Herpes simplex encephalitis: how good are we in diagnosing this condition?

Abstract

INTRODUCTION: Herpes simplex encephalitis (HSE) is the commonest sporadic infective encephalitis in Hong Kong. Early recognition of HSE, which relies on a high index of suspicion, is important as effective treatment is available. Empirical acyclovir is advocated for all cases of clinically suspected viral encephalitis. Electroencephalography (EEG) is a routine investigation in suspected HSE. METHODS: The EEG database of Neurodiagnostic Unit, Queen Mary Hospital, was reviewed retrospectively. All referrals from April 2006 to March 2009 with a diagnosis of suspected HSE treated with empirical intravenous acyclovir were identified. Their presenting features, imaging and laboratory findings, and final diagnoses were reviewed. RESULTS: During the study period, 60 patients (mean age, 51 years; range, 18-90 years, M:F ratio=13:7) underwent EEG for suspected HSE. Presenting features included fever (n=39), confusion (n=39), impaired consciousness (n=31), focal signs (n=15, seizure in 8), and headache (n=13). All patients underwent brain CT and 45 had MRI. The commonest imaging findings were unrelated old changes (n=20) and normal study (n=16). Lobar inflammation was detected in four patients. EEG was normal, showed diffused abnormalities, or focal/ multifocal abnormalities in 16, 31, or 13 patients, respectively. Lumbar puncture was performed in 59 patients. Total cell count was ≤10×106 /L in 68% of patients and CSF protein was <0.8 g/L in 51% of patients. Polymerase chain reaction for herpes simplex virus was positive in one out of 56 requests. Viral encephalitis was the final diagnosis in three patients (HSE=1, Japanese encephalitis=1, other virus=1). Other common diagnoses included meningitis (n=9), non-CNS sepsis (n=9), psychiatric illnesses (n=8), epileptic seizure (n=6), and acute stroke (n=5). CONCLUSION: Our findings demonstrate that we were exercising a high index of suspicion for diagnosing HSE. Our liberal use of empirical acyclovir was also consistent with the IDSA (Infection Diseases Society of America) recommendations. Despite our low threshold of investigating for HSE, only one case was identified over 3 years, suggesting HSE is an uncommon condition.