Study suggests fading sense of smell often occurs years before symptom onset
By Steven Reinberg
WEDNESDAY, Sept. 6, 2017 (HealthDay News) -- Losing your sense of smell may be an early sign of an increased risk of developing Parkinson's disease, a new study suggests.
Researchers say that people with a poor sense of smell may have as much as a five times greater risk of developing Parkinson's.
"Unlike vision or hearing impairment, a poor sense of smell often goes unrecognized," said lead researcher Dr. Honglei Chen. He is a professor of epidemiology and biostatistics at Michigan State University College of Human Medicine in East Lansing.
"Evidence suggests olfactory [sense of smell] impairment may develop years prior to the diagnosis of Parkinson's disease and dementia, the so-called neurodegenerative diseases that we are yet to find a cure for," Chen said.
The researchers found a strong association between smell test results and developing Parkinson's up to six years later. The association remained beyond six years, but was not as strong, he added.
However, Chen stressed that Parkinson's is fairly rare, so not everyone with a poor sense of smell will develop the disease.
Research on the sense of smell may eventually help identify people at high risk for this devastating disease and help scientists understand how Parkinson's develops before a diagnosis is possible, Chen suggested.
James Beck, chief scientific officer at the Parkinson's Foundation, said the sense of smell is a kind of window into the brain.
"Smell testing is indicative of some kind of brain disease. People with Alzheimer's, for instance, experience loss of smell," Beck said. But, he added, having a poor sense of smell doesn't mean that you are doomed to develop Parkinson's or Alzheimer's.
"The loss of sense of smell alone isn't predictive that someone will develop Parkinson's disease -- it's a warning sign that there may be problems ahead," Beck explained.
Testing patients' sense of smell when no early treatment for Parkinson's exists isn't something doctors should be doing, Beck said.
For an average of 10 years, Chen and colleagues followed nearly 2,500 men and women (1,500 white and nearly 1,000 black seniors). At an average age of 75, the study participants took a scratch-and-sniff test that determined their sense of smell.
The test included 12 common odors, such as cinnamon, lemon, gasoline, soap and onion. Participants were asked to pick the correct answer from four choices.
Based on the test, participants were classified as having a poor, medium or good sense of smell.
During the follow-up, 42 people developed Parkinson's disease. Thirty were white and 12 were black, according to the report.
Of the 764 people with a poor sense of smell, 26 developed Parkinson's, compared with seven of the 835 people with a good sense of smell, and nine of the 863 people with a medium sense of smell, Chen's team reported.
The findings stayed the same after researchers took into account other factors that could affect risk of Parkinson's -- including smoking, drinking coffee and a history of head injury.
"While the sample sizes in specific groups are small in this study, especially in blacks and women, the association appears to be stronger in whites than in blacks, and in men than in women," according to Chen.
Dr. David Hiltzik, director of otolaryngology, head and neck surgery at Staten Island University Hospital in New York City, said, "The only area of the brain that has direct contact with the outside world is through the nose."
Many factors can cause loss of smell, so patients shouldn't be worried that losing their sense of smell puts them on the road to Parkinson's, he said.
Some of these causes include vitamin and mineral deficiencies, head trauma, tumors, inflammatory disease and viral infections, Hiltzik said.
Another specialist, Dr. Sami Saba, a neurologist at Lenox Hill Hospital in New York City, agreed that patients needn't worry about Parkinson's disease if they lose their sense of smell.
"Patients should not be very concerned," he said. "Like a lot of things in medicine, this shows that something that is fairly common may increase your risk of something, but Parkinson's is still pretty uncommon," he noted.
"A lot of people have a diminished sense of smell, and those people may be at a higher risk for Parkinson's, but it is still only one in thousands who will develop the disease," Saba said.
The report was published online Sept. 6 in the journal Neurology.
For more about Parkinson's disease, visit the National Parkinson Foundation.
SOURCES: Honglei Chen, M.D., Ph.D., professor, epidemiology and biostatistics, Michigan State University College of Human Medicine, East Lansing; James Beck, Ph.D., chief scientific officer, Parkinson's Foundation; Sami Saba, M.D., neurologist, Lenox Hill Hospital, New York City; David Hiltzik, M.D., director, otolaryngology, head and neck surgery, Staten Island University Hospital, New York City; Sept. 6, 2017, Neurology, online
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