A Simple Smell Test Might Be Able To Predict Parkinson’s Disease 10 Years Before Diagnosis

Since earlier diagnosis is generally linked to better outcomes in most diseases that physicians treat, screening to detect early signs of disease is ideal. In this way, therapies and interventions can be tailored to fit a patient’s specific needs.

One current example is colonoscopy and fecal occult blood testing to screen for colon cancer.  But even more promising are scientific advances such as a “liquid biopsy”, a blood test looking for specific DNA circulating from tumors, to not only screen and detect, but monitor disease progression.

Sense of smell - Parkison

But for certain diseases, early diagnosis is problematic. For example, in Parkinson’s disease, a movement disorder characterized by tremor, rigidity, and progressive muscle weakness, the damage to nerve cells occurs well before such symptoms appear. 

Apart from specialized MRI scans (resting state functional MRI-fMRI), and and potential of blood-based protein markers, the ability to screen for and accurately detect who is a risk for the disease remains elusive.

And often overlooked is the fact that in patients with Parkinson’s, the deterioration of the sense of smell is the second most common symptom noted, after rigidity and slow movement.

The often subtle development of its characteristic resting tremor along with slowed movements and loss of normal posture signals the beginning signs of the disease that over 10-20 years leads to progressive muscle weakness that is crippling as the years progress.

Prior research using smell tests in Parkinson’s suggests that the sense of smell may be compromised as early as 4-5 years prior to a formal diagnosis, well before any signs of tremor or muscle weakness set in.

But now researchers have developed a simple scratch-and-sniff that may have the ability to help identify some people at greater risk of developing Parkinson’s disease nearly 10 years before the disease could be diagnosed.

The research was published online in the journal, Neurology, last week.

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The study established that older persons who demonstrate a poor sense of smell are more likely to develop Parkinson’s disease compared to people who scored better on the smell test. This observed link was also noted to be stronger in men than in women.

The test is pretty straightforward:  persons were asked to smell 12 common odors including gasoline, soap, lemon, cinnamon and onion, and select the right answer from four choices.

What’s also notable is that this study was one of a few select investigations to evaluate the sense of smell and Parkinson’s disease in black people, with recent prior research confirming the link in Whites and Asians.

“Previous studies have shown that black people are more likely to have a poor sense of smell than whites and yet may be less likely to develop Parkinson’s disease,” said study author Honglei Chen, MD, PhD, of the Michigan State University College of Human Medicine. “We found no statistical significance for a link between poor sense of smell and Parkinson’s disease in blacks but that may have been due to the small sample size. More research is needed to further investigate a possible link.”

Over 1,500 white persons and 950 black persons (average age of 75) completed the scratch-and-sniff test and were then tracked for 10 years on average. The researchers determined which study participants had developed Parkinson’s disease during that time period, and then placed them into three groups based on how well they performed on the smell test: poor, medium and good.

Over the course of the study, 42 people developed Parkinson’s disease: 30 white persons and 12 black persons. Study participants who scored poorly were close to 5 times more likely to develop the disease than those who had a good sense of smell.

Overall, looking at the 764 participants who scored poorly on the smell test, 26 of them developed Parkinson’s disease, compared to just 7 of the 835 with a good sense of smell and nine of the 863 people with a medium sense of smell.

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And when researchers adjusted for other confounding factors associated with developing Parkinson’s disease such as prior history of head injury, or habits associated with improvement of the disease–drinking coffee (caffeine), or smoking (nicotine usage)—the results remained the same.

The study demonstrated a strong link between the smell test and development of Parkinson’s up to six years later. While the link remained beyond six years, it was not as solid.

“Earlier studies had shown prediction of Parkinson’s disease about four to five years after the smell test was taken,” said Chen. “Our study shows that this test may be able to inform the risk much earlier than that.”

But one caveat is that not all persons who don’t perform well on the smell test will go on to develop Parkinson’s disease, emphasized Chen.  Additional research will be necessary before such a smell test can be used to screen for Parkinson’s disease in the general population because the disease affects a very small percentage of the population. Moreover, inability to perform well on such a test does not rule out other causes for poor smell such as inflammation, tumors or even post-traumatic.

Chen also explained that one weakness of the study was that patients were only given a diagnosis at the end of the study, as opposed to earlier time points.  As Parkinson’s may take quite a while to actually diagnose, it’s possible that some patients could have been diagnosed earlier while others may have had other symptoms missed prior to formal diagnosis.

“This is an interesting, but limited study that strengthens the notion that especially older men with poor olfaction (smell) are 5.2 times more likely to develop Parkinson’s disease (PD), based on this study.” said Dr. Paul Wright, Chair, Neurology at North Shore University Hospital and Long Island Jewish Medical Center in New Hyde Park, New York.  “This study showed that there is, however, a racial difference between white and black men where it did show the relationship between PD and decreased smell ability, however it was not statistically significant.  This also seemed not to hold true for women.”

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“This is a small but valuable study which shows that there may be gender and racial difference in the predictability of diagnosing Parkinson’s disease based on a simple smell test.  It also showed evidence that PD may be predicted earlier than was believed prior.  A larger study would help us establish how a simple smell test could be accurately integrated into clinical use,” offered Wright.

Arthur Kapell, a Parkinson’s patient who was formally diagnosed by MRI in 2013, explained that he began to note changes in his sense of smell approximately 8 to 9 earlier, which was quite pronounced with inability to smell flowers.  Of note, he also recounted that he did not detect any changes in the taste of his food, although he often spiced it quite well.

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