Cervical dermatomes and myotomes

Cervical Dermatomes - The Skin Map

• C2/C3: Posterior head and neck.
• C4: Shoulder cape and collar area.
• C5: Lateral arm (deltoid patch).
• C6: Lateral forearm, thumb, and index finger.
  • 📌 Mnemonic: Hold up 6 fingers like a "six-shooter" gun; your thumb and index finger form the gun.
• C7: Middle finger.
• C8: Medial forearm, ring and little fingers.
• T1: Medial side of the forearm and arm (axilla).

⭐ High-Yield Fact: Dermatomes have significant overlap. Anesthesia in one dermatome typically requires blocking at least two adjacent spinal nerves.

Cervical Myotomes - Muscle Command Center

• Myotome: A group of muscles innervated by a single spinal nerve root.
• C5: Shoulder abduction (Deltoid) & Elbow flexion (Biceps).
• C6: Wrist extension (Extensor carpi radialis longus & brevis). Think "picking up sticks."
• C7: Elbow extension (Triceps) & Wrist flexion. Think "pushing away."
• C8: Finger flexion (Flexor digitorum profundus). Making a fist.
• T1: Finger abduction & adduction (Dorsal & Palmar Interossei). Spreading fingers.

📌 Mnemonic: Use your fingers to count from 5 to 8!

• 5: Raise arm (abduction)
• 6: "Six-shooter" hand (wrist extension)
• 7: Prayer hands pushing down (wrist flexion/elbow extension)
• 8: Grip an "8" ball (finger flexion)

⭐ Clinical Pearl: The biceps reflex primarily tests the integrity of the C5 nerve root. A weak or absent reflex can be a key sign of C5 radiculopathy.

Clinical Tie-Ins - Reflexes & Wrecks

• Reflex Arc Integrity:

  • Biceps Reflex: Tests C5 (some C6). Diminished in C5 root lesions.
  • Brachioradialis Reflex: Tests C6 (some C5). Tap styloid process of radius.
  • Triceps Reflex: Tests C7 (some C8). Absent in C7 root compression.

• Brachial Plexus Wrecks:

  • Erb-Duchenne Palsy ("Waiter's Tip"):
    • Injury to Superior Trunk (C5-C6).
    • Cause: Lateral neck traction (birth, falls).
    • Arm adducted, internally rotated; elbow extended.
  • Klumpke's Palsy ("Claw Hand"):
    • Injury to Inferior Trunk (C8-T1).
    • Cause: Hyperabduction (grabbing a branch while falling).
    • Affects intrinsic hand muscles.

⭐ High-Yield: Horner's syndrome (ptosis, miosis, anhidrosis) can accompany Klumpke's palsy due to involvement of the T1 sympathetic ganglion.

High‑Yield Points - ⚡ Biggest Takeaways

• C4 dermatome covers the shoulder cape; its myotome controls scapular elevation.
• C5 is crucial for shoulder abduction (deltoid) and sensation over the lateral arm (deltoid patch).
• C6 governs elbow flexion and wrist extension, with sensation to the thumb and index finger.
• C7 controls elbow extension and wrist flexion; it provides sensation to the middle finger.
• C8 is responsible for finger flexion and sensation of the ring and little fingers.
• T1 manages finger abduction and adduction (interossei muscles).