Covid-19 can cause more gray matter loss and more damage to brain tissue than naturally occurring in people who haven’t been infected with the virus, a large new study has revealed.
Published on Monday in the journal Nature, the study is believed to be the first involving people who underwent brain scans both before contracting Covid and months later.
Neurologists not involved in the research found it valuable and unique, but cautioned that the implications of the observed changes are unclear and do not necessarily suggest that people may experience lasting harm or that the changes profoundly affect thinking, memory or other functions.
Conducted with people aged 51 to 81, the study found shrinkage and tissue damage primarily in areas of the brain linked to smell. Some of these areas, according to the scientists, are also involved in other brain functions.
“To me, this is pretty compelling evidence that something changes in the brains of this group of people because of Covid,” said Serena Spudich, director of neurological infections and global neurology at the Yale School of Medicine, who was not involved in the study.
But she added: “I think concluding that this has long-term clinical consequences for patients would be going too far. We don’t want to scare the public and make people think, ‘Oh no, this proves that everyone is going suffer brain damage and will not be able to function normally'”.
The study involved 785 participants from the UK Biobank, a repository of medical data, and more than half a million people in the UK. Each of the participants had two diagnostic imaging brain scans about three years apart, plus some basic cognitive tests. Between the two brain scans, 401 of the participants tested positive for the coronavirus. All were infected between March 2020 and April 2021.
The other 384 participants formed a control group because they were not infected with the coronavirus and had similar characteristics to infected patients, in terms of age, sex, medical history and socioeconomic status.
In the normal aging process, people lose a tiny amount of gray matter each year. For example, in memory-related regions, the typical annual loss is between 0.2% and 0.3%, the researchers said.
But study patients who had Covid and had the second diagnostic imaging brain scan lost more than the uninfected, suffering between 0.2% and 2% additional gray matter loss in different brain regions over the three years between one exam and another. They also lost more total brain volume and showed more tissue damage in certain areas.
“I found it surprising how much has been lost and how widespread this is,” said Spudich, who studies the neurological effects of Covid. “I wouldn’t have anticipated such a huge percentage change.”
The effects may be especially notable because the study primarily involved people who — like most Covid patients in the general population — were only mildly affected by their initial Covid infection, so much so that they do not need to be hospitalized.
The study’s lead author, Gwenaëlle Douaud, a professor in the department of clinical neuroscience at the University of Oxford, said that although the number of study patients who were hospitalized, 15, was too small to provide conclusive data, the results suggested that brain atrophy who suffered was worse than patients who had mild symptoms.
People who had Covid also showed a greater decline than uninfected people in a cognitive test linked to attentiveness and efficiency in performing a complex task.
But outside experts and Douaud noted that the cognitive tests applied were rudimentary, so the study is very limited in what it can point to whether the gray matter loss and tissue damage suffered by Covid patients affected their cognitive abilities.
“None of the participants did cognitive tests thorough enough to find out whether they had significant deficits in the many brain regions where these volume changes were found,” said Dr. Benedict Michael, an adjunct professor of neurological infections at the University of Liverpool, who studies the neuropsychiatric effects of Covid and was not involved in the study.
“We don’t know if this really means anything for the patient’s quality of life or function.”
Translation by Clara Allain.
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