TORONTO -- Researchers in the U.S. have found the first instance of COVID-19 causing recurring blood clots in a patient's upper extremities.

The case study was one of 1,000 patients admitted to hospital with COVID-19 between March and May 2020 who were part of a larger Rutgers Robert Wood Johnson Medical School study.

Prior to this case study there had been reports of deep vein thrombosis in the lower extremities of people who had been infected with COVID-19. This is the first known case of COVID-19 setting off recurring deep vein thrombosis in an upper extremity.

The patient is an 85-year-old man who is described as “active” and had previous diagnosis of upper extremity blood clots.

"The patient presented to his primary care physician with complaints of swelling in his left arm and was sent to the hospital for further management where he was diagnosed with an upper arm blood clot and an asymptomatic COVID-19 infection," Payal Parikh, an assistant professor of medicine at Rutgers Robert Wood Johnson Medical School, who led the study, said in a press release.

The man was admitted to hospital based on concerns related to the upper arm blood clot, not because of his breathing, the press release said.

Most cases of deep vein thrombosis occur in the legs, according to the press release. Roughly 10 per cent of these clots occur in upper extremities, and of those only nine per cent are recurring.

"This is of concern since in 30 per cent of these patients, the blood clot can travel to the lung and be possibly fatal," said Parikh. "Other disabling complications include persistent swelling, pain and arm fatigue."

The research suggests that primary care physicians should be on the lookout for signs and symptoms of deep vein thrombosis in patients who complain of unexplained swelling.

"If you have been previously diagnosed with deep vein thrombosis or have chronic medical illness that predisposes you to blood clots, you have a higher risk for recurrence of a deep vein thrombus in the setting of a COVID-19 infection and thus, should be vigilant," said Parikh.