Fig. 23.15 ). When severe enough to involve the gray matter and white matter, large cystic lesions can develop throughout the hemispheres, a condition termed multicystic encephalopathy.
Certain infections, which may be acquired transplacentally or during birth, have a propensity to cause destructive brain lesions. Agents in this group (commonly referred to as TORCH) include toxoplasmosis and CMV, which can both result in parenchymal calcifications along with the tissue injury; these are discussed in more detail later.
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Fig. 23.15 Perinatalbraininjury.Thisspecimenfromapatientwithperiven-tricularleukomalaciacontainsacentralfocusofwhitematternecrosiswithaperipheralrimofmineralizedaxonalprocesses.
Malformationsofthebraincanoccurbecauseofgeneticfactorsorexternalinsults.
Thedevelopmentaltimingandpositionoftheinjurydetermineitspatternandcharacteristics.
Variousmalformationsstemfromfailureofneuraltubeclosure,improperformationofneuralstructures,andalteredneuronalmigration.
Perinatalbraininjurymostlytakesoneoftwoforms:(1)hemorrhage,oftenintheregionofthegerminalmatrixwiththeriskforextensionintotheventricularsystem;and(2)ischemicinfarcts,leadingtoperiventricularleukomalacia.
Certaininfectionsmayaffectthebrainduringdevelopmentwithsubsequenttissuedestructionandneurologicconsequences.
The brain and its coverings can be sites of infection. Some infectious agents have a relative or absolute predilection for the nervous system (e.g., rabies), while others affect many other organs as well as the brain (e.g., Staphylococcus aureus). Damage to nervous tissue may be the consequence of direct injury of neurons or glial cells by the infectious agent or microbial toxins, or may be a consequence of the host immune response.
Infectious agents may reach the nervous system through several routes: • Hematogenous spread by way of the arterial blood supply is the most common means of entry. There can also be retrograde venous spread through the anastomoses between the veins of the face and the venous sinuses of the skull.
Direct implantation of microorganisms is almost invariably due to open or penetrating trauma; rare cases can be iatrogenic, as when microbes are introduced with a lumbar puncture needle or into a surgical field.
Local extension can occur with infections of the skull or spine. Sources include air sinuses, most often the mastoid or frontal; infected teeth; cranial or spinal osteomyelitis; and congenital malformations, such as meningomyelocele.
Peripheral nerves also may serve as paths of entry for a few pathogens—in particular, viruses such as rabies and herpes zoster.
Basic mechanisms of injury associated with infectious agents are discussed in Chapter 9; in the following sections we discuss infections that are specific to the central nervous system (
Table 23.2