Abstract
BACKGROUND: Miller-Fisher Syndrome (MFS), a distinct subtype of Guillain-Barré Syndrome (GBS), accounts for 5% of GBS cases and classically manifests with the triad of ophthalmoplegia, ataxia, and areflexia. Isolated bilateral mydriasis as the sole presenting feature is exceptionally rare, particularly in pediatric populations. While pupillary abnormalities have been documented in adult MFS cases, their diagnostic significance and management in children remain poorly characterized. We report a novel pediatric case of a 7-year-old girl presenting with 7 days of unexplained bilateral painless mydriasis unresponsive to light accommodation. Initial symptomatic management targeting potential toxic or neuropathic etiologies proved ineffective. Recognition of this atypical presentation prompted serological evaluation for autoimmune neuropathy markers, which demonstrated positivity for GQ1b IgM, GQ1b IgG, and GT1a IgG antibodies, confirming MFS diagnosis. Rapid clinical improvement followed intravenous immunoglobulin (IVIG) therapy. This case highlights the diagnostic challenges posed by incomplete or atypical MFS manifestations and underscores the necessity of early antibody testing in unexplained autonomic or neurological symptoms. CONCLUSION: This report expands the phenotypic spectrum of pediatric MFS by demonstrating isolated bilateral mydriasis as a potential initial manifestation, clinicians evaluating pupillary dilation should consider MFS in differential diagnoses. Future studies should continue to explore the pathophysiological link between anti-GQ1b antibodies and isolated autonomic dysfunction in pediatric MFS.