New York, June 28 (IANS): Even if symptoms of Alzheimer's disease become apparent in old age, the biological processes that cause the mental decline may begin a couple of decades ago in the 'middle age', suggests a new study by an Indian-origin researcher.

Low scores on memory and thinking tests may signal Alzheimer's up to 18 years before the disease can be diagnosed, the findings showed.

"The changes in thinking and memory that precede obvious symptoms of Alzheimer's disease begin decades before," said study author Kumar Bharat Rajan, assistant professor at Rush University Medical Center in Chicago.

"While we cannot currently detect such changes in individuals at risk, we were able to observe them among a group of individuals who eventually developed dementia due to Alzheimer's," said Rajan, an alumnus of Madras Christian College.

For the study, 2,125 people from Chicago, with an average age of 73, without Alzheimer's disease were given tests of memory and thinking skills every three years for 18 years.

The researchers found that those who were finally diagnosed with Alzheimer's scored lower on the scored lower overall on the memory and thinking tests throughout the study.

One unit lower in performance of the standardised cognitive test score was associated with an 85 percent greater risk of future dementia.

"A general current concept is that in development of Alzheimer's disease, certain physical and biologic changes precede memory and thinking impairment. If this is so then these underlying processes may have a very long duration. Efforts to successfully prevent the disease may well require a better understanding of these processes near middle age," Rajan said.

The research was published online in the journal Neurology.