A double scan, one carried out before the pandemic and the other in the worst months of the crisis, shows that those infected by the coronavirus have brain changes not seen in those who have escaped the virus.The study shows that the decrease in gray matter is generalized, and that the most affected areas are those related to smell.The authors of the work do not know if these damages are caused by the virus itself or are a consequence of the disease.They also do not know if temporary or will be forever.There are many studies that link coronavirus infection with neurological problems.Among the most common symptoms, even ahead of the respiratory ones, are anosmia or hyposmia (total or partial loss of smell, respectively).Also, most of those affected who suffer from persistent covid report a certain mental dullness or inability to concentrate.But, beyond analysis of postmortem tissues from fatal cases, there was little work that had looked at what was going on in the brain.Now, a group of researchers from the University of Oxford (United Kingdom) has been able to analyze the brain images of almost 800 Britons.The study is longitudinal, that is, with data from the same group of people at different times.This gives an extra robustness to your results.Half of the participants had covid between one MRI and another, taken more than three years apart, which has made it possible to compare the changes induced by the coronavirus in the brain and compare them with the brain images of those unaffected.The results, published in the journal Nature, show that those infected had a greater loss of gray matter and more alterations in brain tissues.The differences were greatest in parts of the brain involved in the sense of smell, such as the orbitofrontal cortex or the parahippocampal gyrus.All these changes were more pronounced the older the participants were.The researcher at the Oxford Nuffield Department of Clinical Neuroscience and lead author of the study, Gwenaëlle Douaud, highlights that they have also seen "differences in regions not related to the sense of smell, such as the temporal pole, the supramarginal gyrus or the cerebellum".As for global brain volume, "there was an additional 0.3% loss among the infected participants," she adds.That is the average, but there are cases of a total brain reduction of 2%, an even higher percentage in the olfactory regions.Again, the impairment was comparatively greater the older the subject.In parallel to the second resonance, those studied had to carry out a series of cognitive tests that people with neurodegenerative diseases or after a brain trauma do.Although in general those who passed the covid passed almost all the exams, "they showed a greater decline in their mental abilities to execute complex tasks," says Douaud.In particular, their scores were worse than the volunteers in the control group in the speed of completing the tests.The head of the Neurology service at the University Hospital of Albacete, Tomás Segura, has been carrying out a similar study since January with a hundred Spanish patients affected.They have not yet analyzed the images they have taken of their brains, but they have subjected them to a series of neuropsychological tests.They have seen that, as in the British study, "they perform them more slowly and have a decreased ability to inhibit attention," he says.One of the strengths of the Oxford research is that an average of 38 months passed between the first scan and the second.Another is that the second brain scan was done more than four and a half months after infection, which would indicate some permanence of the effect.In addition, the vast majority of the participants, except for 15 of them who were hospitalized, had a mild covid.For David García Azorín, member of the Spanish Society of Neurology, "this offers a unique opportunity to see that the changes in the brain are due to the infection and not to brain aging itself."But only the repetition of taking images over time would make it possible to know if the changes remain or, as García Azorín says, "if the two groups have a similar brain or that of those affected by covid has worsened."What the authors of the study do not know, nor are the Spanish experts consulted clear about, is what causes what.There are at least three possibilities that would explain the changes in the brain of those affected by covid.One would be the direct action of the coronavirus on the central nervous system.Another option could be that it was all due to the inflammation that accompanies the immune response.And there is a third, confirmed by some studies: the virus spreads through the olfactory mucosa, inside the nose, killing both olfactory neurons and supporting cells and, therefore, causing loss of smell.This would cause an atrophy, due to lack of use, of the brain circuit responsible for processing odors.Science has long known that the loss of any sense causes a change in the related part of the brain.Deciphering this enigma would help to know if the observed alterations are temporary or definitive.That's one of the weaknesses of the work: The researchers didn't determine whether people with brain changes reported having or having had a loss of smell.Neither do other disorders of neurological origin, such as the so-called brain fog or headaches.Another weakness is the demographics of the sample.Although they balanced the two groups, infected and uninfected, by age, gender, previous risk factors or race, they limited the study to those older than 50 years.Neuroscientist Segura asks one last question.The enormous scope of the current pandemic has made it possible to study the brains of hundreds of people at the same time.But, "what happens with other viruses such as Epstein-Barr or viral encephalitis, what impact do all the pathogens that attack us throughout our lives have on the brain?"You can follow MATERIA on Facebook, Twitter and Instagram, or sign up here to receive our weekly newsletter.He is co-founder of Materia and writes about technology, artificial intelligence, climate change, anthropology... since 2014. He previously worked for Público, Cuarto Poder and El Mundo.He is licensed in CC.Politics and Sociology.Or subscribe to read without limitsSubscribe and read without limits