Is intravenous aciclovir overused in possible viral encephalitis? a retrospective review

Timely administration of IV aciclovir reduces mortality in herpes simplex virus (HSV) encephalitis (Sköldenberg in Lancet, 1984), (Whitley in N Engl J Med, 1986). Early diagnosis, however, is challenging due to non-specific symptoms and delays in obtaining key investigation results. Empiric treatment with intravenous (IV) aciclovir in cases of suspected meningitis to cover for possible concurrent viral encephalitis is an approach not supported by the National Institute for Clinical Excellence (NICE) ( https://www.nice.org.uk/guidance/ng240 , 2024). Such practice exposes patients to the risk of iatrogenic nephrotoxicity and neurotoxicity.

Our objectives were to evaluate the diagnostic approach to suspected viral encephalitis and appropriateness of aciclovir prescription. This was a retrospective cohort study of 410 patients over 16 years old prescribed IV aciclovir for suspected central nervous system infection at Royal Free London NHS Foundation Trust between December 2021 and February 2024.

29% of patients fulfilled diagnostic criteria for possible or probable encephalitis while 5% did not fulfil any of the criteria. 38% had no microbiological or serological testing for HSV or varicella zoster virus. Discharge diagnoses included 5% with viral encephalitis (2% confirmed on cerebrospinal fluid testing) and 6% with meningitis, while the commonest diagnosis was delirium (11% of patients).

While acknowledging clinical uncertainty and attendant risks of missing a diagnosis of true viral encephalitis, in line with NICE guidelines we suggest a review of routine prescription of IV aciclovir in suspected meningitis and emphasise the importance of altered mental status as a useful distinguishing feature between viral encephalitis and meningitis in immunocompetent patients.