COVID Causes People to Lose Their Sense of Smell Because of Changes in the Brain, Study Suggests

The sudden loss of smell in some COVID-19 patients may be explained by changes to the brain, according to a study.

After the coronavirus that causes COVID-19 first emerged late last year, losing one's sense of smell and taste quickly emerged as a possible symptoms of the disease. The U.S. Centers for Disease Control and Prevention lists these alongside other symptoms such as a fever or chills, cough, shortness of breath, and fatigue.

Study Co-author Maxime Niesen of the Functional Brain Mapping Laboratory at Belgium's ULB university and colleagues set out to explore the hypothesis that some COVID-19 patients lose their sense of smell because the infection affects cells that support certain neurons that are key for this sense working.

It is also thought that inflammation stops certain receptors from picking up smells, and a person's sense of smell is restored when this subsides, Professor Barry Smith, director of the Centre for the Study of the Senses at the U.K's University of London who did not work on the paper, told Newsweek via email. If a person loses their sense of smell totally or partially for a longer period of time, this may be due to the overreaction of the immune system blocking receptors, he said.

After studying 12 COVID-19 patients, the authors of the paper suggested there are a number of reasons why they lost their sense of smell. Parts of the brain involved in processing smells, including what are known as the olfactory cleft, olfactory bulbs, and surrounding neural tissue were affected in some patients.

The team found that the infection appears to affect how parts of the brain involved in this sense metabolize sugar. And the connections of nerve cells that pass on information about smells may also be interrupted by the coronavirus infection.

The research was submitted as a pre-print to the website medRxiv, and therefore has not been peer reviewed.

The study involved 12 patients aged between 23 to 60 years old who suddenly lost their sense of smell, and were diagnosed with the coronavirus. The team compared their scans with those of 26 healthy people, aged between 22 to 52 years old. In seven patients, a loss of smell was their predominant COVID-19 symptom. Five patients fully or almost recovered up to 10 weeks after losing their sense of smell, with seven still having problems up to 16 weeks later.

Participants took a test to measure their senses of taste and smell. They also had MRI scans to look for structural chances in the brain and the organ's pathways related to these senses. PET scans were used to examine brain metabolism.

Experts who did not work on the study said the findings indicate there may be more than one reason why COVID-19 patients lose their sense of smell. Cris S. Constantinescu, professor of neurology at the U.K.'s University of Nottingham Queen's Medical Centre, told Newsweek via email the study was limited because it involved a small number of participants.

Asked why some COVID-19 lose their smell and taste and not others, Constantinescu said it likely has to do with how much the virus spreads in a person's upper respiratory tract. He suspects patients where other parts of their body is affected by the virus, for instance their blood vessels, digestive system, may not lose their sense of smell.

Smith said he was impressed that the imaging techniques the team used enabled them to "trace every part of the nose and brain's response to odors and see where things are going wrong."

smell, lemon, sense, stock, getty
A stock image shows a woman smelling a lemon. Losing one's sense of smell and taste is a symptom of COVID-19.

Smith said: "the study indicates negative results, but important ones, ruling out a single explanation for the patterns of smell loss we see with COVID-19. But this means there is still more to find out about how this virus affects the body and brain."

The study was limited because the team were unable to explore why some people lose their sense of taste, and what might explain this, he said.

Professor Aytug Altundag, an expert in diseases of the ear, nose and throat, at Turkey's Biruni University told Newsweek via email "In my opinion it is much more speculative study and doesn't show directly the mechanism of olfactory [smell] loss in COVID-19 patients."

Altundag, who recently published a paper on smell loss in COVID-19 patients in the journal Otolaryngology–Head and Neck Surgery, said it would be "wrong" to attribute the cause of smell loss to glucose metabolism "with so little data."

Dr. James C. Denneny III, executive vice president and CEO of the American Academy of Otolaryngology–Head and Neck Surgery, told Newsweek the loss of smell following rhinovirus infections also have a number of causes.

He said the study's main limitation was that the team could not study the tissue to check they matched up with the scans, as this is not possible in living patients.
"Even so, this serves as a 'pilot study' that can be repeated elsewhere and with more subjects over time."

This article has been updated to include information about Professor Altundag.