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Dermatomes

The term “dermatome” is a combination of two Ancient Greek words; “derma” meaning “skin”, and “tome”, meaning “cutting” or “thin segment”. It is an area of skin which is innervated by the posterior (dorsal) root of a single spinal nerve. As posterior roots are organized in segments, dermatomes are as well. This is why the term “dermatome” refers to the segmental innervation of the skin.

Neighboring dermatomes often, if not always overlap to some degree with each other, as the sensory peripheral branches corresponding to one posterior root typically go beyond the limit of their dermatome. As such, the thin lines seen in the dermatome maps are more of a clinical guide than a real boundary. This means that if a single spinal nerve is affected, there is likely still some degree of innervation to that segment of skin coming from above and below. For a dermatome to be completely numb, usually two or three neighboring posterior roots need to be affected. In addition, it’s important to note that dermatomes are subject to a large degree of interindividual variation. A graphical representation of all the dermatomes on a body surface chart is referred to as a dermatome map.

This article will discuss the development, distributions, and function of dermatomes.

Key facts about dermatomes
Development Develop from somites
Dermatome map 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 and radiculopathy
Contents
  1. Development
  2. Anatomy
  3. Dermatome maps
    1. Face, head and neck
    2. Upper limb
    3. Thorax and abdomen
    4. Lower limbs and genitalia
  4. Dermatome landmarks
  5. Clinical relations
    1. Herpes Zoster
    2. Radicular pain and radiculopathy
  6. Sources
+ Show all

Development

The rostrocaudal organization of the spinal cord happens early in intrauterine development. Mesodermal tissue is divided into cubed-shaped, bilaterally paired segments called somites. There are about 42-44 pairs of somites: 4 occipital segments, 8 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 sacral segments, and 8-10 coccygeal segments. Several of the coccygeal segments on the embryonic tail degenerate in the course of development, leaving about three that ultimately form the coccyx. Each somite will differentiate into three regions:

Anatomy

Developmentally, each somite has a corresponding spinal nerve, and by extension, a corresponding posterior root of a spinal nerve that innervates a dermatome. Spinal nerve C1 is the exception in that it does not have an associated dermatome due to the reason that the posterior root for spinal nerve C1 is absent in many individuals. Spinal nerves are responsible for providing sensory innervation to the skin of the entire body with the exception of the face. Sensory information from the skin on the face is relayed via the trigeminal nerve (CN V).

Dermatomes are relatively evenly distributed in a roughly ‘striped’ arrangement along the thorax and abdomen, with their course tending to ‘dip’ inferiorly as they course posterior to anterior. The dermatomes of the upper and lower limbs, however, follow a different pattern. This difference is present due to the manner in which the limbs bud and rotate during early embryonic development. To understand this distribution in the upper limb, imagine someone standing upright with their limbs abducted and thumbs pointing superiorly. In this position, the dermatomes are aligned in the way they were prior to the rotation of the limbs. Similarly, lengthening and rotation of the lower limb during development accounts for its dermatomal pattern. Additionally, it is important to note that the region of innervation of peripheral nerves are not equivalent to dermatomes due to the fact the peripheral nerves are derived from various plexuses (brachial, lumbar and sacral), which contain fibers from multiple spinal nerves.

As with all anatomy, dermatomes are subject to interindividual variation as mentioned earlier. This variation can be a result of intrathecal intersegmental anastomoses of posterior spinal rootlets; an occurrence in which the sensory neurons of a dorsal root ganglion (spinal ganglion) enter the spinal cord at a different level.

Dermatome maps

Dermatome maps depict the sensory distribution of each dermatome across the body. Clinicians can assess cutaneous sensation with a dermatome map as a way to localise lesions within central nervous tissue, injury to specific spinal nerves, and to determine the extent of the injury. Several dermatome maps have been developed over the years but are often conflicting. The most commonly used dermatome maps in major textbooks are the Keegan and Garrett map (1948) which leans towards a developmental interpretation of this concept, and the Foerster map (1933) which correlates better with clinical practice. This article will review the dermatomes using both maps, identifying and comparing the major differences between them.

