According to the National Institute of Health, an estimated 50,000 people are diagnosed with Parkinson’s disease each year in the United States, and around half of a million people are living with the disease. Worldwide, that estimate is as high as 10 million people. Parkinson’s disease, a neurodegenerative disorder, is characterized by the loss of nerve cells in the brain that are responsible for releasing dopamine, a neurotransmitter that helps monitor the signals related to pleasure, emotional responses, and movement regulation.
As many as 47 million people worldwide are living with Alzheimer’s disease, a more general degeneration of the brain, characterized by a decline in memory and reasoning skills. An estimated 5.4 million Americans have Alzheimer’s and that number is expected to reach 13.8 million by 2050.
Despite their ubiquity, the causes for both diseases are still mostly unknown making it a challenge to catch the disease early. The symptoms generally develop slowly over many years and are more commonly found in older patients. Early signs of Alzheimer’s cannot always be distinguished from normal memory loss with age. Parkinson’s is usually not diagnosed until there are clear outward symptoms like the tremors, rigidity, or muscle weakness that characterize the disease, which means the nerve cell loss is already taking place.
For both diseases, both genetic and environmental risk factors have been identified. For example, consumption of Vitamin D and caffeine can be associated with a lower risk of Parkinson’s disease. And now, new studies are beginning to take advantage of a less obvious but still common symptom that may serve as a potential earlier indicator for both diseases: the deterioration of the sense of smell.
Sense of Smell and Parkinson’s Disease
In a recent study published in the journal of the American Academy of Neurology, scientists monitored a group of more than 2,400 people over an average of 10 years to look for a possible link between sense of smell and the onset of Parkinson’s disease. First, the study participants were tested on their ability to properly identify common odors like gasoline and citrus, and they were placed into groups of good, intermediate, and poor smellers. Then, after several years had passed, the participants were tested for signs of Parkinson’s.
Even when controlling for other factors that may influence the onset of the disease, like caffeine consumption or smoking, scientists found that on average those with a poor sense of smell were more likely to be diagnosed with Parkinson’s disease up to six years later.
Although the link was clear when averaging over all of the participants, the link was found to be stronger in men than in women. There was also no definitive link found between sense of smell and Parkinson’s in the black participants studied. The efficacy of smell as a potential biomarker may thus depend on race or, as noted by the authors, the study may have suffered from a small sample size. (Only 12 of the approximately 950 black participants developed the disease.) More research is thus needed, and in particular research that separates participants by race and sex.
Sense of Smell and Alzheimer’s
Another recent study, led by graduate student Jennifer Stamps and Professor Kenneth Heilman at the University of Florida’s McKnight Brain Institute, investigated what has since come to be known as the peanut butter test. Patients are asked to close their eyes as a dish of peanut butter is raised closer and closer to one of their nostrils while the other nostril is closed. Using a ruler, the researchers could quantify how well each nostril worked to independently smell the peanut butter (in other words, how close the peanut butter had to come to the nostril before its presence was sensed).
The study concluded that patients with Alzeimer’s could not smell the peanut butter as easily through their left nostril as they could through their right side. There was broad media coverage for this exciting result—an easy-to-implement-at-home test for Alzheimer’s!—and anecdotal evidence suggested the smell test could predict the onset of dementia as early as a decade before a standard diagnosis.
However, experts in statistics warn that the sample size for the study, while an important first step, is not big enough to draw any broad conclusions. In other words, for a sample of people to be representative of the broader population (i.e. the rest of us who weren't involved in the study), a large enough number of people have to be tested so that any other factors that may influence the results, like how much each individual likes to exercise or smoke cigarettes, are averaged out. If the sample of people studied is too small, a few outliers or exceptions to the rule could shift the results.
The researchers monitored 94 people, only 18 of whom were diagnosed with Alzheimer’s. In fact, a study published the very next year in the Journal of Neurological Sciences attempted to replicate the study’s result with a similar sample size but failed to see the same pattern.
Another very important caveat in interpreting the results from any similar study is that other things can cause a lack of sense of smell besides degenerative brain diseases. For example, inflammation, tumors, and even post-traumatic stress will lower our olfactory abilities. Even Stamps, who brought us the peanut butter test, warns against self-diagnosing at home since slight differences between your left and right nostrils can also be normal.
So the search continues for a clear indicator that could warn us of the onset of Alzheimer’s or Parkinson’s early enough that we may have a better chance of staving off the more devastating effects of either disease. The link between our sense of smell and different forms of dementia is well-established but how we can harness that link for an early diagnosis still remains to be seen.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
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