People who are unable to distinguish between different smells, such as lemons and petrol, may be at a higher risk of developing Alzheimer's disease, a study warns.
Damage to brain associated with Alzheimer's disease (AD) occurs up to 20 years before symptoms start showing.
Scientists now believe that simple odour identification tests may help track the progression of the disease before symptoms actually appear, particularly among those at risk.
"Despite all the research in the area, no effective treatment has yet been found for AD. But, if we can delay the onset of symptoms by just five years, we should be able to reduce the prevalence and severity of these symptoms by more than 50 per cent," said, John Breitner from McGill University in Canada.
Researchers asked about 300 people with an average age of 63, who were at risk of developing Alzheimer's Disease (AD) and had a parent who suffered from the disease, to take multiple choice scratch-and-sniff tests to identify scents as varied as bubble gum, gasoline or the smell of a lemon.
One hundred of them also volunteered to have regular lumbar punctures to measure the quantities of various AD- related proteins in the cerebrospinal fluid (CSF).
Researchers found that participants with the most difficulty in identifying odours were those in whom other, purely biological indicators of AD, were most evident.
"This is the first time that anyone has been able to show clearly that the loss of the ability to identify smells is correlated with biological markers indicating the advance of the disease," said Marie-Elyse Lafaille-Magnan, PhD student at McGill University.
"For more than 30 years, scientists have been exploring the connection between memory loss and the difficulty that patients may have in identifying different odours," she said.
"This makes sense because it is known that the olfactory bulb (involved with the sense of smell) and the entorhinal cortex (involved with memory and naming of odours) are among the first brain structures first to be affected by the disease," she added.
The study was published in the journal Neurology.
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