How can Covid-19 affect the human brain?

How can Covid-19 affect the human brain?

The cognitive impairment caused by severe Covid-19 is comparable with the decline that takes place between the ages of 50 and 70, according to a recent study by Cambridge university and Imperial College London.

Researchers said the degeneration was equivalent to losing 10 IQ points. The findings, published earlier this month, were the latest in a series of studies that suggest Covid-19 has an impact on the brain.

The lasting impact of Covid-19 on the millions who have contracted it is still being assessed more than two years into the pandemic, with few areas of uncertainty as urgent and unsettling as the potentially enduring effects on the brain.

Scientists are examining the precise mechanisms causing neurological effects and whether symptoms will prove to be temporary, or the heaviest health burdens may still lie in the future.

What are the most striking findings about Covid-19’s impact on the brain?

Amid a growing body of anecdotal evidence, Alzheimer’s Disease International, a federation of dementia associations, suggested in September that the degenerative effect of coronavirus could fuel the “pandemic of dementia”. The World Health Organization estimates that the number of people living with dementia, currently some 55mn, will rise to around 80mn by 2030 as the elderly population grows.

A study from Oxford university researchers published in March found tissue damage and shrinkage in parts of the brain related to smell in people who had only mild bouts of Covid-19. Researchers, who analysed nearly 800 brain scans from the UK Biobank — one of the world’s largest biomedical databases — found a reduction in whole brain size compared with people who were not infected and, on average, greater cognitive decline.

Loss of the sense of smell, which people began noting in the early days of the pandemic, may have been caused by damage to the olfactory nerve which extends into the brain and conveys this function, according to a study published in JAMA Neurology last month.

Diagram showing the body’s scent mechanisms and how Covid-19 could affect them as shown in the results of recent scientific studies

How concerned are experts?

Dr S. Andrew Josephson, chair of neurology at the University of California, San Francisco, and editor in chief of JAMA Neurology, said people with even mild Covid described symptoms, such as mental fatigue, that might be related to the brain. “We’re seeing more and more studies that show changes in the brain that may be associated with it,” he said.

Difficulties with memory, language and concentration are among a broad range of symptoms that fall under the umbrella term “long Covid”. Defined as suffering symptoms for 12 weeks or more after a Covid-19 diagnosis, medical experts have estimated that it affects more than 100mn people.

But other experts suggest that superficially worrying findings may not be as concerning as they first appeared.

“The majority of patients we’re seeing clinically have . . . a disorder of concentration and the ability to direct your thinking,” said Alan Carson, consultant neuropsychiatrist at Edinburgh university. “It’s very unpleasant, but it’s not a permanent neurodegenerative state — it’s treatable.”

Serena Spudich, professor of neurology at the Yale School of Medicine, said it was not clear how much changes in the brain are specific to Covid, nor their significance. “People may lose some grey matter and it may have little real-life meaning,” she said.

What research is under way to find out more?

Research into the connection between Covid-19 and dementia is in its early stages. Scientists said it was theoretically possible that the disease could affect the brain in a similar way to some other viruses.

A US study in 2020 found that people with HIV had a 50 per cent higher risk of developing dementia. If Sars-Cov-2 travelled “along brain pathways in a manner similar to HIV then it is possible Covid infection may increase the risk of Alzheimer’s disease”, reflected Dennis Chan, who is leading a study on cognitive impairment in long Covid funded by the UK’s National Institute for Health and Care Research.

Other scientists said the belief that the virus could travel into the wider central nervous system via the olfactory nerve now seemed flawed. “It’s proved incredibly difficult to infect the brain with coronavirus,” said Carson.

Josephson said researchers were analysing spinal fluid samples from living patients in search of “unusual antibodies or inflammatory cells” that could shed new light on long Covid.

Discouraging precedents from history had not so far been repeated, experts suggested.

Clinicians feared the pandemic “would be associated with an encephalitic Parkinson’s that had been described after Spanish Flu”, said Anna Cervantes-Arslanian, a neurologist at Boston University School of Medicine.

But a study she led found only 0.5 per cent of people with severe Covid-19 had meningitis or encephalitis. About 10 per cent had altered brain function or structure, according to the research published in April in the journal Critical Care Explorations.

Are new treatments being developed?

Researchers led by Chan are using MRI scans to understand the causes of Covid’s effects on memory, thinking speed and decision making. He said his team would also trial cognitive rehabilitation techniques used to treat memory problems after a stroke, such as setting tasks to increase mental focus.

Other scientists are looking at the possibility of new pharmaceutical treatments. Studies are under way to examine changes in tissues and organs that cause, or are caused by, Covid-19 in order to trial treatments.

Josephson said it remained unclear whether impacts on the brain were caused by an overactive immune system or the reverse. However, he said if this could not be quickly established it may be best to go ahead with trialling drugs that modify the immune system, either by decreasing or enhancing it, to help those whose symptoms suggest cognitive impairment.

But disentangling Covid’s impact from other elements only indirectly associated with the virus remains a puzzle-in-progress for researchers.

“Covid’s effects on the brain are real — some people have very discrete, defined conditions and some have things that we don’t understand quite as well,” said Spudich. “The problem is that there are so many other social factors, pressures, stresses related to these pandemic times that it definitely muddies the waters.”

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