To describe an unusual case of opsoclonus-myoclonus-ataxia syndrome after the diagnosis of breast cancer and initiation of pembrolizumab, a programmed death (PD-1) inhibitor.  

The two main etiologies for adult-onset OMS include idiopathic (I-OMS) and paraneoplastic (P-OMS). Increasing reports of paraneoplastic syndromes with widespread use of immune checkpoint inhibitors (ICIs) are now emerging. Pembrolizumab has been associated with the subacute onset of neurological manifestations, including movement disorders, action-sensitive myoclonus, and ophthalmologic conditions.  

A 67-year-old woman diagnosed with ER- PR- HER2- invasive ductal carcinoma grade 2 presented to the emergency department with nausea, vomiting, lightheadedness, and paresthesias two months after initiating chemotherapy with paclitaxel, carboplatin, and pembrolizumab. During hospitalization, she was noted to have lower extremity weakness, difficulty with ambulation and sustaining a standing posture, and. Her symptoms further worsened over a few days with uncontrollable myoclonus and irregular, rapid eye movements with horizontal, vertical, and torsional components. Magnetic resonance imaging (MRI) of the brain with and without contrast revealed unspecific white matter hyperintensities. A lumbar puncture was performed, and the cerebrospinal fluid (CSF) paraneoplastic panel was negative. A five-day course of intravenous (IV) methylprednisolone 1g daily was trialed, and 2g/kg intravenous immunoglobulin (IVIG) was given. She was started on clonazepam, which was slowly increased to 2 mg TID, and topiramate twice daily, with improved eye tracking and resting myoclonus. Her truncal coordination and opsoclonus improved, and she was discharged to an acute rehab facility.  

With the widespread use of immune checkpoint inhibitors, it is crucial to consider pembrolizumab as a potential etiology OMAS, particularly when serum and cerebrospinal fluid paraneoplastic panels yield negative results and there is a temporal relationship between the administration of pembrolizumab and the onset of symptoms.