Dermatomes in Neurology: C2–S5 Segmental Map and Myotomes

1. Definitions and principles

• A dermatome is an area of skin whose sensation is supplied by a single spinal nerve (or spinal segment).
• A myotome is a group of muscles innervated by a single spinal segment.
• A sclerotome is an area of bone/ligaments innervated by a single segment.
• Adjacent dermatomes overlap — the loss of a single nerve root often results in hypesthesia rather than complete anesthesia.
• Dermatome maps (Keegan, Foerster) differ slightly — a combined approach (ASIA 2019) is used in clinical practice.

2. Key dermatomes

3. Key myotomes

4. ASIA Scale

• International Standards for Neurological Classification of Spinal Cord Injury (American Spinal Injury Association).
• Sensory level — the most caudal dermatome with normal pain and light touch sensation.
• Motor level — the most caudal segment with a muscle strength of ≥ 3 (provided that segments above are normal).
• AIS classification (A–E): A — complete; B — sensory preservation in S4-S5; C, D — motor preservation; E — normal.
• Mandatory component — evaluation of the anal sphincter (voluntary contraction + perianal sensation).

5. Cervical Radiculopathies

• C5 — pain in the shoulder girdle and lateral arm, abduction weakness; ↓ biceps reflex.
• C6 — pain in the thumb, paresthesias, weakness in elbow flexion and wrist extension.
• C7 — pain in the middle finger, triceps weakness, ↓ triceps reflex.
• C8 — pain in the little finger, finger flexor weakness.
• Spurling's test — provocation of pain by lateral flexion and rotation of the head toward the affected side.

6. Lumbar Radiculopathies

• L4 — pain along the anteromedial lower leg, foot dorsiflexion weakness, ↓ knee reflex.
• L5 — pain along the lateral lower leg and dorsum of the foot; great toe extension weakness (positive dorsiflexion test).
• S1 — pain along the posterior thigh, calf, and heel; plantar flexion weakness, ↓ Achilles reflex.
• Lasegue's sign — provocation of radicular pain during straight leg raise at 30–60°.
• Wasserman's sign — femoral nerve tension sign (L3-L4), straight leg raise in the prone position.

7. Shingles (Herpes Zoster)

• Reactivation of varicella-zoster virus (VZV) in spinal/cranial ganglia.
• Classic rash — vesicles on an erythematous base within a single dermatome, without crossing the midline.
• Most frequently — Th3–Th12 and trigeminal nerve (V1 ophthalmic branch).
• Prodrome: itching, burning, hypesthesia 1–3 days prior to the rash.
• Treatment: acyclovir 800 mg × 5/day or valacyclovir 1000 mg × 3/day for 7 days; start < 72 h.
• Postherpetic neuralgia — a frequent complication (gabapentin, pregabalin, amitriptyline).

8. Spinal Cord Syndromes

• Complete transverse cord lesion — complete loss below the level.
• Brown-Séquard syndrome (hemisection) — ipsilateral paralysis and loss of proprioception + contralateral loss of pain and temperature sensation.
• Anterior cord syndrome — bilateral loss of pain/temperature + motor function, preserved proprioception (posterior columns).
• Central cord syndrome (cervical cord) — arms weaker than legs (Schneider).
• Posterior cord syndrome — loss of proprioception and two-point discrimination.
• Cauda equina syndrome — asymmetric leg weakness, loss of bowel/bladder function, anogenital ("saddle") anesthesia.

9. Topical Level Diagnosis

• C1–C4 lesion — tetraplegia, respiratory failure (above C3 — lifelong mechanical ventilation).
• C5–T1 — tetraplegia with partially preserved diaphragm; useful hand functions are partly preserved depending on the level.
• T2–T12 — paraplegia, level corresponding to the segment.
• L1–S2 — lower flaccid paraplegia in conus medullaris or cauda equina lesions.
• S3–S5 — pelvic dysfunction only.

10. Differential Diagnosis of Radiculopathies

• Intervertebral disc herniation — primary cause; MRI.
• Spondylosis, spinal stenosis.
• Oncology: metastases, multiple myeloma, lymphoma.
• Infections: epidural abscess, tuberculosis (Pott's disease), spondylitis.
• Inflammatory: spondylodiscitis, ankylosing spondylitis.
• Red flags: night pain, fever, weight loss, history of cancer, bowel/bladder dysfunction, saddle anesthesia, progressive neurological deficit.

11. Instrumental Diagnostics

• Contrast-enhanced MRI — gold standard (herniations, tumors, myelitis, epidural processes).
• CT — when MRI is contraindicated, or for trauma (bone structures).
• NCS + needle EMG — confirmation of radicular level, ruling out plexopathy and neuropathy.
• X-ray — to assess scoliosis, spondylolisthesis; becoming obsolete in favor of MRI.
• Lumbar puncture — if myelitis, neoplastic meningitis, or infection is suspected.

12. FAQ

Why do dermatomes overlap?
Because adjacent spinal nerves innervate neighboring skin areas with a 25–50% overlap — an evolutionary defense mechanism against complete anesthesia upon partial injury to a single root.

How to quickly memorize key levels?
Mnemonics: T4 — nipples, T10 — umbilicus ("4 × 10 = 40, which is the nipples, 10 is the umbilicus"); "C4 — shoulder, C6 — thumb, C7 — middle finger, C8 — little finger"; "L4 — medial malleolus, L5 — great toe, S1 — little toe".

What is the difference between radicular syndrome and polyneuropathy?
Radicular syndrome strictly follows a dermatome, is asymmetric, and often presents with pain and a single diminished reflex. Polyneuropathy is symmetric, "stocking-glove" distribution, with generalized reflex hyporeflexia.

Why does shingles not cross the midline?
Because each spinal ganglion innervates only its respective half of the body — the virus, upon reactivation in a single ganglion, produces a rash strictly on its own side.

What is conus medullaris syndrome and how does it differ from cauda equina syndrome?
Conus syndrome (S3–S5) — symmetric pelvic dysfunction, minimal motor deficits. Cauda equina syndrome — asymmetric flaccid paraplegia, saddle anesthesia, radicular pain, more pronounced motor deficits. Cauda equina is a surgical emergency requiring decompression.

See also: Spinal cord injury · Disc herniation · Shingles · Cauda equina syndrome · Neurology Catalog