New York City doctors say the coronavirus is triggering a surge in strokes in younger patients, causing alarm among medical experts.

Over a two-week period, Mount Sinai doctors reported five patients who suffered large vessel strokes in patients under the age of 50, according to letter they published in the New England Journal of Medicine.

All five patients tested positive for COVID-19 but had very mild to no symptoms.

“That creates a big alarm,” said Dr. J Mocco, director of the Cerebrovascular Center at Mount Sinai and one of the letter’s authors. “Our spider sense goes up to say that there’s something not right here.”

Out of the five patients, one died, one is still hospitalized, one was discharged from the hospital and two are in rehabilitation. The youngest patient was 33 and only one patient had a history of stroke.

Dr. Shazam Hussain, director of the Cerebrovascular Center at the Cleveland Clinic, said he was surprised when he saw the reports coming from New York. The normal stroke population are typically older patients with high blood pressure or cholesterol problems.

Mocco said these stroke patients in New York were 15 years younger than the normal stroke population, had no risk factors and were statistically more likely to be male.

Reports of this phenomenon first came from Wuhan, according to the NEJM letter, where approximately 5% of patients who were hospitalized with the coronavirus suffered a stroke. The youngest reported patient was 55.

According to the Mayo Clinic, a stroke occurs when blood supply to the brain is interrupted or reduced, preventing oxygenation and causing brain cells to die. Both Mocco and Hussain believe the coronavirus is causing blood clots that block or narrow the blood vessels.

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However, it’s not just stroke doctors that have seen this trend in clotting. Mocco said he’s heard from colleagues in heart, lungs and kidney medicine who have also seen complications in coronavirus patients caused by blood clots.

“People were very focused on the fever and shortness of breath,” Hussain said. “There’s a lot of parts of the body that is affected by the virus as well.”

A study from the Netherlands published in early April looked at 184 patients who were checked into the intensive care unit for COVID-19 pneumonia. Nearly a third of those patients had suffered from thrombotic complications, more commonly known as blood clotting.

Although doctors can’t confirm why the coronavirus seems to induce this, some experts have an educated guess. Mocco said research suggests the virus attaches itself to a host cell that not only exist in the respiratory tract, but also in blood vessels. This means the virus can go anywhere inside the body.

“A big part of the way this disease hurts people is in blood clots whether it’s in the lungs or the kidney or the heart or the brain,” he said.