D. ENCEPHALITIS AND ENCEPHALOMYELITIS

1. Pathogenesis:

a. Hematogenous spread after viremia

b. Spread along nerves (rabies, Herpes simplex)

2. Etiologic Agents:

a. Usually viral: important organisms include HIV, polio, rabies, Herpes simplex, Herpes zoster, equine encephalitis and St. Louis encephalitis viruses.

b. Rickettsia (typhus and Rocky Mountain spotted fever) and protozoa (toxoplasma) also cause a diffuse inflammatory reaction in the central nervous system.

c. Bacterial invasion of the CNS typically results in abscess formation, not encephalitis or encephalomyelitis.

3. Pathological Changes:

a. General description: Multiple and/or widespread areas of CNS involvement occur, as opposed to localized nature of abscess formation. Specific viruses affect specific anatomic areas or subpopulations of cells, a phenomenon called "tropism". An example of this is the predilection of herpes simplex virus to produce lesions in the limbic system, and infection of lower motor neurons by poliovirus. Microscopic changes include: perivascular cuffing by lymphocytes and plasma cells; neuronal necrosis; inclusion bodies; microglial proliferation and glial nodules; hemorrhagic necrosis (common in Herpes simplex encephalitis). Calcification can be detected in some neonates infected in utero with encephalitis-producing agents like CMV and HIV viruses.

b. Examples:

Arthropod-borne viral encephalitis is responsible for most outbreaks of epidemic viral encephalitis. Most of these viruses have animal hosts and mosquito vectors. The pathology due to different viruses is similar, although the severity and area affected (cortex or basal ganglia) varies.

Herpes Simplex Virus Type 1 (HSV-1) produces the most common non-epidemic form with high mortality levels. Acyclovir is an effective treatment, leading to reduction in the mortality rate. The most common presenting symptoms are alterations in mood, memory and behavior, due in part to lesions in the limbic system. Lesions involve the inferior and medial regions of the temporal lobes, and the orbital gyri of the frontal lobes. Pathological changes include necrosis and hemorrhagic regions. Perivascular inflammatory infiltrates and inclusion bodies are usually present.

This coronal section focuses on the cingulate gyrus showing patchy hemorrhagic necrosis in a case of herpes simplex encephalitis.

Rabies is transmitted by the bite of a rabid animal. The virus enters the CNS by ascent along peripheral nerves. Microscopic changes include Negri bodies, pathognomonic findings that consist of inclusions in pyramidal neurons of the hippocampus.

This high power microscopic image shows intracytoplasmic Negri bodies in rabies.

4. Spinal Fluid Changes:

Varying numbers of lymphocytes, usually some increase in protein; glucose usually normal.

5. Complications:

Generalized cerebral/spinal cord edema, with space-occupying effects

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