Ulnar Neuropathy at the Elbow Associated With Focal Demyelination in the Proximal Forearm and Intraoperative Imaging Correlation

Abstract

Ulnar nerve focal demyelination (FD) in the forearm [defined as conduction block (CB) and or temporal dispersion (TD)] has been described with immune-mediated neuropathy and with compression affecting the forearm segment of the nerve. The association of FD in the forearm with entrapment ulnar neuropathy at the elbow, as well as the intraoperative imaging of the abnormal ulnar nerve at the flexor carpi ulnaris muscle level (FCU), has not been reported before. We report a 33-years-old woman presented with only sensory symptoms of the right hand suggestive of right ulnar neuropathy for the last 10 years. On clinical examination, she had reduced pinprick sensation on the little and ring fingers with no motor deficit. Nerve-conduction study showed slowing of conduction velocity across the elbow on the right when recording at the abductor digiti minimi (ADM) and first dorsal interossei (FDI). There was 63% amplitude drop when stimulating below the elbow compared to distal stimulation at the wrist. Increment inching study localized the block at 5 cm distal to the medial epicondyle. During surgical transposition, the ulnar nerve was swollen, and edematous in the segment where the nerve enters the FCU muscle, which provides a physiological explanation for the electrophysiological findings. After the surgery, the patient reported complete resolution of the symptoms. This case demonstrate that ulnar nerve motor potential FD at the proximal forearm could be recorded and it is still compatible with ulnar-nerve entrapment at the elbow.

Keywords: conduction block; cubital tunnel; entrapment; neuropathy; ulnar.

Figure 1

(A) The ulnar nerve compound motor action potential at the abductor digiti minimi (ADM) shows 65% drop in amplitude with 30% drop in the area and 63% prolongation in the duration when the nerve is stimulated below the elbow as compared to stimulation at the wrist. There was a slowing in conduction velocity across the elbow. The last trace, shows no motor response from ADM when stimulating the median nerve at the elbow. (B) The ulnar nerve compound motor action potential at the first dorsal interossei (FDI) shows 47% drop in amplitude with 53% drop in the area and 17% prolongation in the duration when the nerve is stimulated below the elbow as compared to stimulation at the wrist. There was a slowing in conduction velocity across the elbow. (C) The last area where the amplitude was partially maintained before the 63% amplitude drop occurred (when recording from ADM) 6 cm below the elbow, indicating that the maximum area of focal demyelination occurs just proximal to that point at around 5 cm below the elbow.

Figure 2

Surgical photograph of the right elbow demonstrating the released ulnar nerve from the cubital tunnel placed over the flexor muscle mass, in preparation to complete the transposition under a fascial sling. The edematous segment of the ulnar nerve shown is the segment that normally runs between the two heads of FCU muscle. The red vessel loop encircles the posterior branches of the medial antebrachial cutaneous nerve that crosses the surgical incision.