Myotome vs. Dermatome: Spinal Nerve Innervation Maps for Clinical Diagnosis

Myotome and dermatome are segmental innervation maps crucial for neurologic localization.

Definitions

• Myotome: Group of muscles innervated by a single spinal nerve root (motor).
• Dermatome: Area of skin innervated by a single spinal nerve root (sensory).

Clinical Utility

Testing myotomes (muscle strength) and dermatomes (sensation) identifies the affected spinal level in radiculopathy, spinal cord injury, plexopathy.

Key Differences

Feature	Myotome	Dermatome
Function	Motor (muscle activation)	Sensory (skin sensation)
Testing	Manual muscle testing (MMT)	Pinprick, light touch, vibration
Overlap	Less overlap (more discrete)	Significant overlap between adjacent roots
Naming	By spinal nerve (C5 myotome = deltoid)	By spinal nerve or region (C6 dermatome = thumb)

Important Nuances

• Myotomes/dermatomes vary between individuals (anatomic variation).
• Some nerves supply both (e.g., C5 → deltoid + lateral arm sensation).
• Peripheral nerve lesions affect multiple roots (e.g., median nerve = C6, C7, C8).
• Use standardized charts (e.g., Keegan & Garrett, Foerster) with clinical correlation.

Combined myotomal/dermatomal assessment localizes lesions more accurately than either alone.