Sensory testing of dermatomes

Dermatomes - The Body's Wiring

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve root. Testing key dermatomes with light touch or pinprick helps localize neurologic lesions.

• C5: Lateral shoulder/arm (deltoid badge area)
• C6: Lateral forearm, thumb, and index finger
• C7: Middle finger
• C8: Medial forearm, ring and little finger
• T4: Nipple line (📌 T4 at the T-pore)
• T10: Umbilicus (📌 T10 at the belly but-ten)
• L4: Medial leg and medial malleolus
• L5: Dorsum of the foot and great toe
• S1: Lateral malleolus and little toe

⭐ The C1 nerve root has no sensory component, so there is no C1 dermatome.

Key Landmarks - A Dermatome Tour

• C2/C3: Posterior head and neck.
• C4: Shoulder area, clavicle.
• C6: Thumb (radial side of forearm).
• C7: Middle finger.
• C8: Little finger (ulnar side of forearm).
• T4: Nipple line.
  • 📌 Mnemonic: T4 at the teet-pore (nipple).
• T10: Umbilicus.
  • 📌 Mnemonic: T10 at the belly but-ten (belly button).
• L1: Inguinal ligament.
• L4: Medial malleolus (knee and medial leg).
• L5: Dorsum of the foot, big toe.
• S1: Lateral malleolus, little toe.
• S2-S4: Perianal region (saddle anesthesia).

⭐ Exam Favorite: The three middle fingers often have overlapping innervation. C6 classically supplies the thumb/index, C7 the middle finger, and C8 the ring/little finger, but variations are common. Remember the "6-shooter" gesture: thumb and index finger make a "gun" shape, innervated by C6.

Sensory Testing - The How-To Guide

• Patient: Eyes closed to prevent visual feedback. Explain the procedure clearly.
• Baseline: First, test on an area with normal sensation (e.g., the face) to establish a baseline for the patient.
• Procedure: Test key sensory points bilaterally, comparing sides. Move from impaired to normal areas.
• Stimuli: Use a cotton wisp (light touch) and a neurotip (pinprick). Apply stimulus for 1-2 seconds. Test in an unpredictable pattern to avoid patient anticipation.

⭐ For C6, the most reliable sensory testing point is the pulp of the thumb. This is a frequent exam question focus for differentiating cervical nerve root compression.

High‑Yield Points - ⚡ Biggest Takeaways

• C5 covers the lateral shoulder (deltoid badge area).
• C6 supplies the thumb; C7 the middle finger; C8 the little finger.
• T4 is at the nipple line; T10 is at the umbilicus.
• L4 covers the medial malleolus; L5 the dorsum of the foot.
• S1 is tested at the lateral malleolus and heel.
• Always test both spinothalamic (pinprick) and DCML (light touch) tracts.