When the AIDS pandemic first appeared in North America, doctors were puzzled by the sight of previously healthy young men who had inexplicably developed one or more life-threatening infections. They quickly established that these infections flourished because their patients had severely weakened immune systems.
The cause of this apparent immune deficiency would remain mysterious until 1983, when researchers at the Institut Pasteur in Paris, France, would first isolate HIV from the lymph nodes of an affected patient.
In addition to the life-threatening infections and relentless weight loss that soon became the hallmark of AIDS, doctors also noticed that some of their patients had complex neurological problems that affected certain functions, such as:
In extreme cases, other problems occurred, including:
At first, researchers thought that some of these problems arose as consequences of the life-threatening infections that had spread to the brain. However, when these neurologic problems were seen in people who had HIV infection but not AIDS, researchers sought other culprits. Eventually researchers found that HIV, either directly or indirectly, was responsible for many of these problems.
HIV does not appear to infect brain cells. However, this virus does infect cells of the immune system -- such as T-cells, monocytes and macrophages -- that can enter and leave the brain as well as other cells (such as microglia) that reside within the brain.
These cells of the immune system, particularly macrophages, do not quickly die once infected with HIV. Rather, they survive and rove around the brain and body, spewing HIV and HIV-related proteins as well as chemical signals that incite inflammation. Together, these products -- HIV, viral proteins and inflammatory signals -- cause brain cells to become dysfunctional and, in some cases, die.
In the time before potent combination therapy for HIV (commonly called ART or HAART) was available, the severe loss of intellectual functioning was called HIV-related dementia and its devastating impact on personality, behaviour and ultimately survival was greatly feared by HIV-positive people.
Beginning in 1996, ART became widely available in Canada and other high-income countries, and HIV-related dementia became and has remained relatively uncommon today.
There have been isolated reports of a severe decline in neurocognitive function among a small number of HIV-positive people who have been taking ART. In most of these cases, HIV is often well suppressed in blood samples but not in samples of the fluid that bathes the brain and spinal cord -- cerebrospinal fluid, or CSF. In such cases, doctors may change a person's regimen to include more drugs that can enter the CSF and suppress HIV in that part of the body, often resulting in improved neurocognitive function.
Researchers are still trying to understand why there are sometimes differences in the amount of HIV produced in the brain and blood despite the use of effective ART. As yet, researchers have not reached a consensus about the best combination of anti-HIV drugs to ensure suppression of HIV in the brain, perhaps because results from clinical trials to explore this effect have produced somewhat confusing results.
Although ART can greatly suppress production of HIV in the body and brain, it does not cure HIV infection. Also, ART is unable to completely suppress HIV-related inflammation. Some researchers fear that over many years exposure to chronic low-level inflammation has the potential to make many of the body's organ-systems more susceptible to age-related decline; one of those organs may be the brain. However, firm evidence for this remains elusive. Moreover, despite the use of ART, researchers have identified mild neurocognitive issues in some HIV-positive people. Long-term studies need to be done to find out if mild neurocognitive problems continue or are reversed over time with prolonged ART. Fortunately, mild neurocognitive impairment does not seem to affect people's ability to do routine tasks associated with self-care.
In addition to crossword puzzles and numeracy games such as Soduku, some HIV-positive people have found so-called brain-training exercises very useful in improving their memory. These exercises appear to work by stimulating and improving parts of the brain involved in information processing and memory. Results from clinical trials in HIV-negative people suggest that brain-training exercises can improve memory, speech (by helping people to remember and use more words) and speed of thinking. Although large controlled clinical trials of brain-training exercises are lacking in HIV-positive people, some have found them useful.
For more detailed information on brain-training exercises, see this very interesting article in The Positive Side by Maggie Atkinson.
Neurologist Bruce Brew (Sydney, Australia) and colleagues who have done extensive research on HIV's effect on the brain note that there are no data from large randomized, controlled clinical trials to assess different ways of maintaining or improving brain function in HIV-positive people. However, he suggests that based on research with HIV-negative people, there are steps that people with HIV can take that may be "helpful in minimizing the effects of aging as well as perhaps delaying the onset of [neuro-degenerative conditions such as Alzheimer's disease]." These steps include:
Brain studies are complex, in part because scientists do not fully understand how this impressive organ works. Also, for obvious reasons, scientists are not able to routinely remove brain tissue from people. So, instead, neurologic research sometimes involves using brain tissue from animals -- with monkeys, rats and cats being particularly popular with neuroscientists. Other aspects of brain research involve sophisticated scans of the human brain, particularly magnetic resonance imaging (MRI). Still other studies probe the functioning of the brain by having volunteers undergo written, spoken and pictorial assessments. Some studies involve a spinal tap, where a small amount of the fluid that bathes the brain and spinal cord is removed for analysis. A minority of studies involves analysis of human brain samples collected at autopsy.
All of these different ways of trying to find out what is happening within the brain are complicated and time consuming and require skilled technicians and scientists to interpret the results. Brain research, like much bio-medical research, is expensive. This means that the imaging and other assessments mentioned in this report are not routinely used as part of health monitoring for the average person.
In an attempt to get around some of these obstacles, some researchers have tried to assess blood samples to see if this can provide some insight into the health of the brain.
In this issue of TreatmentUpdate, we explore several recent developments concerning HIV's impact on the brain and possible future avenues to reduce this impact.