Loss of smell is a warning sign of Alzheimer’s disease. What happens if you lose your sense of smell due to Covid?

One of the strangest symptoms of covid, loss of the sense of smell, is a symptom that, long before the pandemic, was considered a warning sign of dementia.

The big question for researchers now is whether Covid-related loss of smell might also be associated with cognitive decline. About 5 percent of Covid patients worldwide, some 27 million people, reported a loss of smell that lasted more than six months.

New preliminary findings presented Sunday at the Alzheimer’s Association International Conference in San Diego suggest there may be a link, though experts warn more research is needed.

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Previous research has found that some covid patients develop cognitive impairment after infection. In the new study, which has not been published in a peer-reviewed journal, researchers from Argentina found that loss of smell during covid-19 may be a stronger predictor of cognitive decline, regardless of disease severity.

“Our data strongly suggest that adults over the age of 60 are more vulnerable to post-COVID cognitive decline if they had olfactory dysfunction, regardless of COVID severity,” said study co-author Gabriela Gonzalez-Aleman, professor of the Pontifical Catholic University of Argentina in Buenos Aires, adding that it is too early to know whether the cognitive decline is permanent.

The study followed 766 adults ages 55 to 95 for a year after infection. Almost 90 per cent had a confirmed case of Covid and all completed regular physical, cognitive and neuropsychiatric tests within 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 hard data on the cognitive function status of the patients before contracting COVID to compare with the findings at the endpoint, but they did ask the families of the participants about their cognitive function before infection and did not include to people who had clear cognitive impairment before the study.

According to Jonas Olofsson, a psychology professor at Stockholm University who studies the link between the sense of smell and dementia risk, and who was not involved in the new research, loss of smell is a well-established precursor to cognitive decline. It’s also well established that Covid can cause lasting loss of smell, he said.

“The question is whether those two lines of inquiry intersect,” Olofsson said. “This study is quite tempting, although the information I have seen so far does not allow for any solid conclusions. ”

The smell-brain connection

According to Dr. Claire Sexton, senior director of scientific programs and outreach at the Alzheimer’s Association, “Loss of smell is a sign of an inflammatory response in the brain.”

“We know that inflammation is part of the neurodegenerative process in diseases like Alzheimer’s,” Sexton said. But we need to dig deeper into how exactly they are connected.”

A separate, unrelated to Covid study published last Thursday in the journal Alzheimer’s & Dementia further investigates that connection. Researchers at the University of Chicago found that a decline in the sense of smell over time may not only predict loss of cognitive function, but loss of the sense of smell may also be a warning sign of structural changes in regions of the brain. brain 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 over 22 years. They also measured the volume of gray matter in parts of the brain that were linked to dementia and to smell.

They found that people whose sense of smell deteriorated faster over time ended up with smaller amounts of gray matter in both brain regions. The same was not true for the parts of the brain linked to vision, suggesting that the sense of smell has a unique link to cognition in terms of structural differences.

“Changes in olfactory function over time can not only predict the development of dementia, but can also predict the size of brain regions that are important,” said study leader Dr. Jayant Pinto, director of Rhinology and Allergy at UChicago Medicine.

Smell ‘critical’ for cognition

Covid isn’t the first virus to cause loss of smell, but virus-related loss of smell was rare before the pandemic, Pinto said. That means it’s only recently that scientists can do big studies on how loss of smell caused by a virus can affect cognition.

“The sense of smell is extremely critical for cognition, especially for the brain to handle information about the environment. If you close that channel of communication with the brain, it will suffer,” said Dr. Carlos Pardo, a professor of neurology at Johns Hopkins University, who was not involved in either study.

But it is not yet clear whether Covid-related loss of smell can cause 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 the parts of the brain that process memory. While it’s possible that Covid disrupts the olfactory bulb and then the brain deteriorates around it, Olofsson said this isn’t likely.

“There are a number of other ways that these two things can be related. The cause may be a pathology that is unrelated to the Covid effect, ”she said.

Or Covid may simply amplify existing loss of smell or cognitive decline that went unnoticed before infection, Olofsson said. Patients may have already experienced some cognitive impairment when they contracted COVID, or they may have already had a slight impairment of the olfactory system, making them more susceptible to COVID-related loss of smell.

“It could be that the olfactory function was maintained despite being atrophied, but when the Covid arrived it devastated it,” he said.

If it turns out that loss of smell caused by COVID-19 can cause cognitive decline, understanding the connection could help doctors intervene early on loss of smell and potentially prevent cognitive decline in people at high risk.

“We will be dealing with the endemic circulation of a virus that does not go away,” Pardo said. “If we learn more ways to quickly regain smell, we may be able to minimize the damage that loss of smell can cause with cognitive problems in susceptible people.”

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