Pembrolizumab-Induced Anti-Yo-Antibody-Positive Cerebellitis: A Case Report.

We report a rare case of pembrolizumab-induced anti-Yo-antibody-positive cerebellitis in a patient with stage IV lung adenocarcinoma. Following two cycles of combination therapy, the patient developed nystagmus and ataxia. Despite tumour regression and normal magnetic resonance imaging (MRI) findings, cerebrospinal fluid analysis revealed lymphocyte-dominant hypercellularity, increased protein, and oligoclonal bands. Immune checkpoint inhibitor (ICI)-related cerebellitis was diagnosed after excluding infectious causes, metastases, and cancerous meningitis coupled with a positive anti-Yo antibody. Methylprednisolone pulse was ineffective, but high-dose intravenous immunoglobulin therapy resolved symptoms. To the best of our knowledge, this is the first case report of ICI-related cerebellitis with anti-Yo-antibody in lung adenocarcinoma. This case highlights the importance of paraneoplastic antibody testing in diagnosing ICI-related encephalitis, especially in cases of normal MRI findings.