They said dementia could indeed be a form of second childhood, after they found that people with the disease lost essential skills in the same order in which they developed them as a child.

As Alzheimer's progresses, sufferers find it increasingly difficult and then impossible to perform even the simplest everyday tasks - such as washing, eating and dressing - without supervision.

However, a leading charity has said it may be dangerous to treat people who have lived a full life as children.

'Patients can be happy'

The findings of the study, which was conducted over 20 years, were presented at an international conference of specialists in the disease in Vancouver.

Dr Barry Reisberg, the outgoing president of the International Psychogeriatric Association, led the study.

He said: "Alzheimer's patients actually can be happy and right now there are hundreds of thousands, perhaps millions, who are suffering because we do not know how to care for them."

The World Health Organization estimates that 15 million people worldwide have the disease.

Reverse order

Dr Reisberg said researchers at New York University found that Alzheimer's patients lose physical and mental abilities in exactly the opposite order that children gain them.

Eventually, they return to an infant-like state, he said, and compared adults in the advanced stages of Alzheimer's to children under two years old - able to smile but unable to speak or walk.

The researchers labelled the process "retrogenesis".

Dr Reisburg said the public sees a correlation between the problems of old age and those of children.

Literary precedent

The researchers noted that Shakespeare talked of "second childishness" in As You Like It and Aristophanes wrote that "old men are children twice over".

They suggest that if Alzheimer's patients were treated like infants they would endure less physical suffering, but so far only "adult measures" have been applied.

"It's not a matter of increasing resources. It's a matter of knowing what to do," Dr Reisberg said.

Potentially dangerous approach

The Alzheimer's Disease Society said the research was interesting but was unhappy with elderly patients being treated as children.

"We don't think it's OK to treat people who've lived a very long life, who have a wealth of experience and who've done a lot with their lives as children," a spokeswoman said.