[A case of elderly-onset herpes simplex encephalitis in an 89-year-old woman, who recovered with acyclovir therapy, showed relapse 2 months later, and died].

The number of case reports on elderly-onset herpes simplex encephalitis (HSE) has been increasing. We encountered the case of an 89-year-old woman with HSE, who was probably one of the oldest-onset patients in Japan. She was a bed patient with underlying diseases of old cerebral infarction and cholangitis. These conditions might be risk factors for the onset of HSE. Concerning HSE among the elderly, it is important to pay attention to underlying diseases that weaken their immunity. Although we delayed in diagnosing her case and started treatment 1 month after convulsions appeared, she completely recovered with intravenous acyclovir (ACV) therapy. However, relapse occurred 2 months after the therapy ended. We treated her again with intravenous ACV but she died without improvement. ACV, which was initially effective, was ineffective at relapse. Cases of ACV-resistant herpes simplex virus (HSV) infection have been reported in immunodeficient patients. The immune system of elderly patients is sometimes too weak to suppress the mutation of the virus. In this case, the HSV may have become resistant to ACV. Therefore, the possibility of ACV resistance should be considerd in HSE relapse in the elderly population.