viral meningitis

Viral (aseptic) meningitis is the most common form of meningitis and is generally less severe than bacterial meningitis. Etiologies: Enteroviruses (Coxsackievirus, Echovirus) account for ~85% of cases, especially in summer/fall. Other causes include HSV-2 (more common in adults, associated with genital herpes - Mollaret's meningitis with recurrent episodes), HSV-1, VZV, mumps, HIV (during seroconversion), LCMV, arboviruses (West Nile virus), and adenovirus. Pathophysiology: Viruses typically reach the CNS via hematogenous spread (viremia) or, in HSV, via retrograde neuronal transmission. Inflammation is primarily lymphocytic, sparing the pia-arachnoid to a lesser degree than bacterial infection, generally without the intense purulent exudate seen in bacterial meningitis. Clinical presentation: Classic triad of fever, headache, nuchal rigidity (meningismus), sometimes with photophobia, phonophobia, malaise, and vomiting. Kernig's and Brudzinski's signs may be present but are less specific. Unlike bacterial meningitis, patients often appear less toxic and mental status is usually preserved (helps distinguish from encephalitis, where altered mentation is prominent). Diagnosis: Lumbar puncture shows lymphocytic pleocytosis (though early in the course, neutrophils may predominate), normal or mildly elevated protein, and normal glucose (key distinguishing feature from bacterial meningitis, which shows low glucose). CSF PCR is the gold standard for identifying specific viral etiology (e.g., enterovirus PCR, HSV PCR). Blood cultures and CSF Gram stain/culture should be performed to rule out bacterial causes, especially in atypical presentations. Management: Primarily supportive care (analgesics, antipyretics, hydration) since most cases are self-limited. Empiric antibiotics are often started until bacterial meningitis is excluded. Acyclovir is used if HSV is suspected or confirmed. Prognosis: Generally excellent with full recovery in most cases, especially enteroviral meningitis. HSV meningitis may have a more prolonged course. Neonates and immunocompromised patients are at higher risk for complications. Key distinguishing points from bacterial meningitis: CSF glucose normal (vs. low in bacterial), CSF protein mildly elevated (vs. markedly elevated in bacterial), lymphocytic predominance (vs. neutrophilic in bacterial), and clinical course is typically less severe with lower mortality.