Fig. 2

Level one entrapment neuropathy in a 29-year-old woman with progressive wasting of the interossei. a, b, c Consecutive short-axis 22–8-MHz US images from proximal to distal demonstrate a hypoechoic and swollen ulnar nerve (black arrow) at the proximal part of the Guyon canal, on the radial side of the pisiform (Pis). Distal to this level, the deep branch (white arrow) appears running in an anomalous position inside the carpal tunnel underneath the flexor retinaculum (thin arrows), on the radial side of a hypertrophied pisohamate ligament (asterisk) and, more distal, of the hamate hook, whereas the superficial branch (outlined arrowhead) keeps on running along the regular path on the side of the ulnar artery (arrowhead). Note the edematous changes and the swollen appearance of the deep branch compared to the regular median nerve (outlined arrow). Sca, scaphoid; FT, flexor tendons; APB, abductor pollicis brevis; PB, palmaris brevis; OP, opponens pollicis; FDM, flexor digiti minimi. d Long axis 22-8MHz US image better demonstrates the thickened pisohamate ligament (arrow). e Transverse turbo Spin Echo T2-weighted MRI scan with fat saturation shows the anomalous position of the deep branch (arrow) inside the carpal tunnel on the radial side of the hamate hook. The superficial branch (arrowhead) is normally positioned over the superficial aspect of the carpal ligament. Outlined arrow, median nerve. f The intraoperative picture confirms the swollen appearance of the deep branch (black arrow) before it engages the carpal tunnel underneath the flexor retinaculum (thin arrow). Arrowhead, superficial branch