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Pedestrians make their way along St. Catherine street in Montreal, on Sept. 15, 2020.

Ryan Remiorz/The Canadian Press

Strokes occur relatively frequently among patients with COVID-19 and tend to have devastating consequences, according to a new study that estimates nearly one in 50 patients admitted to hospital with COVID-19 experiences a stroke.

The study, published Tuesday in Neurology, the medical journal of the American Academy of Neurology, found both the proportion of patients with the infectious disease who have a stroke and their mortality rate are “exceedingly high.”

Around 1.8 per cent of patients with COVID-19 experience a stroke either once they’re admitted to hospital or shortly before, the researchers found. It’s a figure that may seem small, but translates into a large number of cases, considering millions of people around the world are hospitalized with the disease, said senior author Luciano Sposato, an associate professor of neurology at the University of Western Ontario.

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“It’s actually a very high number,” he said, adding that, by comparison, only 0.2 per cent of influenza patients admitted to hospital experience a stroke, and 0.75 per cent of patients who were hospitalized with SARS-CoV1, the virus that caused SARS, had strokes.

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The researchers also found in about half of young patients under the age of 50, stroke was the first symptom of COVID-19.

“That means they were totally asymptomatic before they were admitted,” Dr. Sposato said.

The study, which involved a systematic review of all published research on strokes and COVID-19 up until the end of May, adds to scientists' and clinicians' understanding of the incidence of strokes among COVID-19 patients. While there are several proposed mechanisms, some researchers believe the infection causes a hypercoagulable state in certain individuals. In other words, it causes increased coagulation, which can lead to the formation of blood clots in various parts of the body. In strokes, these blood clots interrupt blood flow to the brain.

The research team analyzed 125 cases from their research review, as well as 35 additional unpublished cases from Ontario, Iran and the United States. They found 35 per cent to 45 per cent of COVID-19 patients with stroke died during their hospital stay, Dr. Sposato said. By comparison, he said, the mortality rate is 15 per cent to 30 per cent for stroke patients in Canada admitted into intensive care without the infectious disease.

Among the patients in the study with the highest risk of death, all had at least one risk factor, such as obesity, hypertension or diabetes, and 97 per cent had severe COVID-19. This led the researchers to conclude most COVID-19 patients who experience stroke likely die because of the severity of COVID-19, rather than of the stroke, Dr. Sposato said.

While stroke treatment is the same as for patients without COVID-19, hospital staff need to take extra precautions to prevent the spread of infection, he said.

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Some of these findings echo those of the North American Neurovascular COVID-19 Consortium (NAN-C), a group of researchers collecting and analyzing data on strokes associated with COVID-19 from roughly 30 hospitals in the U.S. and Canada.

Adam Dmytriw, the Canadian lead for the consortium, said he and his colleagues have found patients with COVID-19 fare worse when they have a stroke, compared with either those with COVID-19 alone or patients who experienced stroke before the pandemic. However, it is yet unclear why.

“We think there is an interaction between COVID and stroke that has yet to be described,” said Dr. Dmytriw, a radiologist and clinical fellow in neuroradiology and neurointervention at Brigham and Women’s Hospital, Harvard Medical School.

He and his colleagues have also found that while massive strokes occur in unusually young patients, under the age of 50, older patients with COVID-19 have a greater chance of having a bad outcome after a stroke.

“We’ve known for a long time that older patients don’t do well with the respiratory complications of COVID, and that continues to be the case in our updated findings for stroke,” he said.

Sign up for the Coronavirus Update newsletter to read the day’s essential coronavirus news, features and explainers written by Globe reporters and editors.

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