Acute cerebellar ataxia due to Epstein-Barr virus under administration of an immune checkpoint inhibitor

A 71-year-old male patient with adenocarcinoma of the lung and contralateral lung metastasis under administration of pembrolizumab had symptoms of cerebellar ataxia. We suspected that the symptoms were immune-related adverse events (irAE), but the patient was subsequently diagnosed as cerebellitis due to Epstein-Barr virus (EBV) infection. After steroid pulse therapy, the symptoms of cerebellar ataxia improved immediately. Immune checkpoint inhibitors (ICI) can induce neurological adverse events and cause acute cerebellar ataxia. Initially, irAEs were suspected in this case. His clinical data suggested that reactivation of the virus had occurred because the ICI affected his immune system. This is the first report of a case of acute cerebellar ataxia due to EBV under administration of an ICI.