At least one in every 100 Covid-19 patients admitted to hospital will suffer a stroke, according to fresh data.
Academics found strokes typically occur a few days after infection and may affect those who never show tell-tale symptoms of the coronavirus in the first place.
Those who suffer the life-threatening complication tend to be older, but around six years younger than patients normally seen in hospitals. And strokes from Covid-19 were around twice as deadly as usual strokes.
Underlying ill health, such as that caused by high blood pressure and diabetes, were also found to be a risk factor for coronavirus-linked strokes.
The findings come from the Stroke Research Group at the University of Cambridge, who reviewed the existing evidence on Covid-19 and stroke. In total, the researchers analysed 61 studies, covering more than 100,000 patients admitted to hospital with the coronavirus.
One in every 100 Covid-19 patients will suffer a stroke, data of hospital patients shows
The results of the study are published today in the International Journal of Stroke.
Professor Hugh Markus, who leads the Stroke Research Group at Cambridge, said the incidence of stroke among Covid-19 patients was 'low'.
But he added: 'The scale of the pandemic means many thousands of people could potentially be affected worldwide.'
The research found strokes occurred in 14 out of every 1,000 cases - the equivalent of 1.4 hospitalised patients out of every 100.
Acute ischaemic stroke, caused by a blood clot cutting off blood supply to the brain, was the most common type of stroke.
BRAIN ABNORMALITIES ARE 'COMMON IN COVID-19 PATIENTS'
Brain abnormalities are common in Covid-19, according to experts who have warned any damage to the organ may be permanent.
Throughout the pandemic, doctors across the world have claimed that it is not just the lungs that are affected by Covid-19.
Scores of patients have suffered neurological complications including stroke, headache and seizures, as well as problems with their heart. Neurological findings are reported in 40 per cent of sick patients with Covid-19.
But academics remain baffled as to exactly how Covid-19 affects the brain — despite several studies warning of a clear link.
Researchers based at Baylor College of Medicine and the University of Pittsburgh have now added to the growing evidence.
They looked at brain scans of more than 600 Covid-19 patients who had participated in 18 different studies.
Each had an electroencephalogram (EEG), a scan that records their brain activity. During an EEG, small sensors are attached to the scalp to pick up the electrical signals produced when brain cells send messages to each other. These signals are recorded by a machine and are looked at by a specialist to see if they're unusual.
The main reason a patient would be given an EEG is to identify what is causing certain symptoms, such as seizures, memory problems, confusion or problems with speech.
A total of 88 per cent of the patients sent for an EEG had abnormal readings. The possibility of preexisting EEG abnormalities is 'so low' the researchers believe most of the EEG abnormalities are new.
Approximately one-third of the patients with EEGs had abnormal results from neuroimaging -an MRI or CT scan that gives a visual representation of the brain.
The team admitted this link may be down to patients being older, having underlying neurological conditions or being sicker.
The most common problem found in the EEG scans was 'diffuse slowing', which is linked with difficulties with awareness, attention, memory, and comprehension.
Dr Zulfi Haneef, an assistant professor of neurology at Baylor, said: 'As we know, the brain is an organ that cannot regenerate, so if you have any damage it will more than likely be permanent or you will not fully recover.'
But in the review, 56.8 per cent of the follow up EEG studies reported that patients had seen an improvement. In some, however, symptoms worsened.
Despite having no proof, scientists said they were confident the brain changes were caused by the coronavirus and were not 'just coincidence'.
They believe this because EEG analysis also showed that 30-50 per cent of the abnormalities involved the frontal region, which is the part adjacent to the nasopharynx which is directly behind the nose.
This suggests that the virus enters through the nose into the frontal lobe of the brain where it directly causes harm.
Dr Arun Raj Antony, co-author from University of Pittsburgh told MailOnline the 'abnormalities were absolutely caused by Covid' because 'in most of these patients, EEGs were performed because they were symptomatic, and all the patients had a positive Covid test'.
Dr Haneef said: 'Before, when we saw this in small groups we weren't sure if this was just a coincidence, but now we can confidently say there is a connection.'
The findings were published in Seizure: European Journal of Epilepsy
Brain haemorrhages — which are caused by an artery in the brain bursting — were less common.
Most patients who suffered the deadly events had been admitted to hospital with Covid-19 symptoms, and they then suffered a stroke a few days later.
Patients who had a more severe infection with SARS-CoV-2 – the coronavirus that causes Covid-19 – were also at greater risk.
However, the researchers warned that many cases of Covid-19 are asymptomatic, causing no symptoms at all.
