neurologic disorders
Acute Bacterial Meningitis
Etiology of Acute Bacterial Meningitis
Likely causes of bacterial meningitis depend on Patient age Route of entry Immune status of the patient Neonatal Bacterial Meningitis and young infants, Group B streptococci, particularlyStreptococcus agalactiae Escherichia (E.) coliand other gram-negative bacteria Listeria monocytogenes Inolder infants, children, and young adults,the most common causes of bacterial meningitis are Neisseria meningitidis Streptococcus pneumoniae N. meningitidismeningitis occasionally causes death within hours. Sepsis caused byN. meningitidissometimes results in coagulopathy and bilateral adrenal hemorrhagic infarction (Waterhouse-Friderichsen syndrome). Haemophilus influenzaetype B, previously the most common cause of meningitis in children < 6 years and overall, is now a rare cause in the US and Western Europe, where theH. influenzaevaccine is widely used. However, in areas where the vaccine is not widely used,H. influenzaeis still a common cause, particularly in children aged 2 months to 6 years. Inmiddle-aged and in older adults,the most common cause of bacterial meningitis is S. pneumoniae Less commonly,N. meningitidiscauses meningitis in middle-aged and older adults. As host defenses decline with age, patients may develop meningitis due toL. monocytogenesor gram-negative bacteria. In people of all ages,Staphylococcus aureusoccasionally causes meningitis. Routes of entry include the following: By hematogenous spread (the most common route) From infected structures in or around the head (eg, sinuses, middle ear, mastoid process), sometimes associated with a CSF leak Through a penetrating head wound After a neurosurgical procedure (eg, if a ventricular shunt becomes infected) Through congenital or acquired defects in the skull or spine Having any of the above conditions increases the risk of acquiring meningitis. Overall, the most common causes of bacterial meningitis in immunocompromised patients are S. pneumoniae L. monocytogenes Pseudomonas aeruginosa Mycobacterium tuberculosis N. meningitidis Gram-negative bacteria But the most likely bacteria depend on the type of immune deficiency: Cellular immunity deficiencies (eg, in AIDS, Hodgkin lymphoma, or drug-induced immunosuppression): Humoral immunity deficiencies or splenectomy: Neutropenia : In very young infants (particularly preterm infants) and older adults, T-cell immunity may be weak; thus, these age groups are at risk of meningitis due toL. monocytogenes. 