If it looks like a duck and quacks like a duck, then it's probably not multifocal motor neuropathy with a conduction block: A Case Report of Median Mononeuropathy in a Drummer

Case Diagnosis: Severe Atypical Median Mononeuropathy

Case Description or Program Description: A 64-year-old male professional drummer presents to an outpatient clinic with gradual onset of left hand pain, weakness, and numbness. Physical exam was significant for thenar atrophy, weakness of the abductor pollicis brevis, reduced sensation over thenar eminence, and positive Tinel’s at the wrist. Electrodiagnostic (EDX) testing showed evidence of axon loss and demyelination of median motor fibers at the wrist with normal median sensory studies and a low amplitude ulnar sensory response. MRI of the arm showed diffuse edema of supinator and thenar muscles. Aside from mild degenerative changes, MRI cervical spine was within normal limits and MRI brachial plexus was unremarkable.

Setting: Outpatient sports medicine clinic

Assessment/Results: The patient’s pain and weakness continued to progress despite conservative management with a wrist splint. Due to the ulnar findings and preserved median sensory responses, multifocal motor neuropathy (MMN) and chronic inflammatory demyelinating polyneuropathy (CIDP) were added to the differential in addition to severe atypical median mononeuropathy. Bilateral lower extremity EDX testing showed low amplitude and borderline conduction block of right fibular motor nerve in the leg. GM1 antibodies were negative. The patient underwent an empiric trial of IVIG without benefit, and was subsequently referred to orthopedic surgery for carpal tunnel release surgery for presumed severe atypical median mononeuropathy.

Discussion (relevance): EDX testing and MRI imaging are both prone to false positive results. In this case, the patient's muscle edema was related to his occupation (professional drummer), and his EDX abnormalities in the ulnar and fibular nerves were false positive findings that didn’t correlate with his neurologic signs and symptoms.

Conclusions: Although rare, a motor-only variant of carpal tunnel syndrome can present with entrapment of the recurrent motor branch of the median nerve. Clinical correlation is important when interpreting EDX and MRI findings to avoid unnecessary workup and treatments.