Lower trunk injury of the brachial plexus primarily affects which muscle group?

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

Answer: (A)

Lower trunk injuries affect fibers from C8–T1, which feed the ulnar nerve and the medial portion of the median nerve. Those nerves innervate the intrinsic muscles of the hand—the interossei and the deeper hand muscles responsible for finger abduction, adduction, and fine motor control. When these muscles are lost, the hand shows loss of intrinsic muscle function and often a clawing posture of the digits. Shoulder muscles, including the deltoid, are mainly supplied by nerves arising from the upper trunk, so they are less affected in a lower trunk lesion. Forearm flexors derive substantial innervation from nerves with roots higher up, so a lower trunk injury tends not to abolish forearm flexion as its primary deficit. Horner syndrome can accompany if the T1 fibers carrying sympathetic innervation are involved, but the defining motor deficit centers on the intrinsic hand muscles.