What is deep vein thrombosis and why do so many athletes like Damian Lillard get blood clots?

Damian Lillard has no timeline to return to the Milwaukee Bucks after being diagnosed with deep vein thrombosis in his right calf.

The blood-clotting disorder also recently cut short Victor Wembanyama’s second NBA season for the San Antonio Spurs and a decade ago contributed to ending Hall of Famer Chris Bosh’s basketball-playing career. NFL, NHL and Major League Baseball players have also stepped away from their respective sport for some period of time because of it.

Deep vein thrombosis is an abnormal clot within a vessel where the congealing of blood blocks the flow through on the way back to the heart. It is most common in legs but also can happen in the arms, like Wembanyama's right shoulder.

Any number of things can cause deep vein thrombosis, or DVT, ranging from immobility and trauma to family history and dehydration. Symptoms include inexplicable swelling and discoloration or pain, or some combination of those factors, and experts say the worry is the clot traveling through the blood stream causing a pulmonary embolism or death.

"If you have a blood clot in a vein and a piece dislodges, it just goes with the flow where the blood is going and it ends up in the lung," said Mary Cushman, a hematologist and professor of medicine at the University of Vermont. "When you have a DVT in the leg or the arm, there's a chance that it will break free, and when it breaks free, it can end up lodged in the lung. And if it's large enough, that can kill a person."

Lillard missed multiple games in December because of right calf soreness and has also been listed on the injury report with right hamstring and right groin soreness, though there’s no way of saying if that caused the clot or vice-versa.

No matter the cause, it is diagnosed through ultrasound testing, in which a probe presses against the skin like imaging of a baby inside a pregnant woman.

"When you take that probe and press on a vein, you can see the vein like a black area and when you press on it, it will compress," Cushman said. "When the vein is full of a blood clot, you can't compress. … You're looking for veins that won't compress because they're full of clots."

Lillard is taking blood thinners, which is the most common course of action. Cushman points out that an on-court (or on-field or on-ice) injury that draws blood would make competing while on anticoagulants more of a risk for athletes.

"That does then make certain activities a bit more dangerous,” said Julie Partridge, a professor of sport and exercise psychology at Southern Illinois University and the vice chair of the board of directors for the National Blood Clot Alliance. “Obviously, a contact sport like basketball is going to be probably contraindicated, which we saw in Chris Bosh when he had to completely step away because he couldn't come off of those anticoagulants."

Sometimes surgery is involved or blood thinners for the long term. That is why Cushman called the decision for athletes to take time off - and how long they might be out - "really an individualized decision."

"You have to determine a care plan that's best for that person, so that's going to relate to the circumstances where they had their clot and the risk level they're willing to accept," Cushman said.

There are plenty of notable cases.

Even beyond Lillard, Wembanyama and Bosh, Detroit’s Ausar Thompson missed the end of last NBA season because of a blood clot. In the NHL, Carolina goaltender Frederik Andersen was out four months from late 2023 into 2024, and Vancouver forward Brock Boeser missed Game 7 of a playoff series last spring.

Colorado pitcher Scott Oberg underwent surgery to remove blood clots in his throwing elbow in 2021, and Arizona’s Merrill Kelly had thrombosis in his right calf two years later. New England defensive tackle Christian Barmore had a blood clot scare last summer, not long after center Billy Price retired at age 29 because of complications.

Stephan Moll, professor of medicine in the division of hematology at the University of North Carolina in Chapel Hill, said there is no evidence to suggest elite, professional athletes are more predisposed to blood clots than those of comparable ages in the general population. Cushman said the risk for people in their 20s and 30s of developing deep vein thrombosis or a pulmonary embolism is 1 in 10,000.

Some of the life of sports could play a role in blood clots occurring.

"Clots are always multifactorial," Moll said, cautioning against height being too much of a risk factor. "Maybe if there's multiple travels such as one after the other - a flight first and then a bus ride and then a game with some dehydration and some maybe more not major trauma - maybe it just adds up to thrombotic risk."

Partridge, who had her own blood clots and DVT discovered and treated in 2019, said female athletes are at higher risk if they are on hormonal birth control, get pregnant or undergo hormone replacement therapy.

One likely reason they come to light more often with elite athletes is the amount of trainers, doctors and medical staff around to provide care.

"I do think there's a certain amount of, anytime you have certainly professional athletes, there's a lot of investment in them," Partridge said, "so having access to medical care is obviously going to be beneficial."

Moll also believes there is more talk about blood clots among medical professionals to be conscious of possible concerns.

"People are more aware of DVT, I think, therefore they think about it earlier and they diagnose it earlier," Moll said. “But I don't think it's more common than it used to be."