A study published this Monday in the journal Nature reports a “harmful impact associated with SARS-Cov-2” in the brains of people infected months earlier.
The Covid, especially a respiratory disease, could also affect the brain. Presumably since the start of the pandemic, the neurological effects of the disease now appear to have been proven by recent work, including in the case of mild forms. According to a study published Monday in the journal, we see “a harmful impact linked to SARS-Cov-2”, the virus that forms the origin of Covid, in the brains of people infected months earlier. Nature†
This work is important because it provides the strongest evidence to date that Covid can have long-term effects on the brain, particularly the “grey matter” that comprises neurons.
The idea is not new. It has been almost mentioned since the start of the pandemic in early 2020, when many doctors noticed neurological disorders in patients with Covid.
A study with more convincing results
Since then, many studies have gone in this direction. Some have shown that the proportion of cognitive impairment 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. The second work was performed on a small number of patients, who generally suffered from severe forms. In some cases, it even involved autopsies of deceased patients.
The study published Monday is much more convincing. 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 are 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 predates the appearance of Covid. Indeed, the patients had all undergone brain scans several years earlier, as part of an operation carried out by Biobank, an organization that has been collecting health data in the UK for years.
Damaged brain tissue
What are the results? Former Covid patients have generally seen their brains shrink. On average, infection with the virus results in a 0.2% to 2% loss or lesion of brain tissue several months later, in addition to what is seen 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 the substance every year. the regions associated with memory,” explains Gwenaëlle Douaud, the principal investigator who contributed to this study, in a presentation on her website.
Should we therefore panic and imagine a virus that systematically ascends into the brain and attacks the neurons irreparably? Far from it, and the study does not allow to draw any conclusions about the mechanisms of these brain attacks or about their irreversibility.
An inflammation at the origin of the consequences?
The researchers make a crucial observation, but it can be interpreted in different ways: After a Covid infection, the areas of the brain most affected are those related to the perception of smells. However, the loss of smell is one of the most common symptoms of Covid. This is probably because the olfactory nerve is attacked by the virus or, as a recent study suggests, by the immune response to infection.
Gwenaëlle Douaud therefore formulates several hypotheses: the brain can be affected by inflammation, for example transmitted through the olfactory tract, caused by the virus itself or by the body’s response to it. But it is also possible to do it the other way around. What if it was the loss of smell itself that affected the brain?
“We know that a permanent loss of smell (…) causes a decrease in gray matter in the brain regions associated with smell,” notes Gwenaëlle Douaud.
However, this effect is reversible: “We may think that with the return of the sense of smell, these brain abnormalities will become less pronounced over time,” the researcher concludes.