A clinical and neurobiological case of IgM NMDA receptor antibody associated encephalitis mimicking bipolar disorder.

Autoimmune encephalitis associated with IgG antibodies to the N-methyl-d-aspartic acid receptor subunit NR1 (NMDAR) presents with neurological symptoms, such as seizures, and especially psychiatric symptoms, such as hallucinations, psychosis, agitation and anxiety. To date, however, the pathological relevance of IgM NMDAR antibodies remains elusive. Here, we describe clinical, neuroradiological and neurobiological findings of a 28-year-old male presenting with IgM NMDAR antibodies coincident with autoimmune encephalitis characterized by symptoms of bipolar disorder. After repeated steroid treatment, cognitive and psychiatric abnormalities improved and no NMDAR antibody was detectable. Using primary neuronal cultures, we demonstrate that patient's serum containing IgM NMDAR antibodies reduced the detection of NMDAR on neuronal cells and decreased cell survival. Although NMDAR encephalitis with IgG antibodies is increasingly recognized and diagnosed, atypical presentations with NMDAR antibodies with immunoglobulin subclasses other than IgG pose a diagnostic and therapeutic challenge. Further clinical and neurobiological studies are needed to study the pathophysiological relevance of IgM NMDAR antibodies.