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 the arachnoid villi and cerebral aqueduct, resulting in communicating or obstructive hydrocephalus and increased intracranial pressure. Viral meningitis (aseptic meningitis), most commonly caused by enteroviruses, is generally self-limiting with a more favorable prognosis. Viral pathogens cause lymphocytic infiltration of the meninges without the purulent exudate characteristic of bacterial infection. Fungal meningitis, caused by organisms such as Cryptococcus neoformans, occurs primarily in immunocompromised patients and follows an insidious, chronic course. The classic triad of meningitis includes severe headache, fever, and nuchal rigidity (neck stiffness), though this complete triad is present in fewer than half of patients. Two critical physical examination signs aid in clinical assessment: Brudzinski sign (involuntary flexion of the hips and knees when the neck is passively flexed) and Kernig sign (resistance or pain with passive extension of the knee when the hip is flexed to 90 degrees). Both signs indicate meningeal irritation. Lumbar puncture with CSF analysis is the definitive diagnostic procedure. In bacterial meningitis, CSF findings typically show elevated opening pressure, cloudy or turbulent appearance, markedly elevated white blood cell count with neutrophil predominance, elevated protein, and decreased glucose (less than 40 mg/dL or less than 40 percent of serum glucose). Viral meningitis CSF shows clear fluid, lymphocyte predominance, mildly elevated protein, and normal glucose. The practical nurse must recognize that bacterial meningitis is a medical emergency requiring immediate antibiotic administration -- treatment should never be delayed while awaiting diagnostic confirmation. Complications of bacterial meningitis include cerebral edema, seizures, cranial nerve damage (particularly hearing loss from cranial nerve VIII involvement), cerebral infarction from vasculitis, disseminated intravascular coagulation (DIC), and Waterhouse-Friderichsen syndrome (adrenal hemorrhage associated with meningococcal septicemia). Droplet precautions must be initiated immediately for suspected meningococcal meningitis, and close contacts require chemoprophylaxis with rifampin, ciprofloxacin, or ceftriaxone within 24 hours of exposure.