USMLE – Other Infectious Diseases of the Nervous System
Protozoal diseases (including malaria, toxoplasmosis, amebiasis, and trypanosomiasis), rickettsial infections (such as typhus and Rocky Mountain spotted fever), and metazoal diseases (especially cysticercosis and echinococcosis) may also involve the CNS.
Cerebral toxoplasmosis has assumed greater importance with the AIDS epidemic. Infection of the brain by Toxoplasma gondii is one of the most common causes of neurologic symptoms and morbidity in patients with AIDS. The average incidence of CNS infection in most clinical and autopsy series ranges from 4% to 30%. The clinical symptoms are subacute, evolving during a 1- or 2-week period, and may be both focal and diffuse. CT and MRI studies may show multiple ring-enhancing lesions; however, this radiographic appearance is not pathognomonic, since similar finding may be associated with CNS lymphoma, tuberculosis, or fungal infections.
The brain shows abscesses, frequently multiple, most involving the cerebral cortex (near the gray-white junction) and deep gray nuclei, less often the cerebellum and brain stem, and rarely the spinal cord. Acute lesions consist of central foci of necrosis with variable petechiae surrounded by acute and chronic inflammation, macrophage infiltration, and vascular proliferation. Both free tachyzoites and encysted bradyzoites may be found at the periphery of the necrotic foci. The organisms are usually seen on routine H&E or Giemsa stains, but they can be more easily recognized by immunocytochemical methods. The blood vessels in the vicinity of these lesions may show marked intimal proliferation or even frank vasculitis with fibrinoid necrosis and thrombosis. After treatment, the lesions consist of large, well-demarcated areas of coagulation necrosis surrounded by lipid-laden macrophages. Cysts and free tachyzoites can also be found adjacent to these lesions but may be considerably reduced in number if therapy has been effective. Chronic lesions consist of small cystic spaces containing small numbers of lipid- and hemosiderin-laden macrophages with surrounding gliosis. Organisms are difficult to detect in these older lesions.
Like CMV encephalitis, toxoplasmosis may also occur in the fetus. Primary maternal infection with toxoplasmosis, particularly if it occurs early in the pregnancy, may be followed by a cerebritis in the fetus, with the production of multifocal cerebral necrotizing lesions that may calcify, producing severe damage to the developing brain.
A rapidly fatal necrotizing encephalitis occurs with infection with Naegleria species, and a chronic granulomatous meningoencephalitis has been associated with infection with Acanthamoeba. The amebae may sometimes be difficult to distinguish from histiocytes. Methenamine silver or PAS stains are helpful in visualizing the organisms, although definitive identification ultimately depends on combined immunofluorescence studies, morphology, and culture.