Atypical herpes simplex virus encephalitis diagnosed by PCR amplification of viral DNA from CSF

OBJECTIVE: To determine the frequency of mild/atypical herpes simplex virus encephalitis (HSVE) among patients with CSF specimens submitted to a university diagnostic virology laboratory for HSV PCR. BACKGROUND: HSVE is the most commonly recognized cause of acute sporadic encephalitis in the United States. Recognized clinical features are based on autopsy- or brain biopsy-confirmed cases. This is likely to produce ascertainment bias for features associated with severe disease and under-recognition of mild or atypical cases. Amplification of HSV DNA by PCR from CSF provides a sensitive and specific method for diagnosis of HSVE. METHODS: Results of all HSV CSF PCR tests sent to a university diagnostic virology laboratory (January 1, 1993, to December 31, 1996) were reviewed. Clinical information was prospectively collected and retrospectively reviewed. Patients with positive HSV CSF PCR tests were classified as having meningitis, encephalitis, or neonatal infection. Encephalitis was considered typical or atypical based on published criteria. RESULTS: A total of 7.6% of 1,224 CSF specimens were positive for HSV DNA. CSF HSV DNA-positive patients had meningitis (52%), encephalitis (26%), neonatal infection (17%), or nonclassifiable disease (5%). A total of 17% of HSVE patients had mild or atypical disease characterized by the absence of focal findings and slow progression in the absence of antiviral therapy. Atypical HSVE was associated with HSV-2 infection (two of the four patients), immunosuppression by steroid therapy or coexisting HIV infection (three of the four patients), or disease predominantly involving the nondominant temporal lobe (two of the four patients). CONCLUSIONS: Approximately one-fifth of HSVE patients have mild or atypical disease. CSF PCR for HSV DNA should be performed in patients with febrile encephalopathy even in the absence of focal features, initial CSF pleocytosis, or abnormal CT. Mild or atypical HSVE may be associated with infection with either HSV-1 or HSV-2. Mild or atypical HSVE was frequently associated with immunocompromise or asymmetric HSV infection affecting predominantly the nondominant temporal lobe.