Update on herpes simplex encephalitis.

Herpes simplex encephalitis is the most common identified cause of sporadic viral encephalitis in the United States. Early diagnosis is critical because treatment with the antiviral drug acyclovir dramatically decreases morbidity and mortality. The use of polymerase chain reaction (PCR) techniques to amplify the genome of herpes simplex virus (HSV) from cerebrospinal fluid (CSF) has become the diagnostic procedure of choice. False-positive CSF HSV PCR results are rare when testing is performed in experienced laboratories. Negative CSF HSV PCR results should always be interpreted in the context of the timing of specimen collection and the likelihood of disease. Negative CSF HSV PCR tests can occur within the first 72 hours of illness, with subsequent tests becoming positive. Patients with HSV encephalitis will typically have a negative CSF HSV PCR after 14 days of acyclovir treatment, and a persisting positive PCR should prompt consideration of additional or revised antiviral therapy. Quantitative PCR testing provides information about HSV viral load in CSF, but the potential correlation of viral load with prognosis or other clinical features of disease remains uncertain. Although the neuroimaging abnormalities seen in HSV encephalitis are not unique, more than 90% of patients with proven HSV encephalitis will have magnetic resonance imaging (MRI) abnormalities involving the temporal lobes. Special MRI techniques, including fluid-attenuated inversion recovery and diffusion-weighted imaging, might reveal abnormalities not seen with conventional imaging sequences. Neuroimaging patterns in infants and children differ significantly from those seen in adults and include a higher frequency of extratemporal lesions.