By Kavish Khatib
Researchers at the University of Chicago have found that elderly patients who could not identify common odours were more than twice as likely to develop dementia in the next five years than those with a normal sense of smell.
The paper – a long-term study of nearly 3,000 adults, aged 57 to 85, was published on September 29th, 2017 in the Journal of the American Geriatrics Society. It follows a related 2014 paper, in which olfactory dysfunction was associated with increased risk of death within five years. In that study, loss of the sense of smell was a better predictor of death than a diagnosis of heart failure, cancer or lung disease.
“Five years after the initial test, almost all of the study subjects who were unable to name a single scent had been diagnosed with dementia.”
In the study, 78 percent of those tested were normal i.e. correctly identifying at least four out of five scents. However, about 14 percent could only name just three out of five, five percent could identify only two scents, two percent could name just one, and one percent of the study subjects were not able to identify a single smell.
Moreover, five years after the initial test, almost all of the study subjects who were unable to name a single scent had been diagnosed with dementia. Nearly 80 percent of those who provided only one or two correct answers also had dementia, with a dose-dependent relationship between degree of smell loss and incidence of dementia.
“The loss of the sense of smell was a better predictor of death than a diagnosis of heart failure, cancer or lung disease.”
“These results show that the sense of smell is closely connected with brain function and health,” said the study’s lead author, Jayant M. Pinto, MD, a professor of surgery at the University of Chicago and ENT specialist who studies the genetics and treatment of olfactory and sinus disease. “We think a decline in the ability to smell, specifically, but also sensory function more broadly, may be an important early sign, marking people at greater risk for dementia.”
Pinto also added a greater understanding of the underlying mechanisms needed to be attained. This will allow us to understand neurodegenerative disease and hopefully develop new treatments and preventative interventions.
The loss of the sense of smell is a strong indicator for significant neural damage. A simple smell test could provide a quick and inexpensive way to identify those who are already at high risk.
Jayant Pinto, MD, professor of surgery, with one of the Sniffin’ Sticks used to test a patient’s ability to identify scents for his research on aging. (The University of Chicago)
For both studies in 2014 and 2017, the researchers used a well-validated tool, known as “Sniffin’Sticks”. These pen shaped sticks, are infused with distinct scents. Study subjects smell each item and are asked to identify that odour, one at a time, from a set of four choices. The five odours were peppermint, fish, orange, rose and leather (in order of increasing difficulty).
“The loss of the sense of smell is a strong indicator for significant neural damage.”
The olfactory nerve is the only cranial nerve directly exposed to the environment. The cells that detect smells connect directly with the olfactory bulb at the base of the brain, potentially exposing the central nervous system to environmental hazards such as pollution or pathogens. Olfactory deficits are often an early sign of Parkinson’s or Alzheimer’s disease. They get worse with disease progression.
In an accompanying editorial, Stephen Thielke, a member of the Geriatric Research, Education and Clinical Center at Puget Sound Veterans Affairs Medical Center at the University of Washington, wrote: “Olfactory dysfunction may be easier to quantify across time than global cognition, which could allow for more-systematic or earlier assessment of neurodegenerative changes, but none of this supports that smell testing would be a useful tool for predicting the onset of dementia.”
The olfactory nerve is the first cranial nerve directly exposed to the environment. The cells that detect smells connect directly with the olfactory bulb at the base of the brain.
However, Pinto and his team stated that much more work would be needed to be done to make this into a viable clinical test. But it would help to identify people who are at greater risk, allowing us to enrol them into earlier treatment strategies.
“Of all human senses,” Pinto added, “smell is the most undervalued and under-appreciated – until it’s gone.”
Official name of the study: “Olfactory Dysfunction Predicts Subsequent Dementia in Older US Adults”
- The study: http://onlinelibrary.wiley.com/doi/10.1111/jgs.15048/pdf
- NORC University of Chicago: http://www.norc.org/Research/Projects/Pages/national-social-life-health-and-aging-project.aspx
- Images from: UChicago Medicine: https://sciencelife.uchospitals.edu/2017/09/29/elderly-who-have-trouble-identifying-odors-face-risk-of-dementia/