HSV infection of the CNS is one of few treatable viral diseases. HSV encephalitis of older children and adults is almost always caused by herpes simplex virus type 1 (HSV-1), and in individuals older than 20, is due to HSV reactivation.
Temporal lobe localization is characteristic for HSV encephalitis in individuals older than 3 months, and is responsible for its characteristic presentation, namely bizarre behavior and expressive aphasia.
CSF analysis will usually reveal an elevated protein level, and a lymphocytic cellular predominance.
CSF protein concentration is a function of disease duration, and will continue to rise even with administration of treatment (acyclovir) and may remain elevated after the completion of therapy.
5% of CSF samples will be totally normal, and the diagnosis will only be revealed with positive PCR detection of viral DNA in the CSF, which is the gold standard for diagnosis.
The sensitivity of MRI is similar to CSF analysis, with 5% of patients with HSV encephalitis having a normal MRI on presentation, and subsequently developing abnormalities.
Of note, HSV-2 tends to cause aseptic meningitis rather than encephalitis in adults, and has a benign course.
Bottom Line? Keep a high index of suspicion for HSV encephalitis, and treat the patient empirically despite a normal CSF/MRI pending PCR results.
References
Whitley RJ. Herpes Simplex Virus Infections of the Central Nervous System. Continuum 2015;21(6):1704 13