They now recommend that every stroke patient admitted to hospital is tested for Covid-19 for that reason.
Covid-19 patients who developed stroke were on average 4.8 years older than those who did not. But they were on average six years younger than non-Covid-19 stroke patients, according to the analysis.
Pre-existing health conditions, such as high blood pressure, diabetes and heart disease, also increased the risk of stroke, the research team noted.
However, the experts found no difference between rates of stroke among infected men and women, or smokers versus non-smokers.
The researchers said Covid-19-associated strokes were more severe and had a high mortality than what's usually seen in stroke patients.
Out of 1,655 patients who had enough data to be analysed, 31.5 per cent died. This compares with 11 to 12 per cent of non-Covid stroke patients.
The researchers gave several possible mechanisms for why the coronavirus, once thought to be a purely respiratory disease, causes strokes.
One mechanism might be that the virus triggers an inflammatory response that causes the thickening of the blood, increasing the risk of thrombosis and stroke.
Another relates to ACE2 – a protein 'receptor' on the surface of cells that SARS-CoV-2 uses to break into the cell.
This receptor is commonly found on cells in the lungs, heart, kidneys, and in the lining of blood vessels. If the virus invades the lining of blood vessels, it could cause inflammation, constricting the blood vessels and restricting blood flow.
A third possible mechanism is the immune system over-reacting to the infection, with the body then releasing an excessive amount of proteins known as cytokine.
This so-called 'cytokine storm' could then cause brain damage, and has also been speculated to damage blood vessels, and cause blood platelets to become more 'hyperactive'.
An important question is whether the coronavirus is directly increasing the risk of stroke, or whether patients who suffer them were already more at risk, and Covid-19 indirectly led to the event.
'The picture is complicated,' said study author Dr Stefania Nannoni from the department of clinical neurosciences at the University of Cambridge.
'For example, a number Covid-19 patients are already likely to be at increased risk of stroke, and other factors, such as the mental stress of Covid-19, may contribute to stroke risk.
'On the other hand, we see evidence that Covid-19 may trigger – or at least be a risk factor for – stroke, in some cases.
'Firstly, SARS-CoV-2 more so than other coronaviruses – and significantly more so than seasonal flu – appears to be associated with stroke.
'Secondly, we see a particular pattern of stroke in individuals with Covid-19, which suggests a causal relationship in at least a proportion of patients.'
Professor Markus said: 'Clinicians will need to look out for signs and symptoms of stroke, particularly among those groups who are at particular risk, while bearing in mind that the profile of an at-risk patient is younger than might be expected.'
The research was supported by the Medical Research Council, the National Institute for Health Research (NIHR), the NIHR Cambridge Biomedical Research and the British Heart Foundation.
COVID-19 'COULD AGE THE BRAIN 10 YEARS'
Covid-19 could age the brain by ten years, a non-peer-reviewed study has found.
More than 84,000 people were studied by Dr Adam Hampshire and colleagues at Imperial College London, who found that in some severe cases, coronavirus infection is linked to substantial cognitive deficits for months.
'Our analyses ... align with the view that there are chronic cognitive consequences of having Covid-19,' the researchers wrote in a report of their findings.
'People who had recovered, including those no longer reporting symptoms, exhibited significant cognitive deficits.'
Cognitive tests measure how well the brain performs tasks - such as remembering words or joining dots on a puzzle. Such tests are widely used to assess brain performance in diseases like Alzheimer's, and can also help doctors assess temporary brain impairments.
Hampshire's team analysed results from 84,285 people who completed a study called the Great British Intelligence Test.
The findings, which have yet to be reviewed by other experts, were published online on the MedRxiv website.
The cognitive deficits were 'of substantial effect size', particularly among people who had been hospitalised with Covid-19, the researchers said.
The worst cases showed impacts 'equivalent to the average 10-year decline in global performance between the ages of 20 to 70'.
Scientists not directly involved with the study, however, said its results should be viewed with some caution.
'The cognitive function of the participants was not known pre-Covid, and the results also do not reflect long-term recovery - so any effects on cognition may be short term,' said Joanna Wardlaw, a professor of applied neuroimaging at Edinburgh University.
Derek Hill, a professor of medical imaging science at University College London, also noted that the study's findings could not be entirely reliable, since they did not compare before and after scores, and involved a large number of people who self-reported having had COVID-19, who had no positive test.
'Overall (this is) an intriguing but inconclusive piece of research into the effect of COVID on the brain,' Professor Hill said.