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Infections of the nervous system are classified according to the infected tissue into (1) meningeal infections (meningitis), which may involve the dura primarily (pachymeningitis) or the pia-arachnoid (leptomeningitis); and (2) infections of the cerebral and spinal parenchyma (encephalitis or myelitis). In many cases, both the meninges and the brain parenchyma are affected to varying degrees (meningoencephalitis).

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Acute Leptomeningitis

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Acute leptomeningitis is an acute inflammation of the pia mater and arachnoid. Most cases are caused by infectious agents; rarely, release of keratinaceous contents from an intradural epidermoid cyst or teratoma causes a chemical meningitis. When the term meningitis is used without qualification, it means leptomeningitis.

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Classification

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Acute meningitis may be classified according to etiology.

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Acute Bacterial Meningitis
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The incidence of bacterial meningitis in the United States is five to ten cases per 100,000 persons per year. Approximately 2000 deaths are reported per year. The bacterium involved varies with the age of the patient and other factors (Table 63-1). About 70% of all cases occur in children under 5 years of age.

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Table Graphic Jump Location
Table 63–1. Etiologic Agents in Bacterial Meningitis.
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Neonatal meningitis is acquired during passage of the fetus through the birth canal. Organisms found in the maternal vagina, commonly Escherichia coli and Streptococcus agalactiae (a group B streptococcus), are responsible.

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In children up to 5 years of age, the most common pathogen causing meningitis is Haemophilus influenzae. In adolescents, Neisseria meningitidis (meningococcus) is the most common cause. Streptococcus pneumoniae (pneumococcus) causes meningitis in all age groups. Listeria monocytogenes and gram-negative bacilli are important causes in older, debilitated, and immunosuppressed patients.

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Acute Viral Meningitis
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Viral meningitis has an incidence of 10,000 cases per year in the United States, and 90% of these occur in patients under 30 years of age. This is a mild, benign illness, which rarely causes ...

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