Viral meningitis is part of the aseptic meningitis syndrome but must be distinguished from bacterial meningitis on the basis of a careful examination of the CSF and sound clinical judgment. Enteroviruses probably account for the bulk of cases of aseptic meningitis that occur in the United States and which are reported to the Centers for Disease Control each year. The seasonal pattern of the incidence of aseptic meningitis is largely due to the seasonal variation of enteroviral infections. Early on, the CSF in patients with viral meningitis frequently contains a predominance of polymorphonuclear leukocytes and may even have a low glucose level. The presence of neutrophils in the initial CSF sample is especially common in patients with enteroviral infections. A CSF glucose level lower than 50 per cent of a simultaneously drawn blood glucose determination is not uncommon in patients with viral meningitis due to mumps, LCM, and herpes simplex. In a patient with a predominance of polymorphonuclear leukocytes in the initial CSF specimen and in whom a viral infection is suspected, antibiotics may be withheld if a spinal tap is repeated within 12 hours. A shift from polymorphonuclear leukocytes to mononuclear cells makes viral meningitis the likely diagnosis. Both herpes simplex and varicella-zoster may infect the meninges by means of spread from cervical and dorsal root ganglia in a retrograde fashion much the way they spread in an antegrade fashion to the skin. HSV-2 is more likely to cause the clinical syndrome of viral meningitis, while HSV-1 is more likely to cause a meningoencephalitis with serious brain dysfunction. The identification of a specific viral agent in body fluids, especially the CSF, in a patient with aseptic meningitis is of more than academic interest, since it can shorten duration of hospital stay and eliminate unnecessary antimicrobial therapy. The diagnosis of enteroviral infections depends upon the isolation of a virus from CSF, stool, or throat plus a fourfold antibody response in the serum to the viral isolate. The 60-odd serotypes of enterovirus, each with different antigenic determinants, preclude serologic testing alone as a useful diagnostic test to identify the patient infected with coxsackievirus or echovirus. For infections, due to herpes simplex, varicella-zoster, LCM, and arboviruses, a serologic test alone can be useful. An analysis of CSF for viral antigens, lactate, CKBB isoenzyme, C-reactive protein, and muramidase is not discriminatory enough to distinguish between bacterial and viral meningitis and is no substitute for sound clinical judgment, frequent observation of the patient including repeated examination of CSF, and a willingness to change diagnosis and therapy based upon clinical progress of the patient.
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