Olfactory tests could help doctors spot older adults who are at greater risk of developing dementia, researchers say.
The sense of smell is known to deteriorate with age. However, researchers have previously found it might also hint at health problems: older adults who struggle to identify odours have a greater chance of dying in the near future regardless of how old they are.
Other studies have found older adults who have difficulties in identifying and remembering smells are more likely to have characteristics linked to a greater risk of developing Alzheimer’s disease even if there is no current sign of cognitive decline.
It is thought the sense of smell is one of the first faculties affected by certain neurodegenerative diseases.
Now experts say they have probed further, and those diseases alone do not explain why a poor sense of smell might bode ill.
“My suspicion is [the] process of smell in older adults probably has much broader potential health implications than what we already know about,” said Prof Honglei Chen, a co-author of the research from Michigan State University. He suggested it could be linked to conditions of the immune system and even psychiatric disorders.
Writing in the Annals of Internal Medicine, researchers in the US and Sweden report how more than 2,200 people aged between 71 and 82 undertook smell identification tests near the turn of the millennium , which were then followed up over 13 years. The test involved participants smelling 12 different common odours, such the scent of strawberries, and choosing the identity of each from four possible answers.
The team then grouped participants into “good”, “moderate” or “poor” sense of smell, depending on how many correct answers they gave. They also looked at the causes of death among participants who passed away during the research period. When the team took into account factors including age, sex, race, smoking and general health at the outset, they found that a poor sense of smell was linked to a 46% greater risk of dying within 10 years compared with those ranked as having a good sense of smell.
The researchers concluded that this trend held regardless of the sex or race of participants, but the link was only present among those who were in very good health at the start of the study.
The researchers say that could be because those in poor health had many factors that might influence the length of their life, overwhelming any effect linked to a poor sense of smell. “On the other hand, poor olfaction among older adults with excellent to good health may be an early warning sign for insidious adverse health conditions that eventually lead to death,” the authors write.
They found a poor sense of smell was linked to death from dementia and Parkinson’s disease, with some signs that poor smell might also be linked to death from cardiovascular disease. There was no link between poor sense of smell and death from cancer or respiratory diseases.
Further analysis revealed 22% of the overall increased risk of death among those with a poorer sense of smell was down to neurodegenerative diseases, with 6% down to weight loss.
The team said the remaining association with mortality was probably down to health conditions that had not yet been identified as being linked to smell. Once that was unpicked, said Chen, it could be worth adding a smell test to general health screening.
However, others pointed out it was unclear if the results held among younger adults, while the sense of smell was only tested once. What’s more, the apparent link between the sense and mortality could be down to factors that affect both but were overlooked.
Prof Jayant Pinto, from the University of Chicago, who has carried out previous research in the area, welcomed the study. “While we still need to understand what exactly a poor sense of smell is signalling in terms of specific mechanisms that lead to increased risk of death, it seems clear that impaired olfaction is an early warning sign of both diseases such as Alzheimer’s and Parkinson’s, as well as heart disease,” he said.
“I’m an advocate of more widespread use of smell testing in general practice since it can signal these major health problems in the future,” he added. “Even if we don’t yet have treatments for [Alzheimer’s or Parkinson’s disease] , better scrutiny of such patients might help us care for them and plan for the future for patients and families.”