It’s important to stress that the existing dermatome maps are at best an estimation of the segmental innervation of the skin since the many areas of skin are typically innervated by at least two spinal nerves. For example, if a patient is experiencing numbness in only one area, it is unlikely that numbness would occur if only one posterior root is affected because of the overlapping segmentation of dermatomes. At least two neighboring posterior roots would need to be affected for numbness to occur.

For more details about the dermatomes, including how to examine them, check out this study unit.

Face, head and neck

There are three dermatomes of the face, each of which are innervated by one of the three branches of the trigeminal nerve (CN V) and not spinal nerves.

The other dermatomes of the head and neck are innervated by spinal nerves C2-C4.

  • C2 - posterosuperior aspect of head
  • C3 - anterior neck, posterior aspect of upper neck and head, and supraclavicular fossa
  • C4 - shoulder and skin of infraclavicular fossa and posterior lower neck

C1 - if present, it would cover a small area of the posterior neck by the external occipital protuberance.

There are a number of variations between the Keegan and Garrett and the Foerster maps in the head and neck region:

Head and neck region
Ear, lateral cheek and skin below jaw line and chin K&G: C2/C3
Foers.: C3
Supraclavicular region K&G: C4
Foers.: C3
Shoulder K&G: C5
Foers.: C4

The shoulder region is a key landmark for testing the C4 dermatome in neurological examinations. It is therefore important to note that this dermatome exhibits significant variation between the two maps. On the Keegan and Garrett map, the C4 dermatome consists of a thin band limited mainly to the base of the neck posteriorly and regions above the clavicles on the anterior perspective. In particular, it does not extend laterally towards the shoulders. It is also worth noting that the dermatome just below C4 on this map is the C5 dermatome.

The Foerster map, on the other hand, depicts the C4 dermatome extending laterally from the base of the neck in the posterior view, expanding over the posterior aspect of the shoulder and continuing on to the anterior aspect of the shoulder and medially over the pectoral regions. It is important to note here that unlike the Keegan and Garrett map, the dermatome immediately below the C4 is the T2 dermatome.

Interestingly, both variations are supported in the literature, with the differences likely the outcome of individual variations in the supraclavicular nerves, which are variable in about fifty percent of the population. This group of nerves typically consist of nerve fibers from only the C3 and C4 spinal nerves, which support the Foerster map. However, they may also variably contain fibers from the C5 spinal nerve, in which case would support the Keegan and Garrett map

Upper limb

Dermatomes of the upper limbs are innervated by spinal nerves C5-T2. Here, the organization of dermatomes is a little complex because of how the upper limbs bud during embryonic development. In the Keegan and Garrett map, the C5-T2 dermatomes run in an uninterrupted/continuous manner from the shoulder along the upper limb to the superior region of the axilla. In contrast, on the Foerster map, C5-T2 dermatomes have a discontinuous appearance from the shoulder region. However, both maps do progress in a sequential manner along the limb. Overall, the dermatomal distribution is quite similar in both maps:

  • C5 - spreads over the lateral aspect of the arm
  • C6 - radial side of the forearm and thumb
  • C7 - central aspect of posterior forearm and middle finger
  • C8 - ulnar side of the forearm and hand, and little finger
  • T1 - extends to the medial aspect of the forearm and distal arm
  • T2 - medial and proximal aspect of the arm continuing into the axilla

The main differences in this region are that on the Keegan and Garrett map the C5 dermatome covers the skin below the clavicles and C6 runs longitudinally from the shoulders, down the lateral aspect of the entire upper limb to the hand and thumb. Additionally, the T1 dermatome extends to the level of the upper pectoral region whereas in the Foerster map this area is represented by the T2 dermatome. It is important to note that in the pectoral region the C5 dermatome lies directly adjacent to T1 on the Keegan and Garrett map and is the only region of the body where the dermatomal bands do not overlap and are clearly defined. On the Foerster map, C4 lies adjacent to T2.

