According to the Alzheimer’s Association, around 1 in 9 people over the age of 65 has Alzheimer’s disease (AD) in the United States, a total of 6.7 million people. With an aging population, experts expect this number to exceed 12 million by 2050.
As Medical News Today reports, Alzheimer’s is an incurable, progressive disease, but it is not inevitable in aging. If a person starts to experience memory problems and cognitive deficits, these should not be dismissed as normal signs of aging, as early diagnosis allows treatment that can alleviate symptoms.
Around 13.7% of people worldwide carry a gene variant, APOE ε4, that increases the risk of Alzheimer’s disease. This gene is found in 40% of people who are diagnosed with the condition.
A new study published in Neurology found that people who carry this gene variant may experience an impaired sense of smell before any symptoms of Alzheimer’s disease, such as mild cognitive impairment, appear. The researchers suggest that testing a person’s ability to detect odors may be useful for identifying those at risk.
The study participants came from the National Social Life, Health and Aging Project (NSHAP) — a study of older adults’ social lives and health run by the University of Chicago.
For this study, over 865 people took an at-home survey that included testing their sense of smell. The tests were repeated at 5-yearly intervals, with odor identification being assessed in 2005, 2010, and 2015 and odor sensitivity being tested in 2010 and 2015. In 2010 and 2015, participants also completed thinking and memory tests. In 2010, the mean age of the participants was 72.3 (range 62-95)
The study aimed to determine whether APOE ε4 is linked to a decline in the sense of smell and cognition and, if so, how. The researchers took DNA samples to determine which of the respondents carried the gene variant. Smell tests included odor sensitivity — the ability to detect odors at different concentrations — and odor identification, which assessed people’s ability to determine what the odor was.
Throughout the study, those with the gene were 37% less likely to have good odor detection than people without the gene. At age 65, people with the APOE ε4 variant could detect fewer odors than those without the gene, suggesting that their ability to detect odors had already declined by this age. However, non-carriers, who started with a better ability to detect odors, showed a more rapid decline after age 65.
In contrast, when identifying odors, there was no difference between carriers and non-carriers at 65, but carriers’ ability declined more rapidly, particularly from age 75. Cognition showed a similar pattern, with faster declines in cognition in those with the APOE ε4 variant.
“It is important to note that, in our study, we had no way of keeping track of which patients would ultimately go on to develop Alzheimer’s,” says study author Dr. Matthew S. GoodSmith, of the University of Chicago. “And, since our data was derived from survey data collected from adults living at home who were able to tolerate a long interview process, respondents with severe dementia were unable to participate.”
“That being said,” he added, “our findings shed light on the interplay between smell loss and cognitive decline in patients at high genetic risk for developing Alzheimer’s. We hope that, with additional research, testing the sense of smell may develop into a useful screening or diagnostic tool in the future.
Although they found no link between odor detection and cognition, the researchers’ findings suggest odor identification and cognition are linked.
“This is an interesting study assessing the relationship between declines in odor sensitivity and odor identification in APOE ε4 carriers and the risk of future cognitive decline,” says Dr. Emily D. Clark, D.O., Assistant Professor of Psychiatry and Associate Director of the Alzheimer’s Disease Care, Research, and Education Program (AD-CARE) at the University of Rochester.
So, although these are early findings, Dr. Clark agreed that they may point toward new methods of detecting early Alzheimer’s disease: “These findings, if further validated, provide a novel opportunity for the use of olfactory testing as an early screening marker in clinical research studies. As the clinical research field is focusing more on intervention in early symptomatic and prodromal Alzheimer’s disease, the ability to use something like olfactory performance as an early biomarker would make identifying appropriate populations easier and possibly more cost-effective.”
However, she added a note of caution: “Further validation of these findings and a better understanding of the underlying mechanisms behind these changes are needed before this can be accepted as a screening test in clinical practice.”
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