Fig.23.16A ).Themeningealvesselsareengorgedandprominent,andtractsofpusmayextendalongbloodvessels.Onmicroscopicexamination,neutrophilsmayfilltheentiresubarachnoidspaceor,inlessseverecases,maybeconfinedtoregionsadjacenttoleptomeningealbloodvessels.Inuntreatedmeningitis,Gramstainrevealsvaryingnumbersofthecausativeorganism.Severeinvolvementofleptomeningealveins(phlebitis)mayleadtovenousocclusionandhemorrhagicinfarctionoftheunderlyingbrain.Whenthemeningitisisfulminant,theorganismsandtheassociatedinflammatorycellsmayspreadintothesubstanceofthebrain(focalcerebritis),sometimesleadingtotheformationofabscesses,discussedlater(see
Fig.23.16B ).Extensiontotheventricles(ventriculitis)alsomayoccur.Iftreatedearlyandeffectively,resolvedmeningitismayleavelittleornoresiduum.
Aseptic meningitis is a clinical term for acute illness with meningeal signs and symptoms that is believed to be of viral origin. The clinical course is less fulminant than in pyogenic meningitis and is typically self-limiting. In contrast to pyogenic meningitis, examination of the CSF often shows lymphocytosis, moderate protein elevation, and a normal glucose level, but bacteria cannot be cultured. While an underlying viral agent is suspected, it is often difficult to identify the responsible virus by culture and serologic methods; application of nucleic acid sequencing has the potential to change our understanding of these cases. There are no distinctive macroscopic findings except for brain swelling, which may be seen in only some instances. On microscopic examination, there is either no recognizable abnormality or a mild to moderate leptomeningeal lymphocytic infiltrate.
Several pathogens, including mycobacteria, some spirochetes, and fungi, cause a chronic meningitis; infections with these organisms also may involve the brain parenchyma.
Tuberculous meningitis usually manifests with generalized signs and symptoms of headache, malaise, mental confusion, and vomiting. There is only a moderate increase in CSF cellularity, with mononuclear cells or a mixture of polymorphonuclear and mononuclear cells; the protein level is elevated, often strikingly so, and the glucose content typically is moderately reduced or normal. Infection with Mycobacterium tuberculosis also may result in a well-circumscribed intraparenchymal mass (tuberculoma), which may be associated with meningitis. Chronic tuberculous meningitis leads to arachnoid fibrosis and hydrocephalus from interference with resorption of CSF.
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Fig. 23.16 Bacterialinfections.(A)Pyogenicmeningitis.Athicklayerofsuppurativeexudatecoversthebrainstemandcerebellumandthickenstheleptomeninges.(B)Cerebralabscessesinthefrontallobewhitematter (arrows). (A, From Golden JA, Louis DN: Images in clinical medicine: acute bacterial meningitis. NEnglJMed333:364, 1994. Copyright © 1994 Massachusetts Medical Society. All rights reserved.)
Neurosyphilis, a tertiary stage of syphilis, occurs in about 10% of individuals with untreated Treponema pallidum infection. Infection with HIV increases the risk for developing neurosyphilis, and the disease is often more aggressive in this setting. There are several patterns of CNS involvement by syphilis, which may be present alone or in combination.
• Meningovascular neurosyphilis is a chronic meningitis, usually involving the base of the brain, often with an obliterative endarteritis rich in plasma cells and lymphocytes.