A patient with a posterior cord brachial plexus injury has wrist drop and weakness of shoulder abduction. However, elbow flexion is intact. Which muscles are selectively weakened?

• A All muscles of the posterior compartment of arm and forearm, and deltoid
• B Biceps and coracobrachialis (musculocutaneous) and deltoid only
• C Deltoid, triceps, brachioradialis, and extensors of wrist/fingers (radial nerve distribution) + teres major ✓
• D Triceps and extensors only (no deltoid)

Explanation

The posterior cord (C5–C8) gives rise to the radial nerve, axillary nerve, and thoracodorsal nerve. A posterior cord lesion produces: axillary nerve palsy (deltoid weakness, loss of shoulder abduction, sensory loss over regimental badge area) + radial nerve palsy (wrist drop, triceps weakness, loss of brachioradialis and wrist/finger extensors). Elbow flexion (musculocutaneous/lateral cord) is intact. Teres major (lower subscapular nerve from posterior cord) may also be affected. This combined pattern distinguishes posterior cord from isolated radial nerve injury.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.