Does a Poor Sense of Smell Predict Alzheimer's Disease?

The Sense of Smell Could Be Related to Alzheimer's Disease
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Yuck! What's that smell? Well, according to some researchers, your ability to answer that question may predict your chances of developing Alzheimer's disease.

Many research studies over the last 20 years have demonstrated a connection between an inability to detect odors and a decline in cognition. Several of those studies also demonstrated a predictive relationship where a poor sense of smell in people who did not have dementia predicted an increased likelihood of those same individuals developing symptoms of Alzheimer's disease over time.

For example, one study involved 308 adults ages 46-86 without cognitive impairment who were studied over the course of three years. Researchers found that a decreased ability to tell the difference between odors was strongly associated with an increased risk for the development of cognitive decline.

Researchers have also looked at mild cognitive impairment. Mild cognitive impairment is a condition where people have some difficulty with the cognitive functions of memory, concentration, orientation and communication abilities such as word-finding skills. Some, but not all, cases of mild cognitive impairment progress into Alzheimer's disease.

In a five-year study, scientists tracked 589 people who lived in their own homes. None of the participants had mild cognitive impairment at the beginning of the study. Each person's cognitive functioning as well as their ability to detect smells were tested at the beginning of the study, and annually thereafter.

The results? The participants who demonstrated a difficulty in detecting odors experienced declining scores on the cognitive functioning tests. In other words, an inability to identify the odors predicted who would develop signs of mild cognitive impairment.

What Causes the Sense of Smell to Be Affected in Alzheimer's disease?

What causes this loss?

Studies seem to indicate that the sense of smell is affected by the presence of beta amyloid protein in the areas of the brain that help us detect and perceive odors. Some research demonstrates that these areas of the brain are where the protein accumulates first, impairing the sense of smell before cognitive functioning is affected.

Multiple studies have demonstrated a high correlation between Alzheimer's disease and a buildup of protein pathology in the olfactory system (our sense of smell).

One study reviewed the results of 130 autopsies and found that in all cases of definitive Alzheimer's disease, there was tau pathology in the olfactory system of the brain.

A second research study found that in reviewing 273 autopsies, there was a high correlation between the buildup of tau in the olfactory system and the presence of brain damage related to Alzheimer's disease.

Studies also indicate that as Alzheimer's disease progresses, the ability to smell further decreases.

Does the Loss of Smell Develop with Other Types of Dementia?

Researchers have found that both Alzheimer's disease and Lewy body dementia demonstrated significant effects on the sense of smell, while people with progressive supranuclear palsy and corticobasal degeneration showed no impairment.

Could Aging Be the Reason the Sense of Smell Is Declining?

Despite the preponderance of evidence that seems to exist, one research article raises some questions. In this study, scientists reviewed the many studies that have been conducted on the sense of smell and its tie to a decline in cognitive functioning, and concluded that much of the research failed to show strong proof due to the design of the studies. One of the concerns is that as people age, the sense of smell declines. Therefore, research studies have to take this factor into account in order for the research to prove that the inability to identify smells is not triggered by age but rather by the process in the brain related to loss of cognitive functioning.

Based upon the significant amount of research that has been conducted, however, it remains likely that there is a tie between a poor sense of smell and cognitive decline.

Should You Test Your Sense of Smell to Find Out if You're at Risk for Dementia?

A simple smell test is not recommended as a way to identify those at risk for dementia at this time, although it's possible that at some point it could be included as part of a battery of tests. What may be more productive at this time is focusing on what you can do to reduce your risk of developing dementia, such as mental activity, physical exercise, and a healthy diet.

Additionally, if you remain concerned about your memory, you could take an at-home test such as the SAGE dementia screening test and discuss your results with a physician.

Sources:

Archives of General Psychiatry. 2007;64(7):802-808. Olfactory Identification and Incidence of Mild Cognitive Impairment in Older Age. http://archpsyc.jamanetwork.com/article.aspx?articleid=482353&resultClick=1

Clinical Neuropathology. 2006 Nov-Dec;25(6):265-71. Olfactory tau pathology in Alzheimer disease and mild cognitive impairment. http://www.ncbi.nlm.nih.gov/pubmed/17140156

The Laryngoscope. Volume 122, Issue 7, pages 1455-1465, July 2012. Olfactory identification testing as a predictor of the development of Alzheimer's dementia: A systematic review. http://onlinelibrary.wiley.com/doi/10.1002/lary.23365/abstract

National Institute on Aging. Advances in Detecting Alzheimer's Disease.Accessed October 31, 2013.  http://www.nia.nih.gov/alzheimers/publication/2011-2012-alzheimers-disease-progress-report/advances-detecting-alzheimers#.UnJOohAhGoY

Neuropathology Applied Neurobiology. 2003 Oct;29(5):503-10. Tau pathology in the olfactory bulb correlates with Braak stage, Lewy body pathology and apolipoprotein epsilon4. http://www.ncbi.nlm.nih.gov/pubmed/14507342

Translational Psychiatry. 2012 May, 2(5), e118. Olfactory discrimination predicts cognitive decline among community-dwelling older adults. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3365262/

University of Pennsylvania School of Medicine. 11-Mar-2004. Loss of smell linked to key protein in Alzheimer's disease. http://www.eurekalert.org/pub_releases/2004-03/uopm-los030304.php

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