A mystery that has baffled scientists for decades, the cause of the devastating memory loss suffered by people with Alzheimer's disease, may have been solved by researchers studying the biochemical changes in the brains of patients.

If the breakthrough is confirmed, it raises the possibility of developing drugs that can block the slow process of decline and might even lead to the reversal of memory loss in some Alzheimer's patients.

William Klein, professor of neurobiology and physiology at Northwestern University in Evanston, Illinois, said a small toxic protein in the brain appeared to be the key factor that destroyed memory.

"If we can develop drugs that target and neutralise these neurotoxins, it might be possible to not only slow down memory loss, but to actually reverse it, to bring memory function back to normal," Professor Klein said.

The announcement was made as researchers in Chicago and Atlanta said the US health system could be overwhelmed by the growth in the number of elderly Americans suffering from Alzheimer's. New estimates suggest the numbers affected will rise to 13.2 million by the middle of the century, three times the 4.5 million affected today.

For many years, medical researchers have known that the brains of Alzheimer's patients slowly become clogged with large proteins known as amyloid fibrils. This led to speculation that these protein clumps were responsible for the symptoms of the disease.

Professor Klein's study, published in the Proceeding of the National Academy of Sciences, indicates that the real culprits are much smaller protein molecules called ADDLs. These appear to attack the brain's synapses, the crucial nerve endings where memories are formed, without causing the death of the nerve cells.

Experiments on mice have shown that it is possible to attack ADDLs by making specific antibodies. A similar approach in humans might help to restore memory, Professor Klein said. Clinical trials could begin within three years.

The new estimates for the numbers affected by Alzheimer's, published in Archives in Neurology, are much higher than previous estimates. The reason is the rapid ageing of the American population. While Britain will also see a startling increase in Alzheimer's disease, the rise will be less pronounced because Europe has an older population than the US.