The first major study to compare brain scans of people before and after catching COVID-19 has revealed loss of gray matter and tissue damage, mostly in areas related to smell.
Researchers from the University of Oxford used data from UK Biobank participants to look at changes to the brain on average 4.5 months after mild COVID-19 infection.
The study’s abstract stated that “There is strong evidence for brain-related abnormalities in COVID-191–13. It remains unknown however whether the impact of SARS-CoV-2 infection can be detected in milder cases, and whether this can reveal possible mechanisms contributing to brain pathology.”
Professor Gwenaëlle Douaud and her colleagues investigated changes in the brains of 785 participants in UK Biobank, a large-scale biomedical database and research resource.
The participants (between 51–81 years of age) underwent two brain scans around 38 months apart along with cognitive tests. A total of 401 participants tested positive for infection with SARS-CoV-2 between their two scans, of whom 15 were hospitalised.
Led by the Wellcome Centre for Integrative Neuroimaging at the University of Oxford, the research team identified a number of effects:
- The overall brain size in infected participants had shrunk between 0.2 and 2%
- There were losses in grey matter in the olfactory areas, linked to smell, and regions linked to memory
- Those who had recently recovered from Covid found it a bit harder to perform complex mental tasks
Professor Gwenaëlle Douaud, lead author on the study, said: “Using the UK Biobank resource, we were in a unique position to look at changes that took place in the brain following mild—as opposed to more moderate or severe—SARS-CoV-2 infection. Despite the infection being mild for 96% of our participants, we saw a greater loss of grey matter volume, and greater tissue damage in the infected participants, on average 4.5 months after infection.
The most significant loss of grey matter was in the olfactory areas – but it is unclear whether the virus directly attacks this region or cells simply die off through lack of use after people with Covid lose their sense of smell.
This study represented is perhaps the most authoritative on the question of the impact of COVID-19 on the brain to date. According to Dr Norman Swan on Coronacast, this was because: “This research doesn’t actually confirm it one way or the other. Most people think the virus sets up a reaction in the brain but it’s plausible that it gets into the brain from this study in particular. During the course of this study, they noticed that grey matter thickness declined in particular areas. There was tissue damage to the part of the brain which receives signals of smell and there was a reduction in brain size. They also did cognitive tests which showed a large decline in thinking and memory in people who has COVID-19.”
That said, during that same podcast, Dr Swan cautioned about the implications of the study, stating that: “Essentially what they’re seeing here is inflammation of pathways in the brain, particularly the smell pathway. That’s why we know it’s one of the symptoms of COVID-19. What might be happening here, this is the hypothesis, the tissue in your nose that receives smell information is actually an outgrowth of the brain. So it is possible that the virus infects that tissue and the virus could spread along the nerves there. Or simply just set up inflammation that goes along these pathways of smell.”
“Now if it was only the smell pathway, you wouldn’t be too worried here. It goes into the limbic system which is involved with emotion and behaviour but also how we lay down memory. What they see in the scan fits with the cognitive decline. It is possible that over this inflammation settles and nerves regenerate or new pathways develop to around in the damage.”
Dr Swan also says it could explain aspects of long COVID but that’s not clear at all in this study.
The future vulnerability of the brain regions affected in the study’s participants will be the subject of further investigation, as medical research focuses increasingly on the medium to long-term implications of COVID-19 infection.
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