Abscesses are discrete destructive lesions with central liquefactive necrosis surrounded by a rim of vascularized granulation and fibrous tissue (see
Fig. 23.16B ). Outside the fibrous capsule is a zone of reactive gliosis. Patients almost invariably present with progressive focal deficits as well as general signs related to increased intracranial pressure. While CSF white blood cell count and protein may be elevated, lumbar puncture has little role in the diagnosis of brain abscess since organisms are more reliably cultured by draining the abscess directly. In addition, if there is significant mass effect from the abscess, the decrease in lumbar CSF pressure following lumbar puncture can result in downward herniation of cerebral contents and neurologic deterioration. A systemic or local source of infection may or may not be apparent. The increased intracranial pressure may cause fatal brain herniation, and abscess rupture can lead to ventriculitis, meningitis, and venous sinus thrombosis. Surgery and antibiotics reduce the otherwise high mortality rate, with earlier intervention leading to better outcomes.
Viral encephalitis is a parenchymal infection of the brain that is almost invariably associated with meningeal inflammation (meningoencephalitis). While different viruses show varying patterns of injury, the most characteristic histologic features are perivascular and parenchymal mononuclear cell infiltrates, microglial nodules, and neuronophagia (
Fig. 23.18A–B ). Certain viruses also form characteristic inclusion bodies.
The nervous system is particularly susceptible to certain viruses such as rabies virus and poliovirus. Some viruses infect specific CNS cell types, while others preferentially involve particular brain regions (such as the medial temporal lobes, or the limbic system) that lie along the viral route of entry. Intrauterine viral infection following transplacental spread of rubella and CMV may cause destructive lesions, and Zika virus causes developmental abnormalities of the brain. In addition to direct infection of the nervous system, the CNS also can be injured by immune mechanisms after systemic viral infections.
Arboviruses (arthropod-borne viruses) are an important cause of epidemic encephalitis, especially in tropical regions of the world, and are capable of causing serious morbidity and high mortality. Among the more commonly encountered types are Eastern and Western equine encephalitis and West Nile virus infection. Patients develop generalized neurologic symptoms, such as seizures, confusion, delirium, and stupor or coma, as well as focal signs, such as reflex asymmetry and ocular palsies. The CSF usually is colorless but with a slightly elevated pressure and an early neutrophilic pleocytosis that rapidly converts to a lymphocytosis; the protein concentration is elevated, but the glucose is normal. Thus, CSF examination helps to distinguish viral from bacterial infections of the CNS.
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Fig. 23.18 Viralinfections.(AandB)Characteristicfindingsinmanyformsofviralmeningitisincludeperivascularcuffingoflymphocytes(A)andmicroglialnodules(B).(C)Herpesencephalitisshowingextensivedestructionofinferiorfrontalandanteriortemporallobes.(D)Humanimmunodeficiencyvirus(HIV)encephalitis.Notetheaccumulationofmicrogliaformingamicroglialnoduleandmultinucleategiantcell.(C, Courtesy of Dr.T.W. Smith, University of Massachusetts
Medical School,Worcester, Massachusetts.)
MORPHOLOGYArbovirusencephalitidesproduceasimilarhistopathologicpicture.Characteristically,thereisaperivascularlymphocyticmeningoencephalitis(sometimeswithneutrophils)(see
Fig.