COVID-19-Triggered Miller Fisher Syndrome Masking Leptomeningeal Progression of Merkel Cell Carcinoma: A Fatal Diagnostic Dilemma

Miller Fisher syndrome (MFS) is a rare variant of Guillain-Barré syndrome (GBS) classically characterized by the triad of ophthalmoplegia, ataxia, and areflexia. Its association with viral triggers, including SARS-CoV-2, has been increasingly recognized. We present a complex case of a 72-year-old male with metastatic Merkel cell carcinoma undergoing chemotherapy who presented in March 2026 with progressive weakness and near-syncope in the setting of severe anemia and COVID-19 infection. During hospitalization, he developed ophthalmoplegia, areflexia, and ataxia, raising concern for Miller Fisher syndrome. Despite early initiation of intravenous immunoglobulin (IVIG), the patient experienced rapid neurological deterioration with bulbar involvement, leading to respiratory failure requiring mechanical ventilation. This case highlights the diagnostic challenges and clinical complexity of MFS in the setting of malignancy and concurrent infection, emphasizing the importance of early recognition and management of neuromuscular complications in high-risk patients.