Infectious Diseases

Bacterial Infections Viral Infections Mycotic Infections Syphilitic Infections Parasitic Infections Miscellaneous Infections

Infectious diseases of the central nervous system (CNS) are particularly important because of their increasing incidence and high fatality. Major predisposing factors to infections are diseases and therapies associated with immunosuppression: notably, AIDS, malignancies, poorly controlled diabetes, long-term antibiotic therapy, high doses of corticosteroids, cytotoxic and immunosuppressive drugs, and organ and bone marrow transplants. Blood transfusions and the intravenous use of illicit substances are important sources of infections (Table 6.1). Some diseases occur sporadically; others occur in epidemics. A great variety of pathogens can infect the nervous system.

The assessment of an infectious disease should include the following points: (a) the portal of entry of the pathogen; (b) the route to the neural tissue; and (c) the characteristic inflammatory response of the neural tissue to a particular pathogen.

Some microorganisms are detected in histologic sections stained with hematoxylin-eosin (HE), but most are only visualized using special stains. Other microorganisms are identified only in tissue cultures or by techniques based on polymerase chain reaction (PCR) or using immunologic stains.

Some pathogens evoke an acute inflammation, presenting with a sudden onset and rapid progression of clinical symptoms. Others evoke a slowly evolving inflammation over several weeks, months, or even years, presenting a protracted clinical course. Still other pathogens are opportunistic—they remain dormant in the nervous tissue and only become pathogenic under an altered immune state in the host.

The inflammation may resolve with no residual tissue damage, resulting in complete recovery. But, in severe cases, residual necrosis or scarring leave the patient with neurologic and psychiatric deficits.

TABLE 6.

Risk Factors for CNS Infections

Immunosuppression by:

Diseases

Acquired immunodeficiency syndrome (AIDS)

Malignancies

Primary immunodeficiency states

Therapies

Chemotherapies,

Corticosteroid therapy

Solid organ and bone marrow transplantations

Radiation therapy

Antiviral, antibacterial therapies

Blood transfusion

Intravenous abuse of illicit substances

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