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Meningitis overview

Infections of the central nervous system (CNS) can be divided into 2 broad categories: those primarily involving the meninges (meningitis) and those primarily confined to the parenchyma (encephalitis).

See the images of meningitis below.

Pneumococcal meningitis in a patient with alcoholism. Courtesy of the CDC/Dr. Edwin P. Ewing, Jr. Acute bacterial meningitis. This axial nonenhanced computed tomography scan shows mild ventriculomegaly and sulcal effacement Acute bacterial meningitis. This axial T2-weighted magnetic resonance image shows only mild ventriculomegaly. Acute bacterial meningitis. This contrast-enhanced, axial T1-weighted magnetic resonance image shows leptomeningeal enhancement (arrows).

Meningitis is a clinical syndrome characterized by inflammation of the meninges, 3 layers of membranes that enclose the brain and spinal cord. They consist of the following:

• Dura - A tough outer membrane
• Arachnoid - A lacy, weblike middle membrane
• Subarachnoid space - A delicate, fibrous inner layer that contains many of the blood vessels that feed the brain and spinal cord

Risk factors for meningitis include the following:

• Age of 60 years or greater
• Age of 5 years or less
• Diabetes mellitus, renal or adrenal insufficiency, hypoparathyroidism, or cystic fibrosis
• Immunosuppression, which increases the risk of opportunistic infections and acute bacterial meningitis
• Human immunodeficiency virus (HIV) infection, which predisposes to bacterial meningitis caused by encapsulated organisms, primarily S pneumoniae, and opportunistic pathogens
• Crowding (eg, military recruits and college dorm residents), which increases the risk of outbreaks of meningococcal meningitis
• Splenectomy and sickle cell disease, which increase the risk of meningitis secondary to encapsulated organisms
• Alcoholism and cirrhosis
• Recent exposure to others with meningitis, with or without prophylaxis
• Contiguous infection (eg, sinusitis)
• Dural defect (eg, traumatic, surgical, congenital)
• Thalassemia major
• Intravenous (IV) drug abuse
• Bacterial endocarditis
• Ventriculoperitoneal shunt
• Malignancy (increased risk of Listeria species infection)
• Some cranial congenital deformities

Meningitis is anatomically divided into inflammation of the dura, sometimes referred to as pachymeningitis, which is less common, and leptomeningitis, which is more common and is defined as inflammation of the arachnoid tissue and subarachnoid space. (See Anatomy.)

Meningitis can also be divided into the following 3 general categories:

• Pyogenic (bacterial)
• Granulomatous
• Lymphocytic

The most common cause of meningeal inflammation is irritation caused by bacterial or viral infections. The organisms usually enter the meninges through the bloodstream from other parts of the body. Most cases of bacterial meningitis are localized over the dorsum of the brain; however, under certain conditions, meningitis may be concentrated at the base of the brain, such as with fungal diseases and tuberculosis. (See Etiology.)

Pyogenic (bacterial) meningitis consists of inflammation of the meninges and the underlying subarachnoid CSF. If not treated, bacterial meningitis may lead to lifelong debility or death.^{[5] [6]} The disease was uniformly fatal before the antimicrobial era, but with the advent of antimicrobial therapy, the overall mortality rate from bacterial meningitis has decreased. Nonetheless, it remains alarmingly high, being approximately 25%. (See Epidemiology.)

The emergence of resistant bacterial strains has prompted changes in antibiotic protocols in some countries, including the United States. Apart from dexamethasone, neuronal cell protectants still hold only future promise as adjunctive therapy. (See Treatment and Management and Medication.)

The specific infective agents that are involved in bacterial meningitis vary among different patient age groups, and the inflammation may evolve into the following conditions (see the images below):

• Ventriculitis
• Empyema
• Cerebritis
• Abscess formation Chronic mastoiditis and epidural empyema in a patient with bacterial meningitis. This axial computed tomography scan shows sclerosis of the temporal bone (chronic mastoiditis), an adjacent epidural empyema with marked dural enhancement (arrow), and the absence of left mastoid air. Subdural empyema and arterial infarct in a patient with bacterial meningitis. This contrast-enhanced axial computed tomography scan shows left-sided parenchymal hypoattenuation in the middle cerebral artery territory, with marked herniation and a prominent subdural empyema.

Meningitis can also be also classified more specifically according to its etiology. Numerous infectious and noninfectious causes of meningitis have been identified. Examples of common noninfectious causes include medications (eg, nonsteroidal anti-inflammatory drugs [NSAIDs], antibiotics) and carcinomatosis. (See Etiology.)

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