Several neurological complications can occur in AIDS patients. These can be subdivided into diseases induced by HIV itself, by opportunistic diseases or by medication.

Cryptococcosis is a cosmopolitan infectious disease caused by a yeast, Cryptococcus neoformans. There are two varieties: C. neoformans var. neoformans and C. neoformans var. gattii. These differ in their geographical distribution, ecology and biochemical characteristics. The antigenic specificity of the polysaccharide capsule determines four serotypes: A, B, C and D. Serotypes A and D are found worldwide in bird droppings (avian excreta), especially of pigeons. Serotypes B and C are geographically associated with certain Eucalyptus trees. C. n. neoformans occurs worldwide, whereas C. n. gattii is restricted to the tropics and subtropics. In Australia Eucalyptus camadulensis trees form the natural habitat for the var. gattii. The typical vegetative form of C. neoformans is a yeast with a diameter of 2.5 to 10 µm. The organism can also reproduce sexually. As it is a basidiomycete (Filobasidiella neoformans) it forms sexual spores: “basidiospores”. [Basidiospore: Gr: “basidon”: small base and “sporon”: seed. This indicates the morphology: a club-shaped cell with the haploid spores at the far end.] Infection can probably occur as a result of inhalation of either dehydrated yeast form or basidiospores. Cryptococcus neoformans primarily causes a chronic meningitis. Systemic infections, inflammation of the lungs and cutaneous lesions also occur as a result of these yeasts. The demonstration of cryptococcal meningitis in a patient is at present considered proof of HIV infection and AIDS.

The clinical picture can be very atypical. Mild fever, headache, confusion and emaciation can be found in patients with cryptococcal meningitis. Neck stiffness is present in less than half of infected persons. Photophobia occurs in 25%. Confusion and coma occur later. Focal signs are seldom observed. Blindness can occur (possibly due to concomitant arachnoiditis), although this is less frequent in HIV patients than in immunocompetent individuals with cryptococcal meningitis. The diagnosis of a cryptococcal meningitis is made by centrifuging a few ml of cerebrospinal fluid and mixing the sediment with an equal quantity of East Indian ink. The yeasts are recognized quite easily as round organisms with a thick capsule. The saccharide capsule can be detected via an antigen detection test (latex agglutination test for serum, cerebrospinal fluid and urine). The encapsulated organism can likewise be detected in tissue biopsies.