nvestigators fail to identify an etiologic agent in up to 75% of cases of presumed viral infection of the central nervous system (CNS).85 Indeed, the confusing term aseptic meningitis is a testament to the historic difficulty of definitively isolating an etiologic agent in presumed viral meningitides.
Amplification of viral nucleic acids from cerebrospinal fluid, as done by the polymerase chain reaction (PCR), for example, has considerably improved diagnosis of several acute, subacute, and chronic viral CNS infections. PCR has become the method of choice for rapid, noninvasive diagnosis of herpes simplex virus (HSV) encephalitis. Herpes viruses that can now be reliably diagnosed include:
- herpes simplex virus (HSV)
- cytomegalovirus (CMV)
- varicella-zoster virus (VZV)
- Epstein- Barr virus (EBV)
- human herpesvirus 6 (HHV-6)
In acquired immunodeficiency syndrome (AIDS), PCR can help differentiate lesions due to the human immunodeficiency virus (HIV) itself from those due to opportunistic infections, such as progressive multifocal leukoencephalopathy (PML) caused by Jamestown Canyon virus or CMV-related complications.87
Viral encephalitis is caused by viral infection of brain parenchyma, producing neuronal and glial degeneration, inflammatory infiltration, edema, and tissue necrosis. Viral encephalitis occurs worldwide, with a higher incidence in tropical regions. Between 1,000 and 2,000 cases per year in the United States are reported by the Centers for Disease Control and Prevention (CDC).86 Focal or generalized seizures are among the common presenting signs and symptoms of viral encephalitis (along with fever, headache, stiff neck, delirium, and stupor or coma).
A summary of the principal viral CNS syndromes with potential seizure complications appears as Table: Viral CNS syndromes with potential seizure complications