A new study out of the United Kingdom found “strong evidence” that even mild SARS-CoV-2 infection can have lasting impacts on a patient’s brain – particularly in the olfactory cortex, the part of the cerebrum responsible for smell.

What You Need To Know

  • A new study out of the United Kingdom found “strong evidence” that even mild SARS-CoV-2 infection can have lasting impacts on a patient’s brain – particularly in the olfactory cortex

  • The study, published Monday in the scientific journal Nature, examined the brain scans of 785 individuals; 401 who had tested positive for COVID-19 and 384 individuals who did not

  • While the study found “no signs of memory impairment” in either cohort, those who tested positive for SARS-CoV-2 showed “a worsening of executive function"

  • Experts have some idea what might lead to changes in the brain, but much remains unknown – including how long the brain might remain impacted by the COVID virus

The study, conducted by the University of Oxford published Monday in the scientific journal Nature, examined the brain scans of 785 individuals using information from Biobank, a public biomedical database that disseminates personal data from participants for research purposes. 

Of the 785 individuals included in the study, 401 tested positive for COVID-19, while the 384 individuals who did not contract the disease represented the “control” group. 

Researchers first examined patient outcomes on a number of cognitive tests, including “Trail Making,” where participants draw lines between specific numbers or letters in a given order to compare impairment of executive function and attention span. 

While the study found “no signs of memory impairment” in either cohort, those who tested positive for SARS-CoV-2 showed “a worsening of executive function, taking a significantly greater time to complete” the test.

Researchers also examined the brain scans of each participant twice: once before a participant contracted COVID-19, and a subsequent scan taken approximately four and a half months after the individual tested positive for the disease. 

Scientists compared the post-COVID brain scans to those of individuals in the control group, and found three “significant longitudinal effects” when comparing the two. Individuals who contracted COVID-19 showed a greater decrease in grey matter thickness in their brains, more changes in “markers of tissue damage” connected to the olfactory cortex and an overall reduction in brain size compared to individuals who did not contract the virus.

Researchers saw a particular decline in the grey matter volume in the parahippocampal gyrus, responsible for cognitive processes like memory, and the entorhinal cortex, which both aids memory and helps navigate time and space.

“The infected participants also showed on average larger cognitive decline between the two timepoints,” the study’s authors wrote in part. “Importantly, these imaging and cognitive longitudinal effects were still seen after excluding the 15 cases who had been hospitalised.”

The researchers posit that the loss of taste and smell in COVID-19 patients, a common symptom from the disease, may contribute to some of the brain atrophy, writing in part: “Such loss of sensory olfactory inputs to the brain could lead to a loss of grey matter in olfactory-related brain regions.”

There are a number of limitations to the data, one being the lack of research on patients with severe COVID-19, as those included in the study primarily had mild cases of the virus. Scientists also noted strains of COVID-19 might impact the brain in different ways, a variable not tested during the study. 

And while experts have some idea on what might lead to the brain impact, much remains unknown – including how long the brain might remain impacted by the COVID virus, as participants were not tracked beyond their second scan at the four or five month mark post-positive test. 

“Whether these abnormal changes are the hallmark of the spread of the pathogenic effects, or of the virus itself in the brain, and whether these may prefigure a future vulnerability of the limbic system in particular, including memory, for these participants, remains to be investigated,” the authors wrote. 

Scientists have long wondered what leads to the COVID-associated symptom referred to as “brain fog,” a non-medical term for the sluggish or unclear feeling usually stemming from long-haul COVID cases. For some of the more serious patients, the lack of oxygen to their brain – should they be placed on a ventilator or have other associated issues – can lead to lasting brain damage, as well as long-term conditions like fatigue, exhaustion, anosmia or muscle pain. 

But there is significantly less research on the lasting impairment from less severe cases of the virus.

While the study published Monday is not the first to examine the impact of COVID-19 on the human brain, it is among the largest – and is the first, researchers say, to compare brain scans of individuals taken both before and after the patient contracted SARS-CoV-2 to individuals who did not test positive for the virus.

Other researchers have similarly pointed to the olfactory centers as a possible gateway for COVID molecules to enter the brain, although scientists have noted “there’s not a [ton] of virus in the brain.”