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The Covid, above all a respiratory disease, might also strike the brain. Suspected since the start of the pandemic, the neurological effects of the disease now seem to have been proven by recent work, including in the case of mild forms. We observe “a deleterious impact linked to SARS-Cov-2”, the virus that causes Covid, in the brains of people infected months earlier, according to a study published Monday in the journal Nature. This work is important because it provides the strongest evidence to date that Covid can have long-term consequences on the brain, in particular the ” gray matter » which includes neurons.
The idea is not new. It has been almost mentioned since the start of the pandemic in early 2020, many doctors having then observed neurological disorders in patients with Covid. Since then, many studies have gone in this direction. Some have shown that the proportion of cognitive disorders was higher in former Covid patients. Others have directly observed the brains of patients and found abnormalities there.
However, the first category of studies cannot demonstrate a direct causal mechanism. As for the second work, they were carried out on a small number of patients, generally suffering from severe forms. In some cases, these were even autopsies of deceased patients.
A shrinkage of the brain
The study published on Monday is much more conclusive. She studies a relatively large number of people – several hundred – and is interested in the state of their brains, depending on whether or not they have been affected by Covid. When they caught Covid, most of these people were not hospitalized. This work therefore gives an idea of the neurological effects of a mild Covid, as the vast majority of people have suffered.
Finally, for each case studied, the study has a benchmark that dates back to before the appearance of Covid. Indeed, the patients had all undergone brain imaging several years earlier, as part of an operation carried out by Biobank, an organization which has been compiling health data for years in the United Kingdom.
What are the results ? Former Covid patients have generally seen their brains shrink. On average, an infection with the virus results, several months later, in a loss or lesion of 0.2% to 2% of brain tissue in addition to what is observed in healthy people. ” To get an idea of the magnitude of these effects, we can compare them to what happens during normal aging: we know that people lose between 0.2% and 0.3% of gray matter each year. in regions related to memory”explains Gwenaëlle Douaud, the main researcher who contributed to this study, in a presentation on her website.
No systematic damage
Should we therefore panic and imagine a virus that systematically rises within the brain and irreparably attacks the neurons? Far from it, and the study does not make it possible to conclude either on the mechanisms of these cerebral attacks or on their irreversibility.
The researchers make a crucial observation, but which can be interpreted in several ways: following a Covid infection, the areas of the brain most affected are those related to the perception of odors. However, the loss of smell is one of the most common symptoms of Covid. This is likely because the olfactory nerve is attacked by the virus or, as a recent study suggests, by the immune response to infection.
Ms. Douaud therefore formulates several hypotheses: the brain might be struck by inflammation, for example transmitted by the olfactory channel, caused by the virus itself or by the body’s reaction to it. But it is also possible to take things the other way around. What if it was the loss of smell itself that affected the brain? “We know that a lasting loss of smell (…) causes a decrease in the gray matter in the regions of the brain linked to olfaction”, remarks Mrs. Douaud. But this effect is reversible: We can think that with the return of the sense of smell, these cerebral abnormalities will become less marked over time”concludes the researcher.