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Meningitis is inflammation of the membranes (meninges) covering the brain and the spinal cord. Although the most common causes are infection (either bacterial or viral), chemical agents and even tumor cells may cause meningitis. Encephalitis and brain abscess can complicate infective meningitis.

Major bacteria that cause meningitis are Streptococcus pneumoniae, Haemophilus influenzae, staphylococcus and meningococcus.

The classical symptoms of meningitis are headache, neck stiffness and photophobia (the trio are called "meningism"). An altered level of consciousness or other neurological deficits may be present depending on the severity of the disease. A lumbar puncture to obtain cerebrospinal fluid (CSF) is usually indicated to determine the cause and direct appropriate treatment.

Meningitis is a medical emergency, being a condition with a high mortality rate if untreated. The cause is most commonly a bacterial infection sensitive to antibiotics. Patients with suspected meningitis should optimally initially have a CT scan to help determine if there is a raised intracranial pressure that might cause a serious or fatal brain herniation during lumbar puncture. If there are no signs of elevated central nervous system pressure demonstrated on the CT scan, a lumbar puncture procedure is performed to obtain cerebrospinal fluid for microscopic examination, chemical analysis, and bacterial cultures. Broad spectrum antibiotics should be urgently started before the culture results are available. If lumbar puncture can not be performed because of raised intracranial pressure (likely due to edema or concomitant brain abscess), a broad spectrum intravenous antibiotic should be started immediately (this is often a third generation cephalosporin). When cerebrospinal fluid gram stain, or blood or CSF culture and sensitivity results, are available, the empiric treatment can be refined by switching to more specific antibiotics. In children (but not in adults) the administration of steroids helps reduce the incidence of deafness following meningitis.

Infection of the meninges usually originates through spread from infection of the neighbouring structures (which include the sinuses and mastoid cells of ear). These should be investigated when diagnosis of meningitis is confirmed or suspected.

Convulsions are a known complication of meningitis and are treated with appropriate anti-seizure drugs such as phenytoin.