A new US study suggests that losing one’s sense of smell in old age could be a predictor of mild cognitive impairment (MCI) which is often a precursor of Alzheimer’s disease.

The study is published in July’s issue of the Archives of General Psychiatry, one of the JAMA/Archives journals.

Mild cognitive impairment (MCI), a mild thinking, learning and memory disorder that does not interfere significantly with daily living, is often a precursor to Alzheimer’s diseases, but scientists don’t know much about how to predict its development.

Inability to recognize familiar odours has been associated with more rapid cognitive decline, and with the transition from MCI to Alzheimer’s.

Dr Robert S. Wilson and colleagues from the Rush University, Chicago and also from the University of Pennsylvania decided to test the hypothesis that losing one’s sense of smell could be a predictor of MCI in a longitudinal study.

The participants were 589 older people with an average age of 79.9, who showed no signs of cognitive impairment at the start of the study in 1997. The study ended 5 years later in 2002.

Their sense of smell at the start of the study was measured using a test where 12 familiar odours were placed under their nose. They were asked to identify each smell from four possible options. Also at the start of the study and then every year for five years, they had clinical evaluation that included medical history, a neurological exam and cognitive function testing.

The results showed that:

  • The mean score at the start of the study was 9.3 out of 12 odours correctly identified with a standard deviation of 1.9 (i.e. most of the participants’ scores ranged from 7.4 to 11.2).
  • Over the 5 years, 177 participants (30 per cent) developed MCI.
  • After adjusting for age, sex and education, MCI impairment was negatively correlated with odour detection scores: thus as sense of smell got worse, so did cognitive function.
  • Participants who scored below average (those with the lowest 25 per cent of scores) on the smell test were 50 per cent more likely to develop MCI than those who scored above average (in the highest 25 per cent of scores).
  • The results were much the same after adjusting for cognitive ability, presence of stroke or smoking status at the start of the study, or after only including data from participants whose MCI lasted a year or more.
  • Low ability to correctly identify smells was also linked to lower cognitive ability at the start of the study and more rapid decline in: episodic memory (remembering experiences), semantic memory (remembering ideas, words and symbols) and perceptual speed.

The researchers concluded that:

“Among older persons without manifest cognitive impairment, difficulty in identifying odors predicts subsequent development of MCI.”

They said their findings suggest that loss of sense of smell in older people could be an early sign of Alzheimer’s, and that testing for this could be a useful diagnostic tool.

But, they said, since loss of smell is also linked to other diseases like Parkinson’s, more research is needed to identify exactly what is going on neurologically when “olfactory dysfunction” develops in older people.

According to the Alzheimer’s Association, there are over 5 million people in the US living with Alzheimer’s disease, a degenerative brain disorder that gradually destroys a person’s memory and their ability to learn, reason, make judgments, communicate and do daily things like cook a meal or go shopping.

As the disorder develops, people can change their personality and become anxious, suspicious or agitated. They can also experience delusions and hallucinations.

There is no cure for the disease, but treatments are improving as scientists find out more about its biology. Studies also show that care and support improves quality of life for people with Alzheimer’s and the people who look after them.

“Olfactory Identification and Incidence of Mild Cognitive Impairment in Older Age.”
Robert S. Wilson, Julie A. Schneider, Steven E. Arnold, Yuxiao Tang, Patricia A. Boyle, and David A. Bennett. Arch Gen Psychiatry. 2007;64:802-808.
Vol. 64 No. 7, July 2007

Click here for Abstract.

Click here for Alzheimer’s Association (US).

Written by: Catharine Paddock
Writer: Medical News Today