Upper trunk injury of the brachial plexus predominantly affects which region? (Alternate phrasing)

• A. Trunk muscles
• B. Intrinsic hand muscles
• C. Forearm extensor muscles
• D. Shoulder muscles

Answer: (D)

When the upper trunk of the brachial plexus is injured, the fibers from C5–C6 are affected, which mainly disrupts muscles that control the shoulder girdle. The nerves stemming from these roots supply the shoulder muscles, including the deltoid and the rotator cuff group (via the axillary and suprascapular nerves), so weakness of shoulder abduction and external rotation is prominent. Distal muscles of the forearm and hand rely more on lower trunk fibers (C8–T1), so they are less involved in this injury, making the shoulder region the primary area affected. Clinically this often presents with the characteristic inability to abduct the arm and with the arm held inwardly rotated, reflecting shoulder girdle weakness.