LONDON (AFP) - DEMENTIA is the biggest cause of disability in old people in poorer countries and the problem and its costs for society will grow rapidly as populations age, doctors said on Friday.

British researchers studied 15,000 elderly people in seven low- and middle-income countries and found that, contrary to previous expert opinion, dementia, not blindness, is by far the biggest cause of poor health in old age.

Renata Sousa of the Institute of Psychiatry at King's College London, who led the study, said this was contrary to World Health Organisation estimates that visual impairment and blindness were the biggest problems.

'Chronic diseases of the brain and mind deserve increased prioritisation,' she wrote in the study published in The Lancet. 'Besides disability, they lead to dependency and present stressful, complex, long-term challenges to carers. Societal costs are enormous.'

Despite decades of research, doctors still have few effective weapons against dementia, a brain-wasting disease which effects some 35 million people around the world.

The most common form of dementia is Alzheimer's and Alzheimer's Disease International predicts the number of sufferers will almost double every 20 years - to 66 million in 2030 and more than 115 million in 2050 - with much of the increase coming in poorer nations.