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Researchers have found that cognitive impairment as a result of severe COVID-19 is similar to that of ageing 20 years.

A team of scientists from Cambridge University and Imperial College London found that the degree of impairment was linked to the severity of illness.

Previous studies have found that acute cases of COVID-19 may cause lasting cognitive and mental health problems, also known as long COVID.

Patients have reported symptoms including fatigue, brain fog, problems in recalling words, sleep disturbances, anxiety and even post-traumatic stress disorder, or PTSD, months after infection.

The latest research, published in the eClinicalMedicine journal, carried out cognitive tests on 46 individuals six months after they were admitted to Addenbrooke's Hospital, Cambridge between March and July 2020.

David Menon, a professor at Cambridge and a lead author of the study, said some patients were left with prolonged cognitive decline from which they may not recover.

" (COVID-19) does cause problems with a variety of organs in the body, including the brain and our cognitive function and our psychological health," he said.

"Cognitive impairment is common to a wide range of neurological disorders, including dementia, and even routine ageing, but the patterns we saw - the cognitive 'fingerprint' of COVID-19 - was distinct from all of these."

Hospitalized patients showed impairments such as slower processing speed, the results suggested. "The thing they struggle with most is verbal reasoning," said Menon, a finding that supports the commonly-reported problem of difficulty finding words, the study noted.

The patients underwent detailed computerized cognitive tests using Imperial's Cognitron platform, which measures different aspects of mental faculties such as memory, attention and reasoning. Scales measuring anxiety and depression and PTSD were also assessed.

Results were matched against 460 people, according to characteristics such as age, gender, education and first language, who had never had COVID-19, but had taken the Cognitron tests.

There were no major differences found in those severely ill COVID-19 patients tested after 10 months, although there were hints of an improvement.

The study found cognitive impairment as a result of acute COVID-19 is similar to that sustained between 50 and 70 years of age and is the equivalent to losing 10 IQ points. The effects were strongest for the 16 patients who had required mechanical ventilation.

Researchers said the cognitive damage may be caused by several mechanisms, although the most important may be the body's own inflammatory response.

The study is the first to undertake such rigorous assessment and comparison of the after effects of severe COVID-19.

Menon said the research allows for better understanding about "underlying mechanisms" of the cognitive decline and to produce effective treatments "to prevent it happening and perhaps treat it later".