbs rotate to adapt an erect position during development. Of note, dermatomes S1, S4, and S5 are only on the posterior aspect.
- L1 - posteriorly includes the skin lateral to L1 vertebra and wraps anteriorly to the groin and pelvic girdle area superior to the inguinal canal
- L2 - anteriorly covers the thigh inferior to the inguinal canal
- L3 - evenly spaced between L2 and L4, extending down the medial aspect of the thigh and leg
- L4 - anteriorly curves from the lateral aspect of the thigh to the medial aspect of the leg and foot. Includes the knee, medial surface of big toe, and medial malleolus
- L5 - posterolateral aspect of the thigh wrapping anteriorly at the level of the knee to cover the anterolateral aspect of the leg. Includes the dorsal and plantar aspects of the foot, lateral surface of the big toe, and toes, 2, 3, and 4
- S1 - extends to the posterolateral aspect of thigh, popliteal region, and leg to the lateral malleolus, lateral margin of foot, heel, and little toe
- S2 - extends from the buttocks to the posteromedial aspect of the thigh, popliteal region and leg. Anteriorly includes the penis and scrotum
- S3 - posteriorly includes the medial aspect of buttocks, perineal area; anteriorly includes the penis and scrotum
- S4 - perineal area , and genitals
- S5 - perineal area, and skin of and adjacent to the anus
Herpes Zoster (shingles) is an infection of the peripheral nervous system caused by the reactivation of Varicella–Zoster virus. This same virus causes chickenpox, so only people who have had chickenpox can develop shingles. After someone recovers from the chickenpox, the virus moves to a single dorsal root ganglion and typically remains inactive. Since cell-mediated immunity decreases with age, reactivation of the virus can occur and generally affects individuals over the age of 50. The reactivated virus can leave the dorsal root ganglion and affect the sensory neurons of the skin. The dermatome of the associated infected dorsal root usually presents with discolouration, pain, rash, and a line of blisters. Thoracic and lumbar dermatomes are the most commonly affected.
Shingles is typically treated with antiviral drugs (acyclovir, famciclovir, or valacyclovir) as well as analgesics for pain management. It usually lasts for three to five weeks, however pain can last for years if postherpetic neuralgia (PHN) develops as a complication. Vaccination for chickenpox or shingles is the most effective method of prevention from contracting the disease.
Radicular pain emanates into the lower limb and is localized to the dermatome of the damaged dorsal root. The pain is described as shooting or burning and often presents with numbness and weakness in the same area. It is commonly caused by damage or irritation to a single dorsal root (e.g. spinal stenosis or a herniated disc). The most common type of radicular pain is radiculopathy (sciatica); pain which radiates down the back of the thigh following the course of the sciatic nerve.
Radicular pain is typically treated and managed with physical therapy, medications, and spinal procedures. If these methods are ineffective, surgeries such as a discectomy (removal of an intervertebral disc) or laminectomy (removal of the lamina of a vertebra) can lessen the pain.