Dermatomes, Myotomes & Spinal Nerve Root Anatomy

Dermatome vs Myotome

• Dermatome: the area of skin supplied by a single spinal (dorsal root) nerve.
• Myotome: the group of muscles supplied by a single spinal nerve root (most muscles are supplied by more than one root).
• Sclerotome / viscerotome: bone and viscera supplied by a segment (less examined).

Overlap and Its Consequences

• Adjacent dermatomes overlap considerably, so a single nerve root lesion rarely abolishes sensation in its dermatome - it produces a band of reduced sensation, not anaesthesia.
• Myotomes are multi-segmental, so a single-root lesion weakens but rarely paralyses a muscle.
• This is why patterns (a key muscle plus a reflex plus a sensory point) localise better than any single finding.

ISNCSCI Standardisation

• The International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) define a key sensory point for each dermatome (C2-S4/5) and a key muscle for each of ten paired myotomes (C5-T1 and L2-S1), graded with the ASIA Impairment Scale (AIS).
• This gives a reproducible sensory level, motor level, neurological level of injury and completeness (based on sacral sparing).

Landmark Dermatomes (ISNCSCI key points)

• C2 occiput - C3 posterior neck/supraclavicular fossa - C4 acromioclavicular joint
• C5 lateral antecubital fossa (lateral arm) - C6 thumb - C7 middle finger - C8 little finger - T1 medial antecubital fossa (medial forearm)
• T4 nipple - T6 xiphisternum - T10 umbilicus - L1 inguinal region/groin
• L2 anterior mid-thigh - L3 medial femoral condyle (medial knee) - L4 medial malleolus - L5 dorsum of the foot at the 3rd MTP joint / great toe - S1 lateral heel
• S2 popliteal fossa - S3 ischial tuberosity - S4-5 perianal

Key Myotomes (ISNCSCI key muscles)

Upper limb:

• C5 elbow flexors (biceps) - C6 wrist extensors - C7 elbow extensors (triceps) - C8 finger flexors (FDP to middle finger) - T1 finger abductors (abductor digiti minimi)

Lower limb:

• L2 hip flexors (iliopsoas) - L3 knee extensors (quadriceps) - L4 ankle dorsiflexors (tibialis anterior) - L5 long toe extensors (EHL) - S1 ankle plantarflexors (gastrocnemius-soleus)

Deep Tendon Reflex Levels

• Biceps: C5-C6
• Brachioradialis (supinator): C6
• Triceps: C7
• Patellar (knee): L3-L4
• Achilles (ankle): S1

Localising a Radiculopathy

• Combine the key muscle (myotome) + dermatome + reflex to localise the level.
• C6: weak wrist extension/elbow flexion, thumb sensation, reduced brachioradialis reflex.
• C7: weak elbow extension (triceps), middle-finger sensation, reduced triceps reflex.
• L4: weak dorsiflexion, medial-malleolus sensation, reduced knee reflex.
• L5: weak great-toe extension, dorsum-of-foot sensation, reflexes preserved.
• S1: weak plantarflexion, lateral-foot sensation, reduced Achilles reflex.

The ASIA / ISNCSCI Examination

• Test the key sensory points (light touch and pin-prick) and key muscles bilaterally to derive the sensory and motor levels and the neurological level of injury.
• Assess sacral sparing (S4-5 light touch/pin-prick, deep anal pressure, voluntary anal contraction) - its presence defines an incomplete injury.
• Grade with the ASIA Impairment Scale (AIS A-E); this drives prognosis and communication.