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

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

One of the stranger symptoms of Covid – the loss of the sense of smell – is a symptom that was considered a warning sign of dementia long before the pandemic.

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

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

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Previous research has found that some Covid patients develop cognitive impairment after becoming infected. 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 illness.

“Our data strongly suggest that adults over 60 years of age are more susceptible to post-Covid cognitive impairment if they have had an olfactory disorder, regardless of the severity of the Covid,” said study co-author Gabriela Gonzalez-Aleman, professor at Pontificia Universidad Catolica Argentina in Buenos Aires, adding that it is too early to say if the cognitive impairment is permanent.

The study followed 766 adults aged 55 to 95 for a year after they were infected. Almost 90 per cent had a confirmed case of Covid and all underwent regular physical, cognitive and neuropsychiatric testing over the course of a year.

Two-thirds of those infected had some type of cognitive impairment by the end of the year. The impairment was severe in half of the participants.

The researchers did not have hard data on the state of the patients’ cognitive function before they were infected with Covid to compare with the results at the end, but they surveyed the participants’ families about their cognitive function before they were infected and did not include people who did had significant cognitive impairment prior to the study.

Loss of smell is an established precursor to cognitive decline, according to Jonas Olofsson, a psychology professor at Stockholm University who studies the link between smell and dementia risk — and was not involved in the new research. It is also known that Covid can lead to a permanent loss of smell, he said.

“The question is whether these two research directions overlap,” Olofsson said. “This study is quite compelling, although the information I’ve seen so far is inconclusive. ”

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 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 take a closer look at how they are connected.”

A separate study – unrelated to Covid – published last Thursday in the journal Alzheimer’s & Dementia further investigates this link. Researchers at the University of Chicago found that not only can a decline in the sense of smell over time predict the loss of cognitive function, the loss of the sense of smell can also be a warning sign of structural changes in regions of the brain implicated in Alzheimer’s disease dementia is important.

Using data from Rush University’s Memory and Aging Project, the researchers tracked smell loss in 515 older adults over 22 years of age. They also measured gray matter volume in parts of the brain linked to dementia and smell.

They found that people whose sense of smell declined more quickly over time had less gray matter in these two regions of the brain. The same was not true for parts of the brain associated with vision, suggesting that the sense of smell has a unique connection to cognition in terms of structural differences.

“The change in olfactory function over time can predict not only the development of dementia, but also the size of these important brain regions,” 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 smell loss, but virus-related smell loss was a rare event before the pandemic, Pinto said. This means that scientists have only recently been able to conduct large studies into how the loss of smell caused by a virus can affect cognition.

“The sense of smell is extremely important for perception, especially for the brain to process information about the environment. If you cut off that communication channel 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 whether Covid-related loss of smell can lead to cognitive decline remains unclear.

“That’s an open question — does SARS-CoV-2 injury to the olfactory system cause problems not only in the olfactory system but also in the brain itself?” said Pinto.

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According to Olofsson, the olfactory system — the parts of the brain associated with smell, including the olfactory bulb, the part of the brain that processes smell — is linked to parts of the brain that process memory. While it’s possible for Covid to disrupt the olfactory bulb and the brain around it to deteriorate, Olofsson said that’s not likely.

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

Or Covid may simply exacerbate existing loss of smell or cognitive decline that went unnoticed prior to infection, Olofsson said. Patients may already have had some cognitive decline when they contracted Covid, or may already have had mild olfactory system impairment, making them more susceptible to Covid-induced smell loss.

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

If it turns out that Covid smell loss can lead to cognitive impairment, understanding the link could help doctors intervene early to address smell loss and potentially prevent cognitive decline in those at high risk.

“We’re going to look at the endemic circulation of a virus that isn’t going away,” Pardo said. “If we learn more ways to quickly restore smell, we may be able to minimize the damage that loss of the sense of smell with cognitive problems can cause in susceptible people.”

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