INTRODUCTION
Ulnar nerve compression can occur at any point along the course of the nerve. The medial intermuscular septum (MIMS) is one of the possible anatomical compression sites of the ulnar nerve. We report a case of ulnar neuropathy in the arm, which was diagnosed by electrophysiologic study and imaging studies (ultrasonography and magnetic resonance image), and surgically confirmed.
CASE REPORT
A 44-year-old man complained the left fourth and fifth finger weakness and hypoesthesia for five months. The physical examinations were performed. The motor power of first dorsal interosseous muscle (FDI) and abductor digiti minimi (ADM) was decreased (poor minus and fair, respectively). And those muscles atrophied. The sensory of dorsal ulnar cutaneous and posterior ulnar cutaneous nerve territory was decreased. Tinnel’s sign was positive at the left elbow, 13cm proximal to the medial epicondyle. The claw hand on the on left also represented. In nerve conduction study, the left ulnar motor responses were of prolonged latencies, low amplitudes, and slow conduction velocities with both abductor digiti minimi (ADM) and first dorsal interosseous (FDI) muscles. The Inching test showed abnormal conduction block between medial epicondyle proximal 13cm and 15cm with ADM recording, and between medial epicondyle proximal 11cm and 13cm with FDI recording (100% and 65.6%, respectively). (Table 1) Subsequent ultrasound study found the compression of the ulnar nerve by the fascia from the triceps muscles and accompanying small vessel. (Fig. 1A, B) The MRI study revealed the thickening of the arcade of Struthers and compressing the nerve. (Fig. 1C, D) Two months later, a follow-up nerve conduction study showed no significant improvement in the lesion. (Table 1) The surgical exploration confirmed thickened fascia connecting to the MIMS covered the ulnar nerve, and the muscular branch of the brachial artery compressed the nerve. The surgeon performed the vessel ligation and neurolysis and excised the thickened fascia (Fig. 1E, F, G).
DISCUSSION
There were several case reports related to the arcade of Struthers or MIMS, all of these cases were diagnosed by electrodiagnostic studies. However, this case was the first report using both ultrasound and MRI. Furthermore, this study revealed that not only the thickening of the MIMS but also the collateral vessel could cause compression.
The routine stimulation site for the ulnar motor fibers includes sites below the elbow (3cm distal to the medial epicondyle) and above the elbow (7cm proximal to the medial epicondyle). Stimulation at the proximal site near the axilla can coactivate other near nerves, especially the median nerve. However, when a routine study fails to localize the lesion, the proximal stimulation above the mid-arm should be done to localize the lesion site., Moreover, additive imaging studies, including ultrasound or MRI study, can be helpful to elucidate the etiologies of the ulnar neuropathy.
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