How a Smell Test May Predict Parkinson's Disease

When people think of Parkinson's disease, the first symptoms that usually come to mind are motor symptoms like a resting tremor, rigidity, or a slowness of movement.

But nonmotor symptoms, like mood disorders and sleeping problems, are also common in Parkinson's. One nonmotor symptom that experts are particularly focusing on is a loss of smell, which occurs in approximately 90 percent of people with early-stage Parkinson's disease.

This 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.

So taking this idea a step farther, experts believe that if a person's smell disturbance is detected early, it could provide a clue to their underlying neurological disease—and there is now research that has turned this idea into reality.

Research Behind the Smell Test for Predicting Parkinson's Disease

In a study in Neurology, the sense of smell of over 2500 healthy people was evaluated in 1999-2000. These participants were of the average age of 75 and all lived in the metropolitan areas of Pittsburgh, Pennsylvania and Memphis, Tennessee.

Their sense of smell was examined using the Brief Smell Identification Test (BSIT). In this test, participants first scratched and smelled 12 different odors. They then had to identify a variety of smells like cinnamon, lemon, gasoline, soap, and onion from four multiple-choice answers.

Several data tools were then used to identify people who developed Parkinson's disease through August 31st, 2012.

Results revealed that during an average follow-up period of 9.8 years, 42 incident cases of Parkinson's disease were found, and with that, a link was found between a poor sense of smell and a higher risk of Parkinson's. This means that people who had the poorest sense of smell (meaning they scored in the lowest tertile of the total BIST scores) had the highest risk of developing Parkinson's disease.

Interestingly, when the study was broken down into race and gender, the link was strongest in Caucasian participants, as compared to African-American participants, and in men, as compared to women.

What Do the Results Mean?

The take-home message here is that "sniff tests" may be able to predict a person's risk of developing Parkinson's disease. That said, there are a few caveats to keep in mind.

One is that a loss of smell can be due to other health problems besides Parkinson's. Other neurological diseases like Alzheimer's can cause smell disturbances, as can non-neurological conditions like chronic rhinosinusitis. This is why devising a smell test that is specific for PD is important, and researchers have not yet sorted this all out.

Secondly, "smell tests" must test for the correct smell disturbance. Simply saying a person has a loss of smell is rather vague. Perhaps one person has a hard time discriminating between odors while another cannot identify odors. Or a person may 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.

Lastly, it's critical to remember that a link or association is simply a connection or a finding based on statistics—it's not 100 percent predictive of any one individual. In other words, a person could lose their sense of smell and never develop Parkinson's disease. Likewise, there are people with Parkinson's disease who retain their sense of smell.

Cause of Loss of Smell in Parkinson's Disease

It's unclear why olfactory dysfunction occurs in Parkinson's disease. Experts have found that smell loss correlates with a lower number of cholinergic neurons (nerve cells that release the brain chemical, acetylcholine) in the nucleus basalis of Meynart—a region of the brain that projects to the primary olfactory cortex where you get the sensation of smell.

With this information, smell tests that focus on detecting cholinergic dysfunction may be ideal. It's still too early to tell, though, so more investigation needs to be done.

Additionally, some researchers have suggested that Parkinson's disease may actually begin in the digestive system and the olfactory bulb (a region of the brain that controls the sense of smell), and not the substantia nigra (where it eventually leads to dopamine-producing nerve cell death). This may be why early symptoms, like constipation and loss of smell, begin years prior to motor symptoms like resting tremor and muscle stiffness.

A Word From Verywell

Whether or not scientists eventually develop a smell test to assess the risk of, or even diagnose, Parkinson's disease is unclear. But, at least, this interest in the loss of smell may spark doctors to consider Parkinson's when their patients bring it up. Put differently, it may prevent this symptom from being overlooked, which it commonly is.

Perhaps in the way future, scientists can stop Parkinson's disease in its tracks, when it's lingering in the olfactory bulb before it reaches the substantia nigra.