Opsoclonus-Myoclonus-Ataxia Syndrome Related to Pembrolizumab Treatment for Invasive Ductal Carcinoma: A Case Report

Immune checkpoint inhibitors (ICIs) such as pembrolizumab have become integral to the treatment of various metastatic malignancies. However, their use is associated with a growing spectrum of immune-related adverse events, including rare neurological complications. This report adds to the limited existing literature on ICI-associated opsoclonus-myoclonus-ataxia syndrome (OMAS) and highlights the importance of prompt recognition and multidisciplinary management of such cases. We present a case of a 67-year-old female who presented with rapid, chaotic eye movements, truncal ataxia, and full-body myoclonus two months after beginning a combined immunotherapy and chemotherapy regimen of paclitaxel, carboplatin, and pembrolizumab for triple-negative invasive ductal carcinoma. The case highlights pembrolizumab, an ICI targeting the programmed death-1 receptor, as a cause of adult-onset OMAS. In our discussion, we expand upon the mechanism of action of ICIs, notably the immune pathways underlying their use. We follow this with how this immunotherapy causes damage to peripheral tissues, focusing on the neurological side effects. We finish by discussing currently implemented treatment options for patients who experience these adverse events, and future directions in the field of autoimmune and paraneoplastic neurology. This case describes the onset of OMAS following pembrolizumab therapy in a patient with triple-negative breast cancer, a rare but clinically significant adverse event. As ICIs are increasingly used, clinicians must remain vigilant for uncommon neurological immune-related adverse events. Early recognition and immunosuppressive treatment can mitigate long-term sequelae. This case underscores the need for further research and interdisciplinary awareness in the evolving field of autoimmune and paraneoplastic neurology.