Cognitive decline refers to the decline of mental functions such as remembering, thinking, and reasoning.
While some waning of these abilities can often accompany "normal aging," a more marked decline could be a symptom of dementia.
Impaired sense of smell, otherwise known as olfactory dysfunction, is not uncommon in the general population and "becomes more common" as age advances.
Many individuals who develop neurodegenerative diseases experience loss of sense of smell in the early stages. This is the case, for example, in the majority of people who have Alzheimer's or Parkinson's disease.
Due to this, and the fact that sense of smell testing has become more reliable and straightforward, olfactory function has received increasing attention as a marker of brain decline — particularly because it might help to diagnose neurodegenerative diseases long before more obvious symptoms emerge.
First population study to examine age link
This study is not the first population-based study to find a link between decreased sense of smell and a decline in cognitive performance.
Research led by the Mayo Clinic that was published in 2015, for example, came to this conclusion after studying a large group of men and women, aged 80 years, on average.
However, as the study authors note in their report, this investigation is the first to report on the "age-specific associations of olfactory function and cognitive performance in the general population."
They analyzed data from the Heinz Nixdorf Recall study, which followed a large group of residents of the Ruhr valley in Germany.
The new study, which was set up toward the end of the 1990s, recruited 4,814 volunteers who were aged 45–75 when they enrolled during 2000–2003. The participants, 50 percent of whom were male, were examined on enrolment, and then another two times 5 and 10 years later.
The 65–74 age group showed strongest links
During the third exam, 2,640 participants — who were aged 68.2 years, on average — comprising 48 percent men completed "eight validated cognitive subtests" and underwent a "Sniffin' Sticks Screening Test," which evaluated their sense of smell as a score of 0–12.
The researchers put the participants into three groups, according to their sniffing test score, as follows:
- "anosmic," or no sense of smell, if they scored 6 and under
- "hyposmic," or impaired sense of smell, if they scored 7–10
- "normosmic," or normal sense of smell, if they scored 11 or higher
The team then compared the results of the cognitive tests with the sense of smell categories by age group and gender. The age groups were: 55–64, 65–74, and 75–86 years.
The analysis showed that, overall, women tended to have a better sense of smell than men.
The most striking result was that for those aged 65–74 years, performance in nearly all the cognitive tests differed significantly according to sense of smell.
The worst cognitive performance in this age group was in those who had no sense of smell (the anosmics) and the best was in those who had a normal sense of smell (the normosmics).
Although there was a similar "quantitative" pattern in the other age groups, it was not as strong.
They suggest that the association that they found in the 65–74 age group "may serve as a marker to improve identification of persons at high risk for cognitive decline and dementia."