Anti-GQ1b-Positive Miller Fisher Syndrome Following Pfizer Bivalent COVID-19 Vaccination.

Miller Fisher syndrome (MFS) is a rare, immune-mediated variant of Guillain-Barré syndrome characterized by acute onset of ataxia, areflexia, and ophthalmoplegia. It typically develops following an antecedent event, such as an infection or, in rare instances, a vaccination, which triggers the production of anti-ganglioside antibodies. While cases have been anecdotally linked to COVID-19 vaccination, serologically confirmed reports remain uncommon. We report a case of a 51-year-old, previously independent male who presented with acute neurological deterioration, including ascending numbness, severe ataxia, and diplopia, 10 days after receiving a Pfizer SARS-CoV-2 bivalent vaccine. Diagnostic workup confirmed the clinical diagnosis, with cerebrospinal fluid showing cytoalbuminologic dissociation and a positive serum anti-GQ1b antibody test. The patient was treated with a five-day course of intravenous immunoglobulin and subsequently underwent intensive inpatient rehabilitation, leading to a significant functional recovery. This case provides robust, serologically confirmed evidence of MFS following COVID-19 vaccination, highlighting a rare but treatable neurological complication. Clinicians should maintain vigilance for this syndrome in patients presenting with acute cranial nerve and cerebellar deficits after a recent immunological stimulus.