Ultrasonographic Imaging of Entrapment of the Palmar Cutaneous Branch of the Ulnar Nerve Due to Accessory Abductor Digiti Minimi Muscle

CASE

A 55-yr-old man visited the clinic with paresthesia and mild erythema over his left hypothenar eminence for several years (Fig. 1). He denied any previous hand trauma but declared that he had been performing push-up exercises daily. Physical examination revealed tenderness over the insertion of the flexor carpi ulnaris tendon but no visible muscle atrophy. With the likely diagnosis of hypothenar hammer syndrome, he was referred for an ultrasonography examination. Compared with the contralateral side (Figs. 1A, B), the affected side showed an aberrant muscle emerging from the distal forearm (Fig. 1C, Video 1). The muscle traveled through Guyon canal and approximated distally to the abductor digiti minimi muscle (Fig. 1D, Video 1). Furthermore, the palmar cutaneous branch of the ulnar nerve (PCUN) was found arising from the radial side of its main trunk and radial to the ulnar artery and veins (Fig. 1C, Video 1). On the asymptomatic side, the PCUN perforated the antebrachial fascia proximal to the wrist crease and then pierced the palmar carpal ligament (Video 1). On the painful side, the nerve was squeezed inside Guyon canal by the accessory abductor digiti minimi muscle, rendering the nerve fascicle more flattened than that of the contralateral side. With a positive sono-Tinel sign over Guyon canal, PCUN entrapment due to the accessory abductor digiti minimi muscle was confirmed. The symptoms of the patient were relieved after physical therapy and discontinuation of the daily push-up exercises.

FIGURE 1: Ultrasonographic image of the palmar cutaneous branch of the ulnar nerve (brown dashed area) and main trunk (yellow dashed area) at the level of the forearm (A) and Guyon canal (B) over the asymptomatic side, and at the forearm (C) and Guyon canal (D) over the painful side. ADM, abductor digiti minimi; asterisk, accessory abductor digiti minimi muscle; black arrowheads, erythema over the hypothenar eminence; FCU, flexor carpi ulnaris; FDP, flexor digitorum profundus; P, pisiform; UA, ulnar artery; v, ulnar vein.

DISCUSSION

The PCUN is a distal branch of the ulnar nerve that is found in 60%–90% of the general population.^1,2 It is small and difficult to visualize using common imaging tools, which is the probable reason for its entrapment being rarely reported in the literature. The nerve arises from the radial side of the ulnar nerve approximately 5–10 cm proximal to the wrist crease.¹ It may depart from the main trunk before or after the bifurcation point of the dorsal cutaneous branch of ulnar nerve. The terminal fibers of the nerve course lateral to the pisiform, between the palmaris longus and flexor carpi ulnaris tendons, and then supply the sensation of the hypothenar eminence.

The PCUN also carries a vascular branch to the ulnar artery, that is, the “nerve of Henle,”³ and has been related to certain peripheral vascular disorders that cause arterial constriction and erythema over the hypothenar eminence. The symptoms can be induced through repeated compression (eg, push-up exercise) and can be aggravated by an accessory abductor digiti minimi muscle.

CONCLUSIONS

This case highlights the usefulness of ultrasonography in diagnosing an overlooked cause of medial palm pain attributed to PCUN entrapment by an aberrant hypothenar muscle.