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

One of the strange symptoms of Covid disease – loss of smell – is one that, long before the epidemic, was considered a warning sign of dementia.

The big question for researchers now is whether the loss of smell associated with Covid may also be associated with cognitive decline. About 5 percent of Covid patients worldwide – about 27 million people – have reported a loss of smell for more than six months.

New preliminary findings presented Sunday at the Alzheimer’s Society International Conference in San Diego suggest there may be a link, although experts caution that 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 was not published in a peer-reviewed journal – researchers in Argentina found that loss of smell during Covid may be a stronger predictor of cognitive decline, regardless of the severity of the disease.

Study co-author Gabriela Gonzalez Aleman, professor at Pontificia Universidad Catolica Argentina in Buenos Aires, said it was too early to tell if cognitive impairment was permanent.

The study followed 766 adults between the ages of 55 and 95 for a year after infection. Nearly 90 per cent had a confirmed case of Covid and all completed regular physical, cognitive and neuropsychological tests over the course of one year.

Two-thirds of those diagnosed had some form of cognitive impairment at the end of that year. In half of the participants, the vulnerability was severe.

The researchers did not have solid data on the state of patients’ cognitive function before they contracted Covid in order to compare with the outcome at the end, but they asked the participants’ families about their pre-infection cognitive function and did not include people who had it. Perceptual impairment evident before the study.

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

“The question is whether these two lines of research intersect,” Olofson said. “This study is very confusing, although the information I have seen so far does not allow for any strong conclusions.”

Smell connection to the brain

According to Dr. Claire Sexton, senior director of science and outreach programs at the Alzheimer’s Association, “loss of smell is a signal 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 exactly how they relate.”

A separate study – unrelated to Covid – was published last Thursday in the journal Alzheimer’s & Dementia Investigations that has another link. Researchers at the University of Chicago found that not only could a decreased sense of smell over time predict loss of cognitive function, but the loss of smell could also be a warning sign of structural changes in brain regions important in Alzheimer’s disease and dementia.

Using data from the Rush University Memory and Aging Project, researchers tracked the loss of smell in 515 older adults over a 22-year period. They also measured gray matter volume in parts of the brain associated with dementia and those related to smell.

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

“Change in olfactory function over time can not only predict the development of dementia, but it can also predict the size of important brain regions,” said study leader Dr. Jayant Pinto, director of rhinology and allergology at UChicago Medicine.

Smell “critical” for perception

Pinto said that Covid is not the first virus to cause a loss of smell, but the loss of smell associated with the virus was a rare event before the outbreak of the epidemic. This means that it is only recently that scientists have been able to conduct large studies of how the loss of smell caused by the virus affects cognition.

“The sense of smell is critical to cognition, especially for the brain to deal with information about the environment. If you shut down this communication channel with the brain, you will suffer,” said Dr. Carlos Pardo, a professor of neuroscience at Johns Hopkins University, who was not involved in either study. .

But it remains unclear whether the loss of smell associated with Covid can cause cognitive decline.

“This is an open question – does injury to the olfactory system from SARS-CoV-2 lead to problems not only in the olfactory system, but also in the brain itself?” Pinto said.

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According to Olofson, the olfactory system — the parts of the brain associated with smell, including the olfactory bulb, the part of the brain that processes smell — communicates with the parts of the brain that process memory. While it is possible that Covid will disrupt the olfactory bulb and then deteriorate the brain around it, Olofsson said that is unlikely.

“There are a number of other ways in which these two things can be linked. The cause may be a disease unrelated to the impact of Covid.”

Or, Covid may simply amplify the current loss of smell or cognitive decline that went unnoticed prior to infection, Olofson said. Patients may have already experienced some cognitive decline when they contracted Covid, or they may already have had a slight impairment of their sense of smell, which made them more susceptible to Covid-related loss of smell.

“It could be that the olfactory function was preserved despite its atrophy, but when Covid came, it wiped out,” he said.

If it turns out that loss of Covid smell can cause cognitive impairment, understanding the connection could help clinicians intervene early loss of smell and potentially prevent cognitive decline in high-risk people.

“We will deal with an epidemic of a virus that does not go away,” Pardo said. “If we learn more ways to get us back in smell quickly, we may be able to reduce the damage that loss of smell can do to cognitive problems in susceptible people.”

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