Lower trunk injury of the brachial plexus primarily affects which group? (Alternate phrasing)

• A. Shoulder muscles
• B. Intrinsic hand muscles
• C. Forearm flexors
• D. Leg muscles

Answer: (B)

Lower trunk injuries involve the C8–T1 fibers, which feed the nerves that go to the hand. The intrinsic hand muscles are innervated mainly by the ulnar nerve and by branches of the median nerve that carry C8–T1 fibers. When the lower trunk is damaged, these distal hand muscles lose input, so weakness or paralysis appears most prominently in the intrinsic hand group (interossei, lumbricals to the ring and little fingers, and related intrinsic muscles). In contrast, shoulder and upper arm muscles are supplied mainly by fibers from the upper trunk (C5–C6) and their distal branches, so they’re less affected by a lower trunk lesion. The leg muscles aren’t part of the brachial plexus, so they’re unaffected. This pattern aligns with Klumpke-type involvement, where intrinsic hand function is primarily impaired.