CorpusHuman Anatomy
NervousRegion

Dermatomes

Also known as: dermatome, dermatome map

A dermatome is the area of skin supplied by the sensory fibres of a single spinal nerve (dorsal root). The dermatome map is used to localise root and cord lesions.

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Overview

Each somite's dermatome carries its segmental nerve with it during development. On the trunk the result is orderly horizontal bands. In the limbs the bands are drawn out along the limb axis by limb-bud growth: preaxial (C5–C7, L2–L4) along the lateral arm and medial leg, and postaxial (C8–T1, S1–S2) along the medial arm and posterior leg. There is no C1 dermatome in most people, because C1 usually lacks a dorsal root. The face is supplied by the trigeminal nerve.

Adjacent dermatomes overlap considerably, especially for touch. Section of a single root therefore produces reduced rather than absent sensation, and complete anaesthesia needs at least three consecutive roots to be interrupted. Pain and temperature dermatomes overlap less than touch, so pinprick is more sensitive for defining a level. Dermatomes differ from peripheral nerve fields: a radial nerve lesion produces a patch that crosses several dermatomes. Distinguishing the two patterns is a core clinical skill.

Parts & subdivisions

  1. C2
    Occiput and posterior scalp, and the angle of the jaw (great auricular area).
  2. C3–C4
    Neck, and the "cape" over the shoulders and clavicles. C4 borders T2 on the upper chest.
  3. C5
    Lateral upper arm, including the regimental badge area over the deltoid.
  4. C6
    Lateral forearm and thumb (and index finger). "Six-shooter": thumb and index finger make a pistol.
  5. C7
    Middle finger.
  6. C8
    Ring and little fingers, and the medial hand.
  7. T1 / T2
    T1 covers the medial forearm. T2 covers the medial arm and axilla.
  8. T4
    Nipple line ("T4 at the teat pore").
  9. T6–T7
    Xiphoid process.
  10. T10
    Umbilicus ("belly butTEN"). Appendicitis pain first refers here.
  11. L1
    Inguinal ligament and groin ("L1, inguinal one").
  12. L2–L3
    Anterior and medial thigh; L3 reaches the medial knee.
  13. L4
    Knee and medial leg, down to the medial malleolus and medial edge of the foot ("L4, down to the floor" medially).
  14. L5
    Lateral leg, dorsum of the foot and the great toe / first web space.
  15. S1
    Lateral foot, little toe, heel and sole ("S1, stand on one").
  16. S2
    Posterior thigh and popliteal fossa.
  17. S3–S5
    Perineum and perianal skin, the saddle area. Lost in cauda equina and conus lesions.

Anatomy

Development

Each somite's dermomyotome gives rise to the dermis of a strip of skin. The segmental spinal nerve that grows into that somite keeps the territory, which is then stretched by limb-bud rotation.

Function

  • Provide a segmental map for localising spinal root and cord lesions (sensory level).
  • Explain the distribution of herpes zoster rashes and of referred visceral pain.

Clinical significance

Referred pain

Visceral afferents enter the cord at the same segments as somatic afferents, and the brain misattributes the source. Myocardial ischaemia (T1–T4) refers to the chest and medial arm. Early appendicitis (T10) refers to the umbilicus. Diaphragmatic irritation (C3–C5, phrenic) refers to the shoulder tip.

Sensory level

In cord compression, the highest dermatome at which pinprick sensation changes marks the lesion. Remember that segmental levels are higher than vertebral levels, so a T10 sensory level points to a lesion at about the T7–T8 vertebrae.

Mnemonics

Trunk landmarks

"T4 at the teat pore, T10 at the belly butTEN, L1 at the IL (inguinal ligament)."

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.