As many as 20 percent of people with HIV may be experiencing problems with the brain's executive secretary: remembering, carrying out complex tasks, organizing paperwork, keeping track of the endless minutiae of a day. And the tasks they need to do to stay alive -- filling prescriptions and swallowing pills on a strict schedule -- are potentially threatened. Doctors are worried that HIV patients, surviving longer thanks to effective treatments, will experience dementia at ages far younger than is customary.

Tests can often pinpoint difficulties that might not yet be obvious: subtle problems with memory, word retrieval, attention and the ability to follow complex directions. As the situation worsens, problems become obvious in everyday chores, missed appointments, lost work and forgotten names.

HIV infects monocytes, common blood cells that can move through the brain's protective membrane, triggering inflammation that changes brain cell physiology. HIV itself can also pass into the brain. Treatment can partially suppress HIV in the brain, said Dr. Gary Blick, who has tested dozens of medicines used in highly active antiretroviral therapy (HAART). But regimens can also come with side effects, including cognitive problems.

"It's hard to tease out what's going on," said Dr. Marshall Forstein, medical director of mental health and addiction services at Fenway Community Health. Doctors often overlook memory and thinking problems, or blame them on the stress of the illness. They could also be a signal for depression.

Cognitive decline also has an effect on the drug-taking regimen. Dr. Justin C. McArthur of Johns Hopkins University School of Medicine studied patients with cognitive problems, half of whom were given beepers that prompted "Time to take your medicine" twice a day. Those who were reminded took three times the number of pills as those who did not have the beeper, McArthur said. The study is being repeated using reminders via cell phones.

Examined at autopsy, HIV dementia's damage is to the basal ganglia, the same area affected in Parkinson's disease. Parkinson's destroys the cells; in AIDS dementia the problem is massive inflammation. Indeed, McArthur and colleagues have been testing the benefits of selegiline, a Parkinson's drug that blocks the inflammatory process.

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