Herpes simplex virus

Herpes simplex virus (HSV) is the most common cause of non-epidemic acute focal viral encephalitis in Europe and North America, with an incidence of up to 0 5 per 100 000 population per year.19 This is almost certainly an underestimate as many milder cases pass unrecognised. It occurs throughout the year and except for neonatal infections, almost all cases are caused by HSV type 1. Using serological studies, it is estimated that one-third of these infections occur during primary HSV1 infection and two-thirds result from reactivation of latent virus. It is no more common in those with compromised immune systems and it affects patients of any age. A third are below 20 and half over 50.20 The clinical presentation is enormously variable. Onset is usually insidious, with a prodrome of 4-10 days with malaise, pyrexia, and irritability, followed by frontal and temporal lobe disturbance manifest by personality change, hallucinations, psychiatric upset, and increasing focal signs, seizures, and deteriorating conscious level. In perhaps as many as 87%, focal signs appear.20 In a few, the onset is cataclysmic and the evolution is compressed into only a few days. Abscess, tumour, granuloma, vascular disease, and other forms of viral encephalitis are other diagnoses that have been mistaken for HSE.21 Focal changes on CT, MRI, and EEG are helpful but not pathognomonic. The use of PCR on CSF can provide a rapid, specific, and sensitive diagnosis.

The HSV1 viral load may also provide prognostic information, with high viral loads (greater than 100 HSV copies per microlitre of CSF) correlating with longer illness, greater mortality, and more long term sequelae. Treatment must be given quickly with aciclovir 10 mg/kg eight hourly by intravenous infusion for 14 days. This has been shown to diminish the mortality rate from greater than 70% in untreated patients to below 30%.22,23 Younger patients and those with a higher Glasgow Coma Score have a better outcome than older patients and those with a markedly disturbed conscious level.

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