Makindo Medical Notes

Related Subjects: |Radial Nerve |Median Nerve |Ulnar Nerve |Musculocutaneous nerve |Axillary nerve |Brachial plexus |Dermatomes

Dermatomes of the Human Body: From Back of Head to Perianal

Definition: A dermatome is a region of skin innervated by sensory fibres from a single spinal nerve root. 💡 Key OSCE pearl: Each dermatome is a "segmental map" of spinal innervation, not the same as individual peripheral nerves.

📖 Cervical Dermatomes (C2–C8)

• C2: Back of head, occiput.
• C3: Neck.
• C4: Shoulder tip.
• C5: Lateral upper arm.
• C6: Lateral forearm & thumb 👍.
• C7: Middle finger ☝️.
• C8: Little finger & medial forearm.

🫀 Thoracic Dermatomes (T1–T12)

• T1: Medial forearm.
• T4: Nipple line landmark.
• T7–T9: Upper–mid abdomen.
• T10: Umbilicus 🎯.
• T12: Suprapubic region.

🦵 Lumbar Dermatomes (L1–L5)

• L1: Groin crease.
• L2: Upper anterior thigh.
• L3: Knee region.
• L4: Medial shin, medial malleolus.
• L5: Dorsum of foot, big toe.

🦶 Sacral Dermatomes (S1–S5)

• S1: Lateral foot, little toe.
• S2: Posterior thigh, calf.
• S3–S5: Perianal and perineal region ("saddle anaesthesia").

🧬 Embryological Origin

Dermatomes derive from somites, blocks of mesoderm alongside the neural tube. The dermatome component of each somite forms dermis → explaining the segmental skin innervation that persists into adult life.

🧠 Dermatomes, Myotomes & Reflexes by Nerve Root

⚕️ Clinical Relevance

• Spinal cord injury: Sensory level defines lesion level.
• Disc herniation: Radiculopathy follows dermatomal distribution (e.g. L5 → dorsum of foot pain).
• Shingles: Varicella zoster virus reactivation → painful vesicles in single dermatome.
• Saddle anaesthesia (S3–S5): Red flag for cauda equina syndrome 🚨.

🌟 Summary

Dermatomes are segmental skin maps of spinal nerve sensory innervation. They are crucial for localisation of neurological lesions, diagnosing radiculopathies, and recognising conditions such as shingles or spinal cord injury.

📋 Dermatomes vs Peripheral Nerve Territories

Dermatomes (spinal root level) and peripheral nerve territories (terminal branches of the brachial/lumbosacral plexus) do not always overlap. ⚠️ This distinction is essential in neurology and orthopaedics for localising lesions (radiculopathy vs peripheral neuropathy).

💡 Exam tip: - Radiculopathy (root lesion): Sensory loss follows a dermatome (e.g. L5 → dorsum of foot). - Peripheral neuropathy: Sensory loss follows a nerve territory (e.g. common peroneal nerve palsy → lateral shin, dorsum of foot). Recognising this distinction helps localise the lesion accurately.

📷 Images

Middle Finger = C7

Dermatome Maps