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Sense of Smell Holds Key to Dementia Onset in Parkinson's

The loss of the sense of smell is a strong predictor of oncoming cognitive problems in people with Parkinson’s disease, according to data published in the January issue of the journal Brain.  The new study suggests that the loss of smell may provide a new method to detect which people with Parkinson’s are likely to develop dementia in the near future.

People with Parkinson’s often have problems with their sense of smell, a symptom called hyposmia.  This research suggested that hyposmia might be related to dementia, which affects many people with Parkinson’s disease.  In the new study, researchers led by Atsushi Takeda, M.D., of Tohoku University in Sendai, Japan, specifically tested for such a link between hyposmia and dementia.  The researchers studied a group of people with Parkinson’s who did not have dementia upon entering the study.  They tested them at the beginning of the study and three years later on a number of sensory, behavioral, and cognitive scales, including tests of smell, vision, motor function and memory.

Results

  • Of the 44 people studied, 24 had severe hyposmia at the time of recruitment.
  • Three years later, the people with severe hyposmia had greater motor, cognitive, and visuoperceptual impairment than the people who did not have hyposmia.
  • Three years later, 10 (42 percent) of the people who had severe hyposmia upon enrollment in the study had developed dementia, while none of the people who had a normal sense of smell developed dementia.
  • Overall, people with severe hyposmia upon enrollment had a nearly nineteen-fold increase in their likelihood of developing dementia within three years.
  • Brain scans also showed that people with severe hyposmia had more widespread loss of brain tissue than people without hyposmia, both at the time of enrollment and three years later.

What Does it Mean?

Studies estimate that as many as 80 percent of people with Parkinson’s disease will develop dementia after 20 years of late onset Parkinson’s.  Yet, to date, there is no test that can predict whether a person is likely to develop dementia in the near future.

Smell impairment is also common in Parkinson’s.  In fact, some studies show that smell impairment can appear many years before the onset of motor symptoms (or cognitive symptoms).  In addition, testing for smell impairment requires good cognitive function: participants need to identify a smell in a way that requires good memory and language performance.  People with severe cognitive impairment will probably not score well on olfaction tests just because of their cognitive abilities.  Therefore, cognition and smell are strongly associated.

The strength of this study lies in its design.  There was a three-year interval between olfaction testing and the last cognitive testing.  A limitation of this study is that the onset of hyposmia in this cohort is unknown.  It is possible that those with olfactory impairment developed hyposmia decades before the study.

Given the high frequency of smell impairment in Parkinson’s and of cognitive impairment as the disease progresses, a probable safe conclusion from this study is that normal olfaction is associated with a good cognitive outcome.  Why some individuals with Parkinson’s are protected from smell impairment and cognitive impairment remains to be studied.

In the future, if researchers can develop effective interventions that prevent or delay the development of dementia, it is possible that this test will allow doctors to identify people with Parkinson’s who would benefit from such treatments before they develop cognitive symptoms.  Even in the absence of an effective therapy, being able to predict dementia susceptibility may help those individuals to make informed decisions.

Related Resources

If you would like to learn more about cognition and Parkinson’s, view our online seminar on the topic.
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Reference: Bara, T., Kikuchi, A., Hirayama, K., Nishio, Y., Hosokai, Y., Kanno, S., Hasegawa, T., Sugeno, N., Konno, M., Suzuki, K., Takahashi, S., Fukuda, H., Aoki, M., Itoyama, Y., Mori, E., Takeda, A. (2012). Severe olfactory dysfunction is a prodromal symptom of dementia associated with Parkinson's disease: a 3 year longitudinal study. Brain. 135 (1): 161-169, doi: 10.1093/brain/awr321.

Source Date: Apr 05 2012