Brachial plexus

The lower (inferior) trunk of the brachial plexus originates from the C8 and T1 cervical roots. Injury to this trunk, which typically results from sudden upward pulling on the arm, produces Klumpke's palsy.

primarily affects ulnar nerve which supplies most of the intrinsic muscles of the hand

Weakness and atrophy of the hypothenar and interosseous muscles characterize this palsy, and a "claw hand" deformity may also result.