The idea is not new. It has been almost mentioned since the start of the pandemic in early 2020, a number of 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. 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.
The sense of smell, a central role
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, they can be compared 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. Internet.
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: after a Covid infection, the areas of the brain most affected are those linked 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.