Herpes Simplex Virus Encephalitis–Associated Hyponatremia and Anti-NMDA Receptor Encephalitis

A 78-year-old male presented to the emergency department with a two-day history of high-grade fever and confusion.Cerebrospinal fluid analysis was positive for Herpes simplex virus type 1 (HSV-1) via polymerase chain reaction, and intravenous acyclovir therapy was initiated.During hospitalization, the patient developed hyponatremia, which was attributed to cerebral salt wasting syndrome based on elevated urinary sodium levels and clinical hypovolemia.Hyponatremia and consciousness improved following acyclovir therapy and aggressive isotonic fluid replacement.After being discharged, the patient experienced recurrent cognitive decline, leading to a second hospitalization.Further evaluation resulted in a diagnosis of anti-N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis.Treatment with intravenous immunoglobulin resulted in gradual improvement in consciousness.This case highlights the coexistence of HSV encephalitis-associated cerebral salt wasting syndrome and subsequent anti-NMDAR encephalitis.