Structural Brain Changes Before and After Covid

On March 7, 2022, Nature, a leading scientific journal, published a striking study describing brain changes in Covid-19 cases, including mild cases, such as reduced gray matter, decreased overall brain volume, and cognitive decline.

For the first time, researchers used magnetic resonance imaging (MRI) to compare scans before and after Covid-19, finding “a significant reduction in gray matter thickness and tissue contrast in the orbitofrontal cortex and parahippocampal gyrus, greater changes in markers of tissue damage in regions functionally connected to the primary olfactory cortex, and a significant reduction in overall brain size.” However, the researchers are trying to be cautious when interpreting the findings.

Gwenaëlle Douadu and colleagues from the Welcome Centre for Integrative Neuroimaging at the University of Oxford, UK, describe these brain changes as “modest.”

They wrote on the subject: “It remains unclear whether these abnormal changes reflect the spread of pathological effects in the brain or are a distinctive feature of the virus itself, and whether they will predispose participants to future vulnerability of the limbic system, particularly memory.”

A Reduction in Gray Matter

The researchers analyzed data from UK Biobank, a large biomedical database containing genetic and health information on 500 thousand people living in the United Kingdom.

They identified 785 adults aged 51-81 who underwent brain MRI twice, approximately 3 years apart. Of these, 401 tested positive for SARS-CoV-2 before the second scan, and participants also completed cognitive tests before both imaging sessions.

Although gray matter declined somewhat over time, individuals infected with SARS-CoV-2 showed a 0.2% to 2% reduction in brain tissue in the parahippocampal gyrus, orbitofrontal cortex, and insula—areas largely involving the sense of smell.

NOTE: Gray matter consists of the cell bodies of neurons, the brain's basic cells. Gray matter includes brain regions related to muscle control and sensory perceptions such as sight and hearing, memory, emotions, speech, decision-making, and self-control.

Participants who had Covid-19 also showed greater decreases in overall brain volume and cognitive function than those who had not had Covid-19. Most had only mild or moderate symptoms. The results were unchanged when individuals with Covid illness severe enough to require hospitalization were excluded.

More Research Is Needed

Max Taquet of the National Institute for Health Research at the University of Oxford said that “These findings may help explain why some people experience brain symptoms long after an acute infection .”

Taquet, who was not involved in the study, emphasized the need to identify the causes of these brain changes. Questions remain about “whether they can be prevented or treated, and whether similar changes are observed in hospitalized patients, children, and young people”.

Taquet said that “these brain changes may not be caused by Covid-19, but may instead represent the natural progression of a disease that increases the risk of Covid-19.” Other researchers also voiced concerns and recommended further investigation.

Neuropsychiatrist Alan Carson of the Centre for Clinical Brain Sciences at the University of Edinburgh, Scotland, said, “I am quite concerned about the use of terms such as ‘neurodegenerative’ in the report .”

“The extent and magnitude of the brain changes are quite modest, and such changes can arise from simple changes in mental experience. What this study almost certainly demonstrates is the effect of losing one's sense of smell in terms of neural changes.”