European consensus on viral encephalitis.

Herpes simplex encephalitis (HSE) is one of the commonest viral brain disorders of immunocompetent individuals, with an annual incidence estimated at between 1 in 250 000 and 1 in a million of the population. Most cases are due to herpes simplex virus (HSV) type 1; up to 10% may be caused by HSV-2. Encephalitis is postulated to occur as a consequence of centripetal spread of virus from sites of latent infection in cranial nerve ganglia to the frontal or temporal lobes of the brain. The mechanism of reactivation in HSE is poorly understood. HSE typically results in focal frontal and/or temporal lobe necrosis, and patients present with an acute neurological syndrome characterised by behavioural disturbances, hemiparesis, aphasia, or seizures. If untreated, HSE is usually fatal. Less commonly, patients present with mild, subacute encephalitis, brainstem encephalitis, recurrent meningitis, or myelitis. Unlike most viral encephalitides, for which there is only supportive management, HSE does respond to specific antiviral drugs, and early, aggressive antiviral therapy saves lives and reduces morbidity. Rapid reliable diagnostic techniques and standardised methods of treatment for HSE are therefore critical. The European Union’s Concerted Action on Virus Meningitis and Encephalitis has produced a comprehensive review of the roles of brain imaging, electroencephalography, brain biopsy, and cerebrospinal fluid studies in the clinical evaluation and management of HSE. Imaging and electroencephalography are neither sensitive nor specific, particularly in early disease. Biopsy is both sensitive and specific but is also both traumatic and expensive. Fortunately, nested PCR amplification of HSV sequences from cerebrospinal fluid offers a rapid, sensitive, inexpensive, and less invasive method for establishing the initial diagnosis of HSE and for monitoring the response to therapy. Although nested PCR is more susceptible to contamination artifact (false positive) than traditional methods such as virus isolation or serology, diagnostic PCR has been successfully incorporated into many clinical microbiology laboratories and is likely to become the gold standard. PCR has revolutionised the management of infections with HIV and hantaviruses as well as herpesviruses. Clinicians who suspect HSE frequently begin antiviral therapy before the diagnosis is established. Pilot studies indicate that even if antiviral therapy has been started before collection of cerebrospinal fluid, PCR can still be of diagnostic value; HSV sequences are typically