How the brachial plexus is tested on Step 1

Know the order of the segments and where injuries occur.

RootsC5–T1; dorsal scapular and long thoracic nervesarise hereTrunksUpper (C5–C6), middle (C7), lower (C8–T1)DivisionsAnterior (flexors) and posterior (extensors)CordsLateral, posterior, medial — named by relation toaxillary arteryBranchesMusculocutaneous, axillary, radial, median, ulnar
Brachial plexus. The posterior cord gives the axillary and radial nerves (extensors); the median nerve gets fibers from both lateral and medial cords.

Trunk injuries

The mechanism predicts which trunk stretches.

Erb (upper trunk)C5–C6Neck pulled away from shoulderArm adducted, internally rotatedElbow extended, forearm pronatedDeltoid, rotator cuff, biceps weakKlumpke (lower trunk)C8–T1Arm pulled up overheadClaw handIntrinsic hand muscles weakMay include Horner syndrome (T1)vs
Erb vs Klumpke. A Pancoast tumor at the lung apex can compress the lower trunk and sympathetic chain.

Terminal nerve injuries

Match the injury site to the nerve and the deficit.

NerveTypical injuryMotor lossSensory loss
Axillary (C5–C6)Surgical neck fracture, anterior shoulder dislocationDeltoid (abduction beyond 15°), teres minorLateral shoulder ("regimental badge")
Musculocutaneous (C5–C7)Upper trunk compressionBiceps, brachialis (elbow flexion)Lateral forearm
Radial (C5–T1)Midshaft humerus fracture, crutches, "Saturday night palsy"Wrist and finger extensors (wrist drop)Back of hand, thumb side
Median (C5–T1)Supracondylar fracture (proximal); carpal tunnel (distal)Thumb opposition, lateral finger flexion; "hand of benediction" when proximalPalmar thumb, index, middle, half of ring finger
Ulnar (C8–T1)Medial epicondyle fracture; hook of hamate fractureInterossei, medial lumbricals; claw of 4th and 5th digitsMedial hand, little finger
Long thoracic (C5–C7)Mastectomy, axillary surgerySerratus anterior → winged scapulaNone

Distal ulnar lesions cause a more obvious claw ("ulnar paradox") because the deep finger flexors still work.

Carpal tunnel syndrome

The most common entrapment neuropathy.

  • Mechanism: median nerve compression under the flexor retinaculum.
  • Associations: pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, amyloidosis (dialysis, β2-microglobulin), repetitive wrist use.
  • Signs: night-time tingling in the thumb, index, and middle fingers; Tinel and Phalen signs; thenar wasting late. Palm sensation over the thenar eminence is spared because the palmar cutaneous branch runs above the tunnel.

Worked example

Link the fracture to the nerve.

functions Worked example — a humerus fracture

After a fall, an X-ray shows a midshaft humerus fracture. The patient cannot extend the wrist or fingers and has numbness on the back of the hand near the thumb. Which nerve and artery are at risk?

  1. Wrist drop means the wrist and finger extensors are weak.
  2. These are supplied by the radial nerve.
  3. The radial nerve runs in the radial (spiral) groove at the midshaft of the humerus.
  4. The deep brachial (profunda brachii) artery travels with it.

Answer. Radial nerve (with the deep brachial artery).