New research has found troubling evidence that Covid infection can shrink areas of the brain, resulting in a loss of grey matter and cognitive decline.
Research carried out by the University of Oxford and published in Nature looked at brain scans from 785 people aged between 51 and 81 years old.
The study found a greater reduction in grey matter thickness in the orbitofrontal cortex and parahippocampal gyrus - regions associated with smell and memory of events in those who had caught the disease.
Researchers have said the findings might help explain the cognitive impairment and 'brain fog' associated with Long Covid.
Participants who had Covid-19 displayed evidence of tissue damage in regions associated with the olfactory cortex, an area linked to smell, and an average reduction in whole brain sizes.
The study showed in the time between two brain scans, the brains of people who had contracted Covid shrank more than those who avoided infection, an additional 0.7% overall.
The effect was especially pronounced in regions of the brain involved with smell, which was 1.8% smaller, and movement which saw an additional 0.8% shrinkage.
On average, the participants who were infected with Covid also showed greater cognitive decline between their two scans, associated with the atrophy of a brain region known as the cerebellum, which is linked to cognition.
Dr Max Taquet, NIHR Oxford Health BRC senior research fellow, University of Oxford, said: "This is the first large-scale study to investigate the actual changes in the brain that can occur after a Covid-19 infection.
"It is well established that Covid-19 infection is associated with subsequent risks of neurological and psychiatric problems in some people including brain fog, loss of taste and smell, depression, and psychosis.
"But why this occurs remains largely unknown.
"This study starts to shed light on this important question by showing that brain regions connected to the 'smell centre' of the brain can shrink after Covid-19 in some people.”
Researchers investigated the changes in the brains of 785 UK Biobank participants aged 51-81 who had two brain scans, on average 38 months apart, and also underwent cognitive tests.
The UK Biobank is a large database that includes genetic and health information on half-a-million people living in the UK.
A total of 401 participants tested positive for infection with Sars-CoV-2 between their two scans, of whom 15 were admitted to hospital.
The authors identified various long-term effects following infection - with an average of 141 days between participants receiving a Covid diagnosis and the second imaging scan.
Dr Taquet added: "These brain changes were not observed in every patient and they were mostly subtle.
"These findings might help explain why some people experience brain symptoms long after the acute infection.
"The causes of these brain changes, whether they can be prevented or even reverted, as well as whether similar changes are observed in hospitalised patients, in children and younger adults, and in minority ethnic groups, remain to be determined."
The research, titled Sars-CoV-2 is associated with changes in brain structure in UK Biobank and carried out by Gwenaelle Douaud from the University of Oxford and others, appeared in the Nature journal.
