One of the earliest and most common warning signs of diseases like Alzheimer’s and Parkinson’s is not necessarily memory loss or tremors, but loss of smell. In fact, studies in recent years have shown that olfactory dysfunction is associated with many neurodegenerative diseases.
Researchers hoping to understand this symptom tend to look for how each disease causes loss of smell in its own way. But in a review published in Lancet Neurology, Richard Doty, director of the Smell and Taste Center at the University of Pennsylvania, suggests that there may be a common mechanism causing olfactory dysfunction across illnesses.
Doty thinks that finding common factors causing smell loss could point to processes that initiate a number of neurodegenerative diseases. In the course of his research he reviewed different diseases, looking for a common cause for the varying levels of olfactory loss. After ruling out a number of possibilities, including environmental and physiological factors, he found evidence for one common thread: damage to neurotransmitter and neuromodulator receptors in the brain that affect the brain’s ability to process smells.
Not all smell loss is tied to a neurological disease; it can also be caused by the common cold as well as a history of smoking, regular aging, and other factors. While Doty’s approach may be promising, research into smell loss is difficult because of the lack of early data. “Smell testing isn’t part of a standard check-up, and people don’t recognize a smell problem themselves until it’s already severe,” Doty said in a statement. Some doctors think that should, and could, change.
Here’s David Noonan, reporting for Scientific American:
The simplicity and low cost of a smell test—$26.95 retail, before additional doctor or hospital charges—make it especially appealing in neurology, a field dominated by positron emission tomography (PET) scans, dopamine transporter single-photon emission computed tomography (DAT-SPECT) imaging and other expensive technologies. Because it is so economical, [Davangere Devanand, a professor of psychiatry and neurology at Columbia University] says researchers should incorporate olfaction testing into studies that look at other Alzheimer’s risk factors in large populations.
Research beginning in Japan will look at smell and physiological changes in older populations over the next few years, possibly clarifying any links between those variables.
Establishing a universal cause for smell loss won’t necessarily hasten a cure for neurodegenerative diseases, but researchers are working on other approaches including using bacteriophages to block and even reverse plaque development in the brain, as NOVA Next reported in 2016.