Clinical Reasoning: A 49-Year-Old Man With Progressive Numbness, Gait Instability, and Tremors.

Approaching patients with paraproteinemic neuropathies can be challenging for the practicing neurologist, and a well-defined strategy considering specific etiologies is necessary to arrive at the correct diagnosis. In this case, a 49-year-old man presented with a 2-year history of progressive upper then lower extremity numbness, weakness, gait instability, and tremors. His examination was marked by proximal and distal symmetric upper and lower extremity weakness, large more than small-fiber sensory loss, prominent sensory ataxia, action and postural tremors, and globally absent deep tendon reflexes. His workup was notable for a chronic demyelinating sensorimotor polyradiculoneuropathy and a monoclonal immunoglobulin (Ig) M kappa gammopathy. This case highlights the approach to a patient with a rare subtype of IgM paraproteinemic neuropathy with a review of the differential diagnoses, red flag features of co-occurring hematologic disorders, and guided workup. We further discuss typical features of this rare diagnosis and therapeutic options.