Toxoplasma gondii, a protozoan, is the most common opportunistic infection in AIDS patients, accounting for between 13.4% and 33% of all CNS complications. The characteristic MR appearance is multiple ring-enhancing lesions located at the cortical-medullary junction, but the basal ganglia and white matter are also frequently involved. The amount of peripheral edema is variable. Earlier in the evolution of the abscesses, the nodule may exhibit more homogeneous enhancement with little mass effect or edema. In general, toxo and fungal abscesses evolve more slowly than bacterial ones, but in immunocompromised individuals they can be quite aggressive. Dual infections are common is AIDS patients. In such cases, invariably one of the pathogens is toxoplasma gondii. Toxoplasmosis may also co-exist with lymphoma. Another confounding fact is that the inflammatory reaction to toxoplasmosis may mimic lymphoma on biopsy or CSF cytology.

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