Case Report of Delayed Encephalopathy From Pembrolizumab.

Introduction: Pembrolizumab is an immune checkpoint inhibitor (ICI) used to treat many types of advanced cancer. ICIs can cause encephalopathy, a severe immune-related adverse event, which is a diagnosis of exclusion. Pembrolizumab immune-related adverse events typically develop early (within a few months) in treatment; presentation after a year is extremely rare.

Case presentation: A 70-year-old White female with metastatic endometrial cancer treated with pembrolizumab for 19 months presented with generalized weakness that rapidly progressed to confusion, delusions, and hallucinations.

Discussion: After ruling out other causes of encephalopathy via broad-based testing of blood and cerebrospinal fluid and imaging, her neurologic status improved after treatment with high-dose glucocorticoids and intravenous immunoglobulin.

Conclusions: Pembrolizumab is an ICI that can cause encephalopathy, which is challenging to diagnose. While immune-related adverse events typically emerge soon after starting treatment, pembrolizumab-induced encephalopathy can be delayed significantly.