Anti-GQ1b antibody syndrome: clinical overlap of Bickerstaff brainstem encephalitis and Miller Fisher syndrome—a diagnostic conundrum

This case presents an early adolescent girl who initially presented with clinical features overlapping Bickerstaff brainstem encephalitis and Miller Fisher syndrome, including progressive lower limb weakness, cranial neuropathies, hyperreflexia, positive Babinski sign and hypersomnolence following a gastrointestinal illness. MRI revealed brainstem involvement, and cerebrospinal fluid analysis showed albumin cytologic dissociation. Further investigations revealed a positive serum anti-GQ1b antibody titre of 1:160, leading to a diagnosis of anti-GQ1b antibody syndrome. She was treated with immunotherapy (intravenous immunoglobulin), which resolved her symptoms. These overlapping conditions share anti-GQ1b positivity but differ in clinical and radiological findings, emphasising the importance of recognising this evolving autoimmune spectrum for timely diagnosis and management.