Blood clots are not a new issue in the NHL, but rather one that has re-emerged in recent days.

Buffalo Sabres coach Lindy Ruff said Monday that center Jiri Kulich is out indefinitely after a “pretty serious” blood clot was discovered. Three days prior, the Pittsburgh Penguins announced forward Filip Hallander would miss at least three months due to a blood clot in his leg.

Awareness of blood clots rose earlier this year when two high-profile NBA players, Victor Wembanyama and Damian Lillard, were taken out of action by deep vein thrombosis. DVT is a blood clot that regularly requires medical intervention.

Kulich and Hallander are not the first NHL players to require treatment for blood clots.

In the 2024 Stanley Cup playoffs, Brock Boeser was forced to miss Game 7 of the Vancouver Canucks’ second-round series against the Edmonton Oilers because of a blood clot stemming from a blocked shot in Game 1. Goalies Frederik Andersen (2023-24), Andrei Vasilevskiy (2015-16) and Tomas Vokoun (2013-14) each missed time due to blood clots, with Vasilevskiy requiring surgery to remove his and Vokoun’s essentially ending his career. Jakob Silfverberg’s 2021-22 season ended prematurely because of a right-leg blood clot.

NHL players are most commonly diagnosed with blood clots in the lower body, but there are exceptions.

Pascal Dupuis had lower- and upper-body blood clots. After an ACL surgery, a blood clot reached his lung, forcing him to retire in December 2015. Chris Kreider needed two surgical procedures — one to remove a blood clot in his right arm, and another to correct a deformity in his rib that likely caused the clot — while with the New York Rangers in 2017-18. He missed nearly two months that season.

Penn Medicine chief of vascular surgery Dr. Darren Schneider said, “It’s not clear if there are higher incidents in hockey players or if it just gets picked up and is newsworthy when it happens.”

“Overall, I think it’s still relatively rare in athletes and, in the general population, young, healthy adults,” Schneider added, citing that about 1 in 10,000 people are diagnosed with lower-extremity blood clots annually.

Two NHL players being diagnosed with clots in a matter of four days is considered an outlier.

Schneider said there are factors specific to hockey players that could potentially lead to a “slightly higher chance” of them developing a DVT:

Long periods of immobilization, such as long airplane or bus rides
Dehydration
Lower-extremity injuries

“It’s probably either some combination of those factors or one of those that could be in play that would increase the risk,” Schneider said, speaking specifically of blood clots below the waist.

Symptoms a hockey player might notice are pain, area-specific swelling and purple discoloration of the skin.

“The biggest issue would be if a patient had a blood clot in their leg and it was missed,” Schneider said. “The biggest risk with a DVT that is undiagnosed or not treated is that clot in the leg could break loose and travel with the flow of blood up to the heart or lungs and cause a pulmonary embolism — and if that pulmonary embolism is large enough, it could cause immediate death.”

For the diagnosis of a blood clot, a hockey player would most likely undergo an ultrasound examination that Schneider said is “very accurate.” He added that treatment usually involves blood thinners, most likely oral, that prohibit a person from practicing and playing a contact sport.

“It’s typically three to six months of blood-thinner therapy,” Schneider said.

A hockey player can begin rehab and off-ice workouts soon after beginning blood-thinning therapy. Avoiding contact, which could cause internal bleeding, is critical until cleared by a doctor, Schneider said.

Also, hockey players would need to be closely monitored on the ice before being cleared for contact.

“In addition to being checked and causing potential internal bleeding, the risk of a stick or puck, even blades, that could cut a hockey player,” adds an element of danger unique to hockey players with blood clots, Schneider said.

Hockey players aren’t necessarily more prone to blood clots.

“The biggest risk for having a blood clot is having had a prior blood clot in the leg,” Schneider said.

So, according to Schneider, Kulich and Hallander will likely be closely monitored for signs of potential future clotting.