Writing about their findings in the journal Clinical Infectious Diseases, the researchers claimed that theirs is the first study to have shown that the specific type of HIV has any effect on cognitive impairment, one of the most common complications of uncontrolled HIV infection.
In their study report, the researchers highlight the fact that HIV occurs in multiple forms, distinguished by small differences in the virus' genetic sequence and designated by letters A through K.
They point out that certain subtypes appear to cluster in particular areas of the world, which others have been associated with different rates of progression to full blown AIDS.
They also revealed that the majority of the 35 million HIV-infected people worldwide live in sub-Saharan Africa, where subtypes A, C and D dominate.
According to them, nearly half of patients with advanced HIV infections have at least mild cognitive impairments, and about 5 percent have the severe form of cognitive impairment, known as dementia.
Dr. Ned Sacktor, professor of neurology at the Johns Hopkins University School of Medicine and a clinician at the Johns Hopkins Bayview Medical Center, previously found that about 31 percent of patients visiting an infectious disease clinic in the Ugandan capital, Kampala, where subtypes A and D dominate, had dementia.
The finding led him to wonder whether patients with different subtypes had different rates of dementia.
He and his colleagues then studied 60 HIV-infected patients from a Kampala clinic, all of whom had been part of a different study testing the effect of anti-retroviral drugs on cognitive impairment, but had not begun taking the drugs.
After determining each patient's HIV subtype, the researchers performed a battery of neurological and cognitive tests to assess each patient's brain function.
They observed that the majority of the patients had HIV subtypes A or D.
Further analysis by the researchers revealed that, of the 33 subtype A patients, seven had dementia, or about 24 percent.
However, out of the nine patients with subtype D, 8 had dementia, about 89 percent.
"We were amazed to see such a dramatic difference in dementia frequencies between these two subtypes. 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," Sacktor says.
He says that the study indicates that some biological property of each subtype seems to influence the likelihood that infected patients will develop dementia.
Based on their findings, his team hypothesize that subtype D may cause more inflammation and injury in the brain, a possibility they are currently investigating.