Paraneoplastic Opsoclonus Without Myoclonus Secondary to High-Grade Serous Carcinoma in an Adult

Opsoclonus is a rare disorder characterized by chaotic, multidirectional eye movements and most commonly presents as part of the opsoclonus-myoclonus syndrome (OMS). Although typically described in pediatric populations, particularly in association with neuroblastoma, adult presentations are exceptionally rare and are most often paraneoplastic or infectious in origin. We describe the case of a woman in her 60s who developed isolated opsoclonus associated with a newly diagnosed poorly differentiated metastatic high-grade serous carcinoma. The clinical course, diagnostic evaluation, antibody profiling, and immunomodulatory treatments, including intravenous immunoglobulin (IVIG), corticosteroids, and plasmapheresis, are reviewed. This case highlights the diagnostic challenges and therapeutic considerations associated with adult-onset paraneoplastic opsoclonus.