Anti-Yo positive paraneoplastic cerebellar degeneration is a rare syndrome associated with breast cancer or gynecological malignancies, and patients typically present with subacute cerebellar dysfunction. Acute onset or rapidly progressive cases are extremely rare. Pembrolizumab is an immune checkpoint inhibitor with known immune-related adverse events. 

The patient was treated with a course of high-dose methylprednisolone followed by plasma exchange with mild improvement of her symptoms. Pembrolizumab was discontinued and the patient was started on chemotherapy and completed one more course of plasma exchange without clear change of the symptoms on her last clinic follow-up, about four months after the onset of her symptoms.