One of the strangest symptoms of Covid – loss of smell – is a symptom that long before the pandemic was thought to be a warning sign of dementia.
The big question for researchers now is whether Covid-related loss of smell could also be associated with cognitive decline. About 5% of Covid patients worldwide – some 27 million people – have reported a loss of smell lasting more than six months.
New preliminary results presented at the Alzheimer’s Association’s international conference in San Diego on Sunday suggest there may be a link, although experts warn that more research is needed.
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Previous research has found that some Covid patients develop cognitive impairment after their infection. In the new study – which has not been published in a peer-reviewed journal – Argentinian researchers found that loss of smell during Covid may be a stronger predictor of cognitive decline, regardless of the severity of the illness. sickness.
“Our data strongly suggest that adults over 60 are more vulnerable to post-Covid cognitive impairment if they had smell dysfunction, regardless of Covid severity,” the co-author said. study Gabriela Gonzalez-Aleman, a professor at Pontificia Universidad Catolica Argentina in Buenos Aires, adding that it is too early to say whether the cognitive impairment is permanent.
The study followed 766 adults aged 55 to 95 for one year after their infection. Almost 90% 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 this year. In half of the participants, the impairment was severe.
The researchers did not have hard data on the state of the patients’ cognitive function before contracting Covid to compare with the results at the end, but they asked the families of the participants about their cognitive function before the infection and did not include people who had clear cognitive impairment before the study.
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, loss of smell is a well-established precursor to decline. cognitive. It’s also well established that Covid can cause lasting loss of smell, he said.
“The question is whether these two lines of research intersect,” Olofsson said. “This study is quite enticing, although the information I have seen so far does not allow for solid conclusions to be drawn.”
The smell-brain connection
According to Dr. Claire Sexton, senior director of science 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 disease,” Sexton said. But we need to dig deeper into exactly how they are connected. »
A separate study – unrelated to Covid – published last Thursday in the journal Alzheimer’s & Dementia further explores this link. Researchers from the University of Chicago have found that not only can a decrease in sense of smell over time predict loss of cognitive function, but loss of sense of smell can also be a harbinger of changes structures in brain regions important in Alzheimer’s disease and dementia.
Using data from the Memory and Aging Project at Rush University, the researchers tracked the loss of smell in 515 people over the age of 22. They also measured gray matter volume in parts of the brain linked to dementia and those linked to smell.
They found that people whose sense of smell declined more rapidly over time ended up with smaller amounts of gray matter in these two brain regions. The same was not true for parts of the brain related to vision, suggesting that smell has a unique connection to cognition in terms of structural differences.
“Not only can the 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 Dr. Jayant Pinto, director of rhinology and allergies at UChicago Medicine.
“Critical” smell for cognition
Covid is not the first virus to cause loss of smell, but virus-related loss of smell was a rare event before the pandemic, Pinto said. This means that it is only recently that scientists have been able to conduct large studies of how loss of smell caused by a virus can impact cognition.
“Smell is extremely critical for cognition, especially for the brain to process information about the environment. If you close this channel of communication with the brain, it will suffer,” said Dr. Carlos Pardo, professor of neurology at Johns Hopkins University, who was not involved in either study.
But it’s unclear whether Covid-related loss of smell can cause cognitive decline.
“It’s an open question – does damage to the olfactory system due to SARS-CoV-2 lead to 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 related to smell, including the olfactory bulb, the part of the brain that processes smell — connects to the parts of the brain that process memory. While it’s possible that Covid is disrupting the olfactory bulb and causing the brain to deteriorate around it, Olofsson said that’s not likely.
“There are a number of other ways these two things can be linked. The cause may be a pathology unrelated to the Covid effect,” he 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 decline when they contracted Covid, or may have already had mild impairment of the olfactory system, which made them more susceptible to Covid-related loss of smell.
“It could be that the olfactory function was maintained despite its atrophy, but when Covid came along it wiped it out,” he said.
If it turns out that smell loss from Covid can lead to cognitive impairment, understanding the link could help doctors intervene early in smell loss and potentially prevent cognitive decline in people at high risk.
“We will be dealing with endemic circulation of a virus that is not going away,” Pardo said. “If we learn more ways to quickly recover the sense of smell, we may be able to minimize the damage that loss of smell can cause with cognitive problems in sensitive people.”
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