By Kaitlin SullivanNBC
News
Loss of smell is one of the strangest symptoms that COVID-19 can cause, but long before the coronavirus pandemic broke out, it was considered an alarm signal for Alzheimer's.
The big question researchers are now asking is whether the disappearance of the ability to smell due to COVID-19 could also be associated with cognitive decline.
About 5% of coronavirus patients worldwide - some 27 million people - have reported having no sense of smell for more than six months.
New preliminary results presented Sunday at the Alzheimer's Association International Conference in San Diego, California, suggest there may be a link, though experts caution that more data is needed.
Loss of smell after contracting COVID-19 could be a sign of cognitive decline. The Washington Post via Getty Images
Previous research has found that some COVID-19 patients develop cognitive decline after infection.
In the new study, which has not been published in a peer-reviewed journal, scientists from Argentina found that loss of smell during coronavirus may be a stronger predictor of cognitive decline, regardless of disease severity.
"Our data clearly suggest that adults over 60 years of age are more vulnerable to cognitive decline after COVID-19 if they have had olfactory dysfunction, regardless of the severity of COVID-19," explained study co-author Gabriela González- Aleman, a professor at the Pontificia Universidad Católica Argentina in Buenos Aires, who added that it is too early to know if this cognitive decline is permanent.
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The study followed 766 adults between the ages of 55 and 95 for a year after becoming infected.
Almost 90% had the disease at least once, and all completed regular physical, cognitive, and neuropsychiatric tests over the course of a year.
Two-thirds of those infected had some type of cognitive impairment at the end of that year.
In half of the participants, the impairment was severe.
The researchers did not have enough data on the state of the patients' cognitive function before contracting COVID-19 to be able to compare it with the final result, but they asked their families about their cognitive capacity before they were infected: no one had shown signs of suffering clear cognitive impairment before the study.
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According to Jonas Olofsson, a psychology professor at Stockholm University who discusses the relationship between the sense of smell and dementia risk, and who was not involved in the new research, loss of the ability to smell is a well-established precursor to impairment. cognitive.
It's also well established that COVID-19 can lead to lasting loss of smell, he stressed.
"The question is whether those two lines of inquiry intersect," Olofsson mused.
"This study is quite tempting, although the data I've seen so far doesn't allow for strong conclusions," he said.
Dr. Claire Sexton, senior director of scientific and outreach programs for the Alzheimer's Association, said, "Loss of smell is a sign of an inflammatory response in the brain."
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"We know that inflammation is part of the neurodegenerative process in diseases like Alzheimer's. But we need to delve into exactly how they are connected," he explained.
An independent study unrelated to COVID-19 and published last Thursday in the journal Alzheimer's & Dementia explores that connection.
Researchers at the University of Chicago found that not only can a diminished sense of smell over time predict loss of cognitive function, but loss of the sense of smell can also be a warning sign of structural changes in brain regions important in Alzheimer's disease and dementia.
Using data from Rush University's Memory and Aging Project, the researchers tracked smell loss in 515 older adults for 22 years.
They also measured gray matter volume in parts of the brain linked to dementia and those linked to smell.
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They found that people whose sense of smell declined more rapidly over time had less gray matter in these two brain regions.
The same was not true for parts of the brain related to vision, suggesting that the sense of smell has a unique link to cognition in terms of structural differences.
"Not only can change in olfactory function over time predict the development of dementia, but it can also predict the size of those brain regions that are important," said study leader Dr. Jayant Pinto, director of Rhinology and Allergy at UChicago Medicine.
COVID-19 isn't the first virus to cause loss of smell, but losing smell linked to the virus was a rare occurrence before the pandemic, Pinto said.
This means that only recently have scientists been able to conduct large studies on how this virus-caused problem can affect cognition.
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"The sense of smell is extremely critical for cognition, especially for the brain to handle information about the environment. If that channel of communication with the brain is closed, it suffers," said Dr. Carlos Pardo, professor of Neurology at Johns Hopkins University, which was not involved in either study.
But it is not yet known whether COVID-19-related loss of smell can lead to cognitive decline.
"That's an open question: Does SARS-CoV-2 damage to the olfactory system cause problems not only in the olfactory system, but also in the brain itself?" Pinto said.
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According to Olofsson, the olfactory system - the parts of the brain related to smell, including the olfactory bulb, the part of the brain that processes smell - connects with parts of the brain that process memory.
Although it's possible that COVID-19 disrupts the olfactory bulb and then the brain deteriorates around it, Olofsson said this isn't likely.
"There are many other ways these two things can be related. The cause can be pathology unrelated to the effect of COVID-19," he said.
Or COVID-19 may simply amplify existing loss of smell or cognitive impairment that went unnoticed before infection, Olofsson stressed.
It is possible that the patients were already experiencing some type of cognitive impairment when they contracted COVID-19, or that they already had a slight impairment of the olfactory system, which made them more susceptible to COVID-19-related loss of smell.
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"It could be that the olfactory function was maintained despite being atrophied, but when COVID-19 hit, it overrode it," he said.
If it turns out that loss of smell from COVID-19 can cause cognitive decline, understanding the connection could help clinicians intervene with smell loss early and potentially prevent cognitive decline in people at high risk.
"We will face the endemic circulation of a virus that is not going to disappear," said Pardo.
"If we learn more ways to quickly regain smell, we might be able to minimize the damage that loss of smell can cause with cognitive problems in susceptible people," she said.
