Early-Onset Ramsay Hunt Syndrome After Pembrolizumab Administration in a Patient With Lung Adenocarcinoma

Immune checkpoint inhibitors (ICIs) are associated with a broad spectrum of adverse effects referred to as immune-related adverse events (irAEs). Some infectious diseases can be precipitated by ICIs due to dysregulated inflammatory immune responses referred to as immune reconstitution inflammatory syndrome (IRIS), which were originally described in human immunodeficiency virus (HIV). Here, we report a case of Ramsay Hunt syndrome (RHS) considered to have occurred as "non-HIV IRIS" after the administration of pembrolizumab. A 64-year-old man with stage IVb lung adenocarcinoma was treated with pembrolizumab as first-line chemotherapy. He presented with a headache in two days and right facial paralysis with some vesicles on the rash in the right external auditory canal six days after the administration. We diagnosed ICI-induced RHS and initiated valacyclovir and systemic corticosteroid therapy. The degree of facial paralysis was severe, but it recovered gradually. Although ICI rarely causes RHS, clinicians should be aware of the possibility of RHS, as the prognosis of paralysis depends on early treatment.