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One nonmotor symptom that experts are particularly looking at is a loss of smell, which occurs in approximately 90% of those with early-stage Parkinson’s disease.

VeryWell Health’s recent article, “How a Smell Test May Predict Parkinson’s Disease,” says that loss of smell (called hyposmia or olfactory dysfunction) not only impairs a person’s quality of life but it’s one of the earliest symptoms of Parkinson’s Disease.

Experts think that if a person’s smell disturbance is detected early, it could give a clue to their underlying neurological disease—and there is now research that has turned this idea into reality. In a study in Neurology, the sense of smell of over 2500 healthy people was evaluated in 1999-2000.

The participants were of the average age of 75, and all lived in the metropolitan areas of Pittsburgh, Pennsylvania, and Memphis, Tennessee. The Brief Smell Identification Test (BSIT) examined their sense of smell. In this test, participants first scratched and smelled 12 different odors and then were asked to identify a variety of smells like cinnamon, lemon, gasoline, soap and onion from four multiple-choice answers.

The results showed that during an average follow-up period of 9.8 years, 42 incident cases of Parkinson’s disease were detected. A link was found between a poor sense of smell and a higher risk of Parkinson’s. Those with the poorest sense of smell had the highest risk of developing Parkinson’s disease.

However, a loss of smell can be due to other health problems besides Parkinson’s. Alzheimer’s can cause smell disturbances, as can non-neurological conditions like chronic rhinosinusitis. This is why devising a specific smell test for PD is important; researchers still need to sort this out.

“Smell tests” must also test for the correct smell disturbance. Just saying a person has a loss of smell is rather vague. Perhaps one person has a hard time discriminating between odors while another can’t identify odors. A person may also have a higher threshold for detecting odors.

With that, research suggests that in Parkinson’s, there is a favorable decline in odor identification rather than odor detection, meaning they can “smell it” but not say what it is.

Finally, it’s critical to remember that a link is simply a connection, or a finding based on statistics—it’s not 100% predictive of any individual. Therefore, a person could lose their sense of smell and never develop Parkinson’s disease. Many people with Parkinson’s disease retain their sense of smell.

Reference: VeryWell Health (July 8, 2022) “How a Smell Test May Predict Parkinson’s Disease”