A Case of Limbic Encephalitis Presenting as Paraneoplastic Neurological Syndrome with Possible Involvement of Immune Checkpoint Inhibitors

━━ Background. There have been few reports of limbic encephalitis in patients with cancer. We herein report a case of limbic encephalitis that developed after treatment with immune checkpoint inhibitors for lung cancer and improved after steroid and anticancer drug therapy. Case. A 72-year-old man visited our hospital complaining of hoarseness. Chest computed tomography (CT) showed a left hilar mass, and a transbronchial lung biopsy of the mass revealed an undifferentiated carcinoma. He was diagnosed with stage IVB non-small-cell lung cancer and treated with anticancer drugs, including pembrolizumab, from November of year X­3 to August of year X­1. Since the therapeutic effect was evaluated as a partial response, he was followed closely without treatment thereafter. At the end of January of year X, amnesic symptoms appeared, and head magnetic resonance imaging showed a high signal in both temporal lobes, leading to a diagnosis of limbic encephalitis. This encephalitis was considered a complication of treatment, being either paraneoplastic neurological syndrome (PNS) or an immune-related adverse events, so steroids were administered. Consequently, his symptoms were alleviated. On chest CT, new mediastinal lymph node enlargement (#4R) was observed, so endobronchial ultrasound-guided transbronchial needle aspiration was performed. The biopsy specimen revealed the histology of the tumor to be so undifferentiated. However, its resemblance to the histology of the primary tumor led to a diagnosis of metasta-sis. Chemotherapy was started as second-line treatment and resulted in further improvement of the neurological symptoms. Conclusion. Symptoms of PNS often precede the diagnosis of cancer, and the response to steroid treatment is considered poor. Since the delayed onset of neurological symptoms and the presence of a response to steroids in this case were not typical of PNS, we suspect that pembrolizumab influenced the development of limbic encephalitis.