To report a case of late onset autoimmune encephalitis secondary to immune checkpoint inhibitor (ICI) nivolumab.

ICI therapy is associated with a wide spectrum of immune-related adverse events (irAEs), including neurologic events ranging from mild headache to potentially life-threatening encephalitis occurring in 1-3% of cases. Nivolumab is a programmed death 1 (PD-1) inhibitor which can cause autoimmune encephalitis within few days to weeks of therapy.

Imaging revealed metastasis to the lungs and inferior vena cava prompting medical management over cytoreductive nephrectomy. He was initially treated with Sunitinib but was later switched to nivolumab due to tumor progression. He continued nivolumab for 38 cycles with gradual improvement of tumor burden. However, this was complicated by subacute onset of proximal leg weakness of 1 month duration and altered mental status of 2 days duration for which he presented to the emergency room. Neurological examination revealed comatose state with intact brainstem reflexes. Reflexes were diminished throughout with upgoing toes. Imaging of the brain and spine was unremarkable. Cerebrospinal fluid analysis and exhaustive serological workup for infectious and inflammatory etiologies was unremarkable. Electroencephalography showed generalized slowing with spikes concerning for ongoing seizure like activity. Paraneoplastic panel was found to be unremarkable. He initially received broad spectrum antibiotics given the suspicion for aseptic meningoencephalitis but was later treated with intravenous immunoglobulins and plasma exchange for possible autoimmune encephalitis in the setting of nivolumab. He was subsequently treated with pulsed corticosteroids without adequate benefit.

We discuss a case of late onset of autoimmune encephalitis secondary to nivolumab which has not been reported before. Both oncologists and neurologists need to be aware of serious but uncommon neurologic irAEs associated with ICI.