Pathophysiology
Clinical meaning
Meningitis is an acute inflammation of the meninges, the three-layered protective membranes (dura mater, arachnoid mater, and pia mater) that surround the brain and spinal cord. The subarachnoid space between the arachnoid and pia mater contains cerebrospinal fluid (CSF), which cushions the central nervous system and serves as a medium for nutrient transport and waste removal. Infectious organisms reach the meninges through several routes: hematogenous spread (most common), direct extension from adjacent infections (sinusitis, otitis media, mastoiditis), or direct inoculation through trauma or neurosurgical procedures. Bacterial meningitis is the most clinically dangerous form, with Neisseria meningitidis (meningococcus) and Streptococcus pneumoniae (pneumococcus) being the most common causative organisms in adults, while Group B Streptococcus and Escherichia coli predominate in neonates. Once bacteria cross the blood-brain barrier, they multiply rapidly in the CSF because this fluid lacks significant immune defenses such as complement proteins and immunoglobulins. The bacterial cell wall components trigger a massive inflammatory cascade, releasing cytokines (interleukin-1, tumor necrosis factor) that increase blood-brain barrier permeability, leading to vasogenic cerebral edema. Exudate accumulates in the subarachnoid space, obstructing CSF flow through...