Chapter 24. Central Nervous System Infections

The incidence of community-acquired central nervous system (CNS) infections is around 35 per 100,000 person-years.1 These infections are often differentiated by the type (ie, meningitis or encephalitis) and cause of infection. Meningitis (aseptic more than bacterial) is the most common type of CNS infection, followed by viral encephalitis.1,2

Bacterial Meningitis

Bacterial meningitis is most common in the pediatric population. Specifically, the incidence ranges from 21 per 100,000 in neonates, 12.5 per 100,000 in children, and 2.6 per 100,000 in adults.3,4,5 The pathogens associated with bacterial meningitis also vary based upon the age of the patient. In neonates and infants less than 3 months of age, the most common isolated bacteria is Streptococcus agalactiae (also known as group B Streptococcus), followed by Escherichia coli, Streptococcus pneumoniae and Listeria monocytogenes.2,6,7 Specifically, early onset meningitis, that with onset in the first week of life is usually associated with pathogens found in the maternal genital tract (eg, S. agalactiae,E. coli, L. monocytogenes). While meningitis occurring from 1 to 4 weeks of age is associated with maternal and community-associated pathogens (eg, S. pneumoniae) or in hospitalized infants, nosocomial pathogens (eg, Staphylococcus aureus, Enterobacter spp.). From 2 to 3 months of age, the likelihood of pathogens from the maternal genital tract decrease and the community-associated pathogens become more prevalent. In children over 3 months old and adults, the most common causes of bacterial meningitis are S. pneumoniae and Neisseria meningitidis.1,2,5,8 Common bacterial causes of meningitis in patients older than 50 years include S. pneumoniae, N. meningitidis, and L. monocytogenes.9

In order to cause meningitis a bacteria must be able to avoid the patient's 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.10 In the neonate, the blood-brain barrier is not well developed and allows bacteria to cross more readily than in an older patient. Neisseria meningitidis and S. pneumoniae are common causes of bacterial meningitis in older infants and adults because they are surrounded by a polysaccharide capsule that assists in avoiding the usual immune system response to bacterial invasion.10 Additionally, when patients are immunosuppressed (by disease, drugs, or functional/anatomical asplenia) or have barriers, such as skin interrupted (eg, cochlear implants, recent trauma) bacteria are more likely to be able to cause disease.