Probable immune-mediated brainstem syndrome presenting as a stroke mimic with isolated serum anti-GM2 IgM positivity: a case report

Autoimmune brainstem encephalitis is an uncommon inflammatory disorder that may present with acute focal neurological deficits mimicking posterior circulation ischemic stroke, posing substantial diagnostic challenges. Although anti-GM2 antibodies have been reported predominantly in peripheral nervous system disorders, their clinical significance in central nervous system inflammatory diseases remains uncertain. A 28-year-old previously healthy man presented with acute-onset vertigo, diplopia, right facial numbness, and gait ataxia suggestive of posterior circulation stroke. Neurological examination localized the lesion to the brainstem, whereas brain magnetic resonance imaging and computed tomography angiography revealed no corresponding abnormalities. Cerebrospinal fluid analysis demonstrated mildly elevated protein and immunoglobulin levels without pleocytosis. Extensive investigations did not identify an infectious, vascular, demyelinating, paraneoplastic, or other autoimmune neurological etiology. Serum anti-ganglioside antibody testing identified isolated anti-GM2 IgM positivity, while anti-GM2 IgG and other antiganglioside antibodies were negative. Intravenous methylprednisolone was initiated, with clinical improvement first noted 2 days later. The overall presentation was considered a probable immune-mediated brainstem inflammatory syndrome, with autoimmune brainstem encephalitis as the leading clinical consideration; the isolated serum anti-GM2 IgM finding was considered of uncertain clinical significance. This case highlights the diagnostic challenges of an MRI-negative brainstem syndrome presenting as a stroke mimic and the need for cautious interpretation of isolated serum anti-GM2 IgM positivity. Whether this serological finding is incidental or reflects an underlying immune-mediated process remains uncertain and warrants further investigation.