(Kiosk 3) The Importance of Intravenous Immunoglobulin Response in Multifocal Motor Neuropathy Diagnosis

Case Diagnosis: Multifocal motor neuropathy

Case Description: A 41-year-old presented with left arm weakness beginning in 2016, initially as an inability to get his wallet from his pocket due to difficulty extending his fingers, and progressed over several years. He denied other limb weakness, sensory complaints, or signs of cranial nerve or sphincter dysfunction. Exam revealed weakness in the multiple nerve distributions in the left upper limb only, with no sensory findings. Electrodiagnostic studies showed conduction block across the plexus of the radial nerve and axon loss in the C6 and 7 distributions. Anti GM-1 antibodies were normal. Neuromuscular ultrasound showed enlargement in multiple nerves outside of entrapment sites. He had a nearly complete recovery of strength with intravenous immunoglobulin (IVIg) treatment for presumed MMN.

Discussion: MMN is a rare autoimmune disease of the lower motor neurons producing asymmetric motor deficits primarily in the upper limbs. MMN diagnoses are based upon recognition of clinical symptoms, signs, electrodiagnostic findings of conduction block, and anti-GM1 antibodies. Standard treatment is IVIg infusions.

Setting: Outpatient neuromuscular practice

Assessment/Results: Despite negative anti-GM1 antibodies and without clear conduction block on multiple nerves, his clinical presentation including clinically deduced proximal conduction block neuromuscular ultrasound, and ultimately response to IVIg treatment suggest a diagnosis of MMN.

Conclusion: MMN is a rare disorder that can be difficult to diagnose. Most cases will have demonstrable conduction block of motor fibers and anti-GM1 antibodies, but atypical cases may not. In these cases, clinical deduction of proximal conduction block and neuromuscular ultrasound can be helpful in the diagnosis of this rare but treatable condition.