Major differences between the Keegan and Garrett and the Foerster maps in the upper limb:

Upper limb
Infraclavicular region K&G: C5
Foers.: C4
Lateral-most aspect of shoulder, arm, forearm and thumb K&G: C6
Foers.: C4,C5,C6 (sequentially)
Upper pectoral region K&G: T1
Foers.: T2

Thorax and abdomen

The dermatomes of the thorax and abdomen are from T2-T12. The dermatomal distribution in this region is fairly straightforward. Anteriorly, each is quite evenly spaced, with T2-T9 being nearly horizontal lines, and T10-T12 having the lower borders dip inferiorly. Posteriorly, each dermatome is evenly spaced and oriented as infero-lateral sloped lines from the spine. Both the Keegan and Garrett and the Foerster maps are largely consistent in this region.

  • T3 - anteriorly and posteriorly 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

Want to learn the dermatomes as efficiently as possible? Make sure you're not making any of these common anatomy learning mistakes.

Lower limbs and genitalia

The dermatomes of the lower limbs are innervated by the spinal nerves L1-S5. Here, there is some significant deviation between the Keegan and Garrett map, which shows a spiral arrangement of the dermatomes, and the Foerster map which has a more segmental distribution. Like the upper limb, this is attributed to how the limbs bud, elongate and rotate during development to adapt an erect position. Of note, dermatomes S4, and S5 are only on the posterior aspect.

In the Keegan and Garrett map:

  • L1 - posteriorly includes the skin lateral to L1 vertebra and wraps anteriorly to the groin and pelvic girdle area overlying 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

The location of L2 and L3 dermatomes in the Foerster map is mostly in the anterior thigh and largely similar to that of Keegan and Garrett. The main differences are observed in the L4-S1 dermatomes which are mostly distal to the knee in the Forester map.

Main variations between the Keegan and Garrett and the Foerster maps in the lower limb:

Lower limb
Medial aspect of leg K&G: L3, L4
Foers.: L4
Lateral aspect of leg K&G: L5, S1
Foers.: L5
Posterior aspect of leg K&G: S1, S2
Foers.: S2
Medial aspect of ankle and foot K&G: L4, S2
Foers.: L4, S1
Plantar aspect of foot K&G: L4, L5, S1, S2
Foers.: L5, S1
Posterior thigh K&G: S1,S2
Foers.: S2
Gluteal region K&G: L5-S5
Foers.: L2, S2-S5

Dermatome landmarks

Clinically, a good understanding of dermatomes is important when performing a neurological examination. Unfortunately, due to differences in existing dermatome maps and the extensive overlap of dermatomes, this is sometimes challenging. These factors coupled with considerable individual variation can affect dermatome identification and recording of findings. Dermatome landmarks are dermatomal areas of the body that are largely uniform despite the various variations and provide a more practical approach to the assessment of dermatomes in a consistent manner.

The major dermatome landmarks include:

Head and neck regions
C2 Occipital protuberance
C3 Supraclavicular fossa
C4 Shoulders
Upper limb
C5 Lateral aspect of arm
C6 Thumb
C7 Middle finger
C8 Little finger
T1 Medial aspect of the arm
T2 Axilla
Thorax and abdomen
T3 Between the level of the nipple and axilla
T4 Nipple
T5 - T9 Distributed evenly between the nipple and the umbilicus
T10 Umbilicus
T11 - T12 Distributed evenly between the umbilicus and the inguinal region (groin)
Lower limbs and genitalia
L1 Inguinal region (groin)
L2 Hip
L3 Knee
L4 Ankle
L5 Big toe
S1 Little toe
S2 Popliteal fossa, external genitalia anteriorly
S3 Buttocks, external genitalia anteriorly
S4-S5 Anal region

Solidify your knowledge on the dermatomal distribution of the body with our quiz:

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