In a study that followed almost 3,000 older people with normal cognition, researchers found that a simple smell test was able to identify those at higher risk of dementia.
Senior author Jayant M. Pinto, a professor of surgery at the University of Chicago in Illinois, and colleagues report their findings in the Journal of the American Geriatrics Society.
They found that participants who could not identify at least 4 out of 5 odors in the simple smell test were twice as likely to have dementia 5 years later.
"These results show that the sense of smell is closely connected with brain function and health," says Prof. Pinto, who is also an ear, nose, and throat specialist.
He explains that losing one's sense of smell is a strong indicator of "significant damage," and that this "simple smell test could provide a quick and inexpensive way to identify those who are already at high risk."
Dementia is a collection of diseases
Dementia is a progressive brain disorder that erodes many aspects of cognitive function - for instance, it diminishes a person's ability to remember, reason, solve problems, and hold a conversation.
As it progresses, dementia encroaches on daily life, eventually robbing people of their independence and personality.
Memory loss is not the only sign of dementia; many people can experience it without having dementia. Dementia is not a normal part of aging, although it is much more common in older people. To be diagnosed with dementia, a person must show impairment in two or more core mental functions, of which memory can be one.
Dementia is not one disease, but a collective term for several diseases that affect the brain. Alzheimer's disease is the most common type of dementia, but other types include vascular dementia, Lewy body dementia, and frontotemporal disorders. It is quite common for people to have more than one type.
The number of people worldwide with dementia is growing. Currently, around 47 million people are thought to be living with the disease. This number is likely to approach 75 million by 2030 and 132 million by 2050.
At present, there are no treatments that can cure dementia or alter its course, but there are some in clinical trials that might have an impact. In the meantime, much can be done to improve the quality of life for people with dementia, as well as the people who love them and care for them. In this respect, early diagnosis is essential.
Humans sense smell through the olfactory nerve, which links the cells that detect odors in the nose directly to the olfactory bulb located at the base of the brain. This is the only cranial nerve exposed to the outside world, offering a potential route through which the central nervous system might be harmed by pollution, pathogens, and other hazards in the environment.
The olfactory system has self-generating stem cells and the researchers suggest that perhaps loss of sense of smell is an early sign that the brain is losing its ability to self-repair. Loss of sense of smell is often an early indicator of Alzheimer's and Parkinson's disease.
In the new study, a "nationally representative sample" of 2,906 men and women aged between 57 and 85 underwent home interviews and completed a simple smell test.
For the "validated five-item test," they had to identify five odors, one at a time, by sniffing a device similar to a felt-tip pen. Each time, they were given four choices, from which they had to pick out the correct one.
The five different odors were: peppermint, fish, orange, rose, and leather, with peppermint being the easiest, and leather the hardest, to identify.
The researchers found that the vast majority of participants were able to correctly identify at least 4 out of 5 odors. Of the rest, 7 percent identified 2 or 3 out of 5 smells, 2.2 percent identified just one, and 1 percent could not identify any of them.
'Important early sign' of dementia
After 5 years, the participants were interviewed again to find out if they had been diagnosed with dementia. A proxy stood in if the participant was too sick to be interviewed or had died during the follow-up.
The team analyzed the results of the smell test against the follow-up information, adjusting them to rule out any effects from age, sex, race, ethnicity, education, other illnesses, and level of cognition at study baseline.
They found that the participants who had not been able to identify at least 4 out of the 5 odors at baseline were more than twice as likely to be among those who had developed dementia during the 5-year follow-up.
They also found that the lower the number of odors correctly detected at baseline, the higher the chances of dementia being diagnosed during the follow-up period.
On the findings, Prof. Pinto says, "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."
In a linked editorial, Dr. Stephen Thielke - from the Psychiatry and Behavioral Sciences Faculty at the University of Washington in Seattle - acknowledges that problems with sense of smell may be "easier to quantify across time than global cognition," and that this could make it easier to assess early decline in the brain.
However, Dr. Thielke also notes that this does not mean that "smell testing would be a useful tool for predicting the onset of dementia."
Prof. Pinto accepts this point, noting, "Our test simply marks someone for closer attention." He and his colleagues say that more work is now needed to turn the test into one that can be used in clinical practice.
Nevertheless, he believes that the test could help to find patients who might be at higher risk for dementia, who can then be put forward for trials of treatments to prevent dementia in the early stages.
"Of all human senses, smell is the most undervalued and underappreciated - until it's gone."
Prof. Jayant M. Pinto