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Central nervous system (CNS) infections are often differentiated by the cause and type of infection (meningitis or encephalitis). Meningitis is a CNS infection characterized by inflammation of the meninges, or the layers of tissue that surround the brain and spinal cord. In contrast, encephalitis is an infection and inflammation of the brain. Meningitis is the most common type of CNS infection, followed by viral encephalitis.


Pathogens associated with bacterial meningitis vary based upon the age of the patient. In order to cause meningitis, bacteria must be able to evade normal barriers to infection and cause systemic infection. Next, the bacteria are able to cross the blood-brain barrier, multiply, stimulate inflammation in the subarachnoid and ventricular space, and cause damage. In the neonate, the blood-brain barrier is not well developed and allows bacteria to cross more readily than in an older patient. Additionally, when patients are immunosuppressed (by disease, drugs, or functional/anatomical asplenia) or have interrupted barriers, such as broken skin (eg, cochlear implants, recent trauma), bacterial infections are more likely. A summary of the common causes of bacterial meningitis is shown in Table 25-1.

TABLE 25-1Common Causes of Bacterial Meningitis by Age and/or Risk Factors and Empirical Antimicrobial Therapy

Aseptic meningitis occurs more often than bacterial meningitis and represents meningeal inflammation caused by pathogens that do not grow in the microbiology laboratory when cultured (eg, viruses, bacteria, fungi) or other noninfectious causes (Table 25-2...

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