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


Nerve Root	Dermatome (Sensation)	Myotome (Motor)	Reflex
C5	Lateral shoulder (regimental badge area)	Shoulder abduction (deltoid)	Biceps (with C6)
C6	Lateral forearm, thumb & index finger	Elbow flexion, wrist extension	Biceps / brachioradialis
C7	Middle finger	Elbow extension (triceps), wrist flexion	Triceps
C8	Little finger, medial forearm	Finger flexion & thumb extension	-
T1	Medial arm & elbow	Finger abduction/adduction (interossei)	-
L2	Anterior thigh	Hip flexion	-
L3	Medial thigh/knee	Knee extension (quadriceps)	-
L4	Medial leg & ankle	Ankle dorsiflexion	Patellar (knee jerk)
L5	Dorsum of foot, great toe	Great toe extension (EHL)	-
S1	Lateral foot & heel	Ankle plantarflexion	Ankle (Achilles)
S2	Posterior thigh	Knee flexion (hamstrings)	-
S3–S5	Perianal region ("saddle anaesthesia")	Anal sphincter	Anal wink / bulbocavernosus

⚕️ 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).

Landmark	Dermatome	Peripheral Nerve Supply
Thumb	C6	Median (palmar), Radial (dorsal)
Middle finger	C7	Median nerve
Little finger	C8	Ulnar nerve
Lateral forearm	C6	Musculocutaneous (lateral cutaneous nerve of forearm)
Medial forearm	T1	Medial cutaneous nerve of forearm
Dorsum of foot, big toe	L5	Deep peroneal nerve (webspace between 1st–2nd toe)
Lateral foot & little toe	S1	Sural nerve
Perianal region	S3–S5	Pudendal & perianal branches

💡 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

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Dermatomes

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