People who could not tell at least four out of five common odours had more than double the risk as those with a normal sense of smell to develop dementia within five years, a long-term study of nearly 3,000 adults, aged 57 to 85, reports.
Even though 78 percent of individuals tested were normal - correctly identifying at least four out of five scents - about 14 percent could 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.
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.
Sense Of Smell Loss
5 years following the first 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.
Lead author Jayant M. Pinto, MD, a professor of surgery at the University of Chicago, said:
“These results show that the sense of smell is closely connected with brain function and health. We think smell ability specifically, but also sensory function more broadly, may be an important early sign, marking people at greater risk for dementia. We need to understand the underlying mechanisms, so we can understand neurodegenerative disease and hopefully develop new treatments and preventative interventions."
Pinto is an ENT specialist who studies the genetics and treatment of olfactory and sinus disease.
The researchers used a well-validated tool, known as “Sniffin’Sticks.” These look like a felt-tip pen, but instead of ink, they are infused with distinct scents.
Study subjects smell each item and are asked to identify that odor, one at a time, from a set of four choices. The five odors, in order of increasing difficulty, were peppermint, fish, orange, rose and leather.
Losing the ability to smell can have a substantial impact on lifestyle and wellbeing, said Pinto, a member of the Section of Otolaryngology-Head and Neck Surgery at UChicago Medicine.
“Smells influence nutrition and mental health. People who can’t smell face everyday problems such as knowing whether food is spoiled, detecting smoke during a fire, or assessing the need a shower after a workout. Being unable to smell is closely associated with depression as people don’t get as much pleasure in life,"
“Of all human senses,” Pinto added, “smell is the most undervalued and underappreciated - until it’s gone."
Adams, D. R., Kern, D. W., Wroblewski, K. E., McClintock, M. K., Dale, W. and Pinto, J. M. Olfactory Dysfunction Predicts Subsequent Dementia in Older U.S. Adults J Am Geriatr Soc. doi:10.1111/jgs.15048