Persistent loss of smell is a better predictor of lingering cognitive symptoms after covid-19 infection than disease severity.
A common symptom of covid-19 is a sudden loss of smell. Previous research has found that smell loss can be an early warning sign of Alzheimer’s and other conditions. Evidence suggests that covid-19 can also lead to long-lasting neurological problems like difficulty concentrating and memory loss.
To see if loss of smell due to covid-19 is associated with persistent cognitive symptoms, Gabriela Gonzalez-Alemán at the Pontifical Catholic University of Argentina in Buenos Aires and her colleagues analysed data from 766 adults aged 60 years or older who had no history of cognitive impairment. Each had taken a PCR test at a covid-19 testing clinic in northern Argentina, and nearly 90 per cent tested positive.
The researchers collected data on disease severity, and they had participants complete a sniff test and a series of cognitive assessments at least three months after the covid-19 test.
They found that two-thirds of the people who tested positive for covid-19 had some form of memory impairment, and for half of this group, it was severe enough to interfere with their daily lives.
What’s more, the people who still had complete loss of smell three months post-infection were roughly 1.5 times more likely to have lasting cognitive impairments compared with those who never lost or had already regained their sense of smell. This was the case regardless of the severity of their infection or whether they had required hospitalisation, says Gonzalez-Alemán.
These results support the idea that coronavirus may the brain by way of the nose, says Frederic Meunier at the University of Queensland in Australia. “Scientists could waste a lot of time looking at other possibilities for how the virus enters the brain, but it seems there is something here that is worth pursuing.”
Given that most brain cells lack the ACE2 receptor the virus normally uses to break into cells, it is unclear whether cognitive symptoms are due to direct infection of the brain. We know that covid-19 can infect cells in the nose: the resulting inflammation of these cells can disrupt scent-detecting olfactory neurons, causing sudden smell loss. But a study in human cells published last month suggests that the coronavirus may then build tiny tunnels in nose cells and shuttle through these to infect brain cells. If this is the case, it could provide a possible explanation for the link between loss of smell and some cognitive symptoms.
Gonzalez-Alemán and her team will continue to recruit patients and plan to follow up with them for four years. Their hope is not only to learn more about long term effects of covid-19 infection on cognition, but also to better understand the mechanism linking sense of smell with memory problems more broadly.
“There are lots of signs that make us think maybe the biological processes in covid-19 cognitive impairment could be the same in neurodegenerative disorders,” says Gonzalez-Alemán.
The research was presented on July 31 at the Alzheimer’s Association International Conference in San Diego, California.
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