Intrathecal IgG synthesis as a biomarker for CNS involvement in anti-GQ1b antibody syndrome: a case report and hypothesis-generating treatment framework.

Anti-GQ1b antibody syndromes span a clinical continuum from peripheral Guillain-Barré syndrome (GBS) to central Bickerstaff brainstem encephalitis (BBE), but reliable biomarkers of central nervous system (CNS) involvement that could guide treatment intensity are lacking. We report a 37-year-old woman with a relapsing GBS-Miller Fisher syndrome (MFS)-BBE overlap phenotype. In the first episode (March 2024), plasma exchange, intravenous methylprednisolone and IVIG were administered after anti-GD1a IgG was detected. After self-discontinuation of immunotherapy, she was readmitted in May 2024 with a two-month course of progressive weakness acutely worsened over four days, ophthalmoplegia, dysarthria, intermittent choking and bilateral extensor plantar responses. CSF showed albuminocytologic dissociation (protein 2.21 g/L; total cell count 40×106/L with a normal nucleated cell count) together with evidence of intrathecal IgG synthesis (IgG index 1.13; synthesis rate 147.10 mg/24h; oligoclonal bands type III with homologous serum bands). Electrophysiology revealed an acute motor axonal neuropathy pattern. IVIG and methylprednisolone were administered, with substantial improvement but persistent mild dysarthria and ataxia at three-month follow-up. The diagnosis was based mainly on the clinical course and electrophysiological findings, supported by serology and CSF analysis. This case shows that intrathecal IgG synthesis may complement albuminocytologic dissociation as a marker of CNS involvement in anti-GQ1b syndromes and supports the hypothesis-requiring prospective testing-that CSF immunological profiling could inform treatment intensity.