Anti-GQ1b Antibody Syndrome: A Clinician-Oriented Perspective on Diagnostics, Therapy, and Atypical Phenotypes—With an Illustrative 16-Case Institutional Series

Anti-GQ1b antibody syndrome (AGABS) unifies triad-defined Miller Fisher syndrome (MFS), Bickerstaff brainstem encephalitis (BBE), and the ophthalmoplegic variant of Guillain-Barré syndrome (GBS-O) under a post-infectious immune mechanism centered on IgG to disialosyl gangliosides. The spectrum also encompasses triad-minus phenotypes-acute ophthalmoparesis without ataxia, acute vestibular syndrome, optic involvement, and acute sensory-ataxic neuropathy. A molecular-mimicry model with complement-mediated nodal/paranodal dysfunction explains severe early deficits despite bland limb nerve conduction studies (NCSs), the cranial/proprioceptive predilection, and generally favorable treatment responsiveness to immunotherapy. In practice, a serology-first strategy, complemented by targeted electrophysiology-blink and H-reflex testing, and, where feasible, paired SEP-ABR showing a literature-supported dissociation (normal ABR with impaired median-nerve cortical SEPs), which, in our series, was documented in one illustrative BBE case-and by structured neuro-otologic examination, mitigates the "normal-NCS trap" and enables timely treatment. Intravenous immunoglobulin (IVIg) is first-line; plasma exchange (PLEX) is an alternative in severe or IVIg-ineligible cases; and intravenous methylprednisolone (IVMP) may be added selectively for central/optic-weighted phenotypes without routine oral taper. We consolidate actionable diagnostic and therapeutic steps and examine them in an institutional series of 16 consecutive seropositive patients (2015-2025): all were anti-GQ1b-positive with frequent GT1a co-reactivity; most reported an antecedent infection-typically upper respiratory, less often gastrointestinal-within the two weeks before onset; limb NCSs were often nondiagnostic whereas reflex/evoked-potential studies were informative; two required intubation in addition to IVIg; outcomes were generally favorable with early immunotherapy. The practical message: order anti-GQ1b at first contact, pair targeted electrophysiology with neuro-otology, and treat early to exploit reversible nodal/paranodal dysfunction.