Viral encephalitis

Acute viral encephalitis may be caused by a wide range of viruses but the most important is herpes simplex encephalitis (HSE) because of its severity, especially if untreated, and its good response to specific treatment with acyclovir. The outcome of any CNS viral infection is dependent on both the immune status of the host and the virulence of the infecting virus. In evaluating a patient with suspected viral encephalitis there are 3 essential steps, namely the identification of a true parenchymal virus infection of the brain rather than a non–infective encephalopathy, the distinction of an infectious viral encephalitis from an acute disseminated encephalomyelitis (ADEM), and then the determination, where possible, of the specific virus involved. In practice, the precise viral cause of the encephalitis may never be established. Analysis of the CSF for herpes simplex virus (HSV) DNA using the Polymerase Chain Reaction (PCR) has been a significant advance in the diagnosis of HSE as this test has a very high sensitivity and specificity especially with appropriate sample timing. It is essential to commence early treatment with intravenous acyclovir in patients suspected of having HSE because of the remarkable safety and efficacy of this drug and the dangers of delaying potentially effective treatment of life threatening disease. This review outlines the general management approach in patients suspected of having viral encephalitis.