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. Aseptic meningitis occurs secondary to pathogens or other causes that do not grow in the microbiology laboratory when cultured (eg, viruses, atypical bacteria, fungi, drug causes). Meningitis (aseptic more than bacterial) 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 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. Specifically, early onset meningitis (first week of life) is associated with pathogens found in the maternal genital tract (eg, S. agalactiae, E. coli, L. monocytogenes). Meningitis occurring from 1 to 4 weeks of age is associated with maternal and community-associated pathogens (eg, S. pneumoniae) or with nosocomial pathogens (eg, Staphylococcus aureus, Enterobacter species) in hospitalized infants. From 2 to 3 months of age, the likelihood of pathogens from the maternal genital tract decrease and the community-associated pathogens become prevalent. In children over 3 months old and adults, the most common causes of bacterial meningitis are S. pneumoniae and Neisseria meningitidis. Common bacterial causes of meningitis in patients older than 50 years include S. pneumoniae, N. meningitidis, and L. monocytogenes.
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. 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. 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), bacteria are more likely to be able to cause disease.
Aseptic Meningitis and Viral Encephalitis
Viral meningitis is the most frequent infectious cause of aseptic meningitis and is common in infants less than 1 year old. The most common cause of viral meningitis in children are enteroviruses. Two common types of enteroviruses are coxsackie and echoviruses. Additionally, nonviral infectious causes of aseptic meningitis include mycobacterial and spirochetal (eg, Borrelia burgdorferi).