It's an Achilles' heel of HIV therapy: The AIDS virus can sneak into the brain to cause dementia, despite today's best medicines.

Now scientists are beginning to test drugs that may protect against the memory loss and other symptoms of so-called neuroAIDS, which afflicts at least one in five people with HIV and is becoming more common as patients live longer.

With almost 1 million Americans, and almost 40 million people worldwide, living with HIV, that's a large and underrecognized toll.

"That means HIV is the commonest cause of cognitive dysfunction in young people worldwide," says Dr Justin McArthur, vice-chairman of neurology at Baltimore's Johns Hopkins University, who treats neuroAIDS. "There's no question it's a major public health issue."

While today's most powerful anti-HIV drugs do help by suppressing levels of the virus in blood so that there's less to continually bathe the brain they can't cure neuroAIDS. Why? HIV seeps into the brain very soon after someone is infected, and few anti-HIV drugs can penetrate the brain to chase it down.

"Despite the best efforts of (anti-HIV) therapy, brain is failing," says Dr Harris Gelbard, a neurologist at the University of Rochester Medical Center. He is part of a major new effort funded by the National Institutes of Health to find the first brain-protecting treatments.

What's now called neuroAIDS is much different from the AIDS dementia of the epidemic's early years, when patients often had horrific brain symptoms similar to end-stage Alzheimer's, unable to move or talk. They'd die within six months.

Today, anti-HIV medication has resulted in a more subtle dementia that strikes four years or more before death: At first, patients forget phone numbers and their movements slow. They become less able to juggle multiple tasks.

Some worsen until they can't hold a job or perform other activities, but not everyone worsens and doctors can't predict who will. In a vicious cycle, the memory loss makes many forget their anti-HIV pills, so the virus rebounds. Gelbard estimates that neuroAIDS reduces patients' mental function by 25%.

If HIV patients live long enough, many specialists worry, nearly all of them may suffer at least some brain symptoms. "They're living longer with HIV in the brain," explains Kathy Kopnisky of the NIH's National Institute of Mental Health, which is spending about $60 million investigating neuroAIDS.