Blood clots in the brain or the lungs could explain why fatigue and ‘brain fog’ are so commonly reported as symptoms of long Covid, research has suggested.
The study, led by teams from the universities of Oxford and Leicester, argued that higher levels of two blood proteins, fibrinogen and D-dimer, could contribute to patients having trouble thinking, concentrating or remembering.
Such brain fog is reported in around 16% of people with long Covid, although the researchers cautioned that it was early days in terms of reaching any definitive conclusions and more research was needed. Their findings were also relevant only to patients admitted to hospital.
Dr Max Taquet and colleagues from the University of Oxford looked at blood tests from 1,837 people who had been hospitalised with Covid-19 to find potential proteins (biomarkers) associated with subsequent cognitive problems.
The paper, published in the journal Nature Medicine, identified two separate profiles of biomarkers for the proteins, with other aspects of the profiles also suggesting they are likely to reflect blood clots. The main findings were then replicated using electronic health records in a separate population.
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Dr Taquet said: “Both fibrinogen and D-dimer are involved in blood clotting, and so the results support the hypothesis that blood clots are a cause of post-Covid cognitive problems.
“Fibrinogen may be directly acting on the brain and its blood vessels, whereas D-dimer often reflects blood clots in the lungs and the problems in the brain might be due to lack of oxygen. In line with this possibility, people who had high levels of D-dimer were not only at a higher risk of brain fog, but also at a higher risk of respiratory problems.
“The ultimate goal is to be able to prevent and reverse the cognitive problems seen in some people after Covid-19 infection. Although our results are a significant advance in understanding the basis of these symptoms, more research is needed into the causes and effects before we propose and test interventions,” he added.
The participants involved in this research were part of the UK Research and Innovation-funded PHOSP-Covid (Post-hospitalisation Covid-19) study, led by the University of Leicester.
Their memory was assessed at six and 12 months after hospitalisation using both a formal test and by asking them their own subjective view about their memory.
A participant in the study said: “Since my illness I have been plagued by brain fog, concentration-induced fatigue, poor vocabulary, poor memory. I am unable to process the amount and scale of work that I would previously have done ‘stood on my head’.”
Professor Paul Harrison, from the University of Oxford, who supervised the study, added: “Identifying predictors and possible mechanisms is a key step in understanding post-Covid brain fog. This study provides some significant clues.”
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