[A case of multifocal motor neuropathy with IgM lambda anti-GM1 antibody and IgM kappa paraprotein reacting exclusively with GM2].

A 57-year-old previously healthy woman visited our clinic complaining of frequent muscle cramps and progressive weakness in the hands and fingers for 3 years. On examination, cranial nerves were unremarkable. There were moderate weakness and mild muscle wasting with fasciculation in the left thumb flexor and interossei on both sides. Tendon reflexes were hypoactive. There were no pathologic reflexes or sensory deficit. The cerebrospinal fluid was unremarkable. Nerve conduction studies demonstrated conduction block in the right ulnar nerve. Compound muscle action potential in the left median nerve was low-normal. Distal motor latencies, motor and sensory nerve conduction velocities were normal in all nerves tested. A diagnosis of multifocal motor neuropathy was made. Two courses of intravenous immunoglobulin infusion gave no beneficial effects. The patient had IgM kappa monoclonal gammopathy of undetermined significance. Her serum IgM reacted with GM2, GM1, and GA1 but not with GD1a, GD1b, GD3, GalNAc-GD1a, GT1b, GQ1b, galactocerebroside, or sulfated glucuronyl paragloboside. IgM kappa paraprotein reacted exclusively with GM2. Only IgM lambda bound to GM1 and GA1, suggesting the possibility that another paraprotein, though undetectable by immunoelectrohoresis, had a reactivity with GM1 and GA1. This case showed previously unreported antigenic specificity of paraproteins in cases of MMN.