Ultrasonography of the ulnar nerve loop in relation to the flexor carpi ulnaris tendon

The purpose of this study is to present the ultrasound appearance of a previously undescribed-at-imaging variant of the ulnar nerve near the Guyon's canal.

The ultrasound database of one of the authors, who has more than 10 years of experience in peripheral nerve ultrasound, was searched for the phrases "UN variant" or "UN loop." The other two authors, with more than 10 years of experience in nerve imaging, retrospectively reviewed the static sonograms as well as the videoclips stored, to confirm the findings.

Between October 2019 and October 2023, six cases were found in five patients, where ultrasound demonstrated previously undescribed-at-imaging anatomical variations of the distal ulnar nerve on six distal forearms. The variant was symptomatic in three cases in which possible distal compression was clinically suspected. A branch of the ulnar nerve made a loop around the flexor carpi ulnaris, which also presented with a variation of normal anatomy. In symptomatic patients, the diameter of the aberrant branch (retrospectively measured) was significantly enlarged.

High-resolution ultrasound can accurately demonstrate ulnar nerve variants even when only small nerve fascicles are involved, which may be clinically relevant and misinterpreted. Loops of the ulnar nerve are rare, but likely under-recognized. We think that symptoms are generally tolerated by patients if the motor branch of the ulnar nerve is not affected, as in our cases.