Many studies of HIV's impact on the brain have been conducted since 1996 when ART became widely available in high-income countries. In these studies, researchers have enrolled HIV-positive people who had serious symptoms, such as AIDS, arising from a weakened immune system. It is therefore possible that such studies produced a skewed profile of the impact of HIV infection on the brain, perhaps portraying excessive damage.
To uncover what HIV does to the brain, it is important to study a wide variety of people, including people with HIV who have not had serious symptoms such as those seen in AIDS. Two studies have focused on HIV-positive people who have minimal symptoms of disease. One suggested that the rate of neurocognitive impairment in symptom-free HIV-positive people is not different from that of HIV-negative people. Another study suggested that mild neurocognitive impairment is relatively common among symptom-free people. As a result of these conflicting results, some neuroscientists argue that it is not certain whether HIV can cause deterioration in neurocognitive ability among "medically stable" people who are free from HIV-related symptoms.
An additional concern faced when trying to assess neurocognitive impairment is the effect of aging. Some researchers suspect that the natural process of aging may intensify HIV's impact on the brain, and vice versa. Studies exploring this issue have produced mixed results.
In part, this mix of results arises from the confounding impact of co-morbidities in some older people, including depression, substance and alcohol abuse, cardiovascular disease and diabetes.
Researchers at King's College in London, UK, have conducted extensive neurocognitive assessments as well as MRI scans on 95 volunteers, some of whom were HIV positive. The researchers described the HIV-positive people as "medically stable." By this, they meant that not only were participants symptom free but that they were taking ART, had very low viral loads and relatively high CD4+ cell counts, and did not have a history of substance use or serious mental health issues.
The King's College team concluded that "HIV disease by itself does not significantly impair cognitive functions when patients are [free from symptoms of HIV disease and are medically stable]."
Researchers enrolled 95 gay and bisexual men. They were divided into the following four groups:
The health information gathered from each HIV-positive man was matched to that from an HIV-negative man of similar age and educational background.
Researchers did not enroll anyone who had any of the following diagnoses:
Researchers conducted extensive neurocognitive assessments, and blood tests were done to screen for many infections and conditions that could have an impact on neurocognitive assessments, such as diabetes, untreated thyroid disease and so on. High-resolution MRI scans were also done.
People with HIV infection did not have impaired neurocognitive function compared to HIV-negative people. HIV infection did not heighten age-related decline in neurocognitive function.
The researchers found that, in general, older people compared to younger people had some neurocognitive impairment, particularly affecting memory. This was considered a normal consequence of aging by the researchers.
High-resolution scans detected changes in some regions of the brains of older participants. Again, these were considered a normal consequence of the aging process.
HIV-positive people had slightly reduced gray matter in one part of the brain, the frontal gyrus.
The London researchers found that "in general, there was no significant [neurocognitive] impairment in our stable HIV-1 patient group." Furthermore, they stated that their findings "suggest that stable HIV-1 [symptom-free] participants with long-term suppression of viral load and CD4+ counts above 200 cells do not necessarily show cognitive decline."
The London team also states that previous studies that found neurocognitive impairment in HIV-positive people may not have taken into account factors such as alcohol and substance use, psychiatric conditions and other medical conditions.
According to the study team, in the present study participants had relatively high IQs and were in "good medical and psychiatric health." The researchers suggest that it is possible that that these factors may have played a role in protecting the men from neurocognitive degeneration.
Long-term studies are needed in order to learn what happens to such stable men as they age with HIV. Also, future studies need to include a broader range of HIV-positive people, including women.
If the results from the London study are confirmed, then dealing with co-morbidities that affect cognition (such as alcohol and substance use, metabolic problems such as diabetes and co-infections such as hepatitis C virus) may become more important to help reduce their impact on the brain.