Aug. 28 (HealthDay News) -- The subtype of HIV a person has may determine their odds for progressing to AIDS-linked dementia, Johns Hopkins researchers report.

Cognitive difficulties, even dementia, are a common hallmark of HIV infection. But during their work in Africa, researchers led by Dr. Ned Sacktor noted that people in areas where HIV subtypes A and D were predominant seemed to have especially high rates of dementia.

To learn more, Sacktor and colleagues tracked rates of dementia in 60 HIV-positive patients at a clinic in Kampala, Uganda. All patients had not yet begun to receive HIV medications.

Reporting in the September issue of Clinical Infectious Diseases, the team found that seven of 33 patients with HIV subtype A had dementia (24 percent), but of the nine patients infected with subtype D, eight had dementia (89 percent).

"We were amazed to see such a dramatic difference in dementia frequencies between these two subtypes," Sacktor says. "If this is the case in all of sub-Saharan Africa, HIV-associated dementia may be one of the most common, but thus far unrecognized, dementias worldwide."

The researchers speculated that some biological property -- such as an increased ability of the virus to cause inflammation or injury in the brain -- may cause certain subtypes of HIV to lead to dementia more often. An investigation to determine whether that might be the case is currently underway, they said.

SOURCE: Johns Hopkins Medicine, news release, Aug. 28, 2009