Viral meningitis: often associated with encephalitis (meningoencephalitis)
Enteroviruses (especially coxsackieviruses and echoviruses): the most common cause of all types of meningitis in all patient groups
Herpesviruses: HSV (meningitis is more commonly caused by HSV2 than HSV1)
Pathophysiology
Pathways of infection
Most pathogens that cause meningitis colonize the nasopharynx or the upper airways before entering the CNS via:
Hematogenous dissemination
About 5–10% of the US adult population are colonized with Neisseria meningitidis. The bacteria attach to the nasopharyngeal mucosa, where they can persist for long periods of time. Hematogenous dissemination may then occur subsequent to mucosal infiltration facilitated by infection with another pathogen capable of infiltrating the mucosa, e.g., adenovirus.
Contiguous spread of infections in nose, eyes, and ears
Retrograde transport along or within peripheral or cranial nerves
Direct infection (e.g., due to trauma or head surgery)
Subtypes and variants
Primary amebic meningoencephalitis
Pathogen: Naegleria fowleri (colloquially referred to as “brain-eating amoeba”), found in warm freshwater (e.g., ponds, hot springs)
Route of infection: via contaminated water entering the nose (e.g., while swimming) → invades the CNS directly via the olfactory nerve (perforates the cribriform plate)
Clinical features: causes fulminant meningoencephalitis with rapid onset
Diagnosis
CSF analysis
Findings similar to those of bacterial meningitis (e.g., neutrophilic pleocytosis, hypoglycorrhachia, increased CSF protein)
