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Etiology

Initially, an infectious agent colonizes or establishes a localized infection in the host. This may be in the form of colonization or infection of the skin, nasopharynx, respiratory tract, GI tract, or genitourinary tract. The organism invades the submucosa by circumventing host defenses (eg, physical barriers, local immunity, phagocytes/macrophages).

The following 3 major pathways exist by which an infectious agent (ie, bacterium, virus, fungus, and parasite) gains access to the CNS and causes meningeal disease:

• Invasion of the bloodstream (ie, bacteremia, viremia, fungemia, parasitemia) and subsequent hematogenous seeding of the CNS.
• A retrograde neuronal (ie, olfactory and peripheral nerves) pathway (eg, Naegleria fowleri, Gnathostoma spinigerum)

Direct contiguous spread (ie, sinusitis, otitis media, congenital malformations, trauma, direct inoculation during intracranial manipulation)

Invasion of the bloodstream, and subsequent seeding, is the most common mode of spread for most agents (eg, meningococcal, cryptococcal, syphilitic, and pneumococcal meningitis).

Rarely, infected contiguous structures invade via septic thrombi or osteomyelitic erosion; meningeal seeding may also occur with a direct bacterial inoculate during trauma, neurosurgery, or instrumentation. Meningitis in the newborn is transmitted vertically from colonized pathogens in the maternal intestinal or genital tract or horizontally from nursery personnel or caregivers at home.

Local extension from contiguous extracerebral infection (eg, otitis media, mastoiditis, or sinusitis) is a common cause. Possible pathways for the migration of pathogens from the middle ear to the meninges include the following:

• A systemic route in the bloodstream
• Along preformed tissue planes (eg, posterior fossa)
• Temporal bone fractures
• The oval or round window membranes of the labyrinths

In HIV-positive/AIDS patients, consider cryptococci, Mycobacterium tuberculosis, syphilis, HIV aseptic meningitis, and Listeria species. If the pathogen is unknown after an ED workup, draw a serum/CSF cryptococcal antigen and treat empirically as in adults older than 50 years (pending results of all blood and CSF tests) to cover the bacterial pathogens, particularly S pneumoniae and L monocytogenes, for which this patient population is most at risk.

Go to HIV-1 Associated CNS Conditions - Meningitis for complete information on this topic.

In patients who have had trauma or neurosurgery, the most common microorganisms are S pneumoniae (if CSF leak is present), Staphylococcus aureus, coliforms, and P aeruginosa. In patients with infected ventriculoperitoneal (atrial) shunt, the most common microorganisms are Staphylococcus epidermidis, S aureus, coliforms, Propionibacterium acnes, and diphtheroids (rare). Consult a neurosurgeon, since early shunt removal is usually necessary for cure.

In patients with aseptic meningitis (CSF pleocytosis and normal CSF glucose, negative bacteria on Gram stain), the most common microorganisms are enteroviruses, human herpesvirus-2 (HHV-2), lymphocytic choriomeningitis virus (LCM), HIV, and other viruses.

Other etiologies include drugs (NSAIDs, metronidazole, IV immunoglobulin) and, rarely, leptospirosis. Manage by repeating LP if necessary to rule out partially treated bacterial meningitis.

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