Mary Cushman, M.D., M.Sc., UVM Professor of Medicine and Director, Thrombosis and Hemostasis Program, UVM Medical Center. (Photo: COM Design & Photography)

Comments, below, are from a University of Vermont Medical Center Blog post by Mary Cushman, M.D., M.Sc., UVM professor of medicine and director of the Thrombosis and Hemostasis Program at the UVM Medical Center.

We keep hearing news stories about elite athletes developing abnormal blood clots. Perhaps the most well-known athlete to experience a blood clot is the tennis icon Serena Williams, who had life-threatening pulmonary embolism (PE) in 2011 after a foot injury and cross-country air travel.

In recent sports news, we have heard about NBA and NHL players with clots. Retired Portland Trail Blazers legend Jerome Kersey died in 2015 at age 52 of a PE about a week after minor leg surgery. Miami Heat star power forward Chris Bosh had a PE in February 2015 and missed the rest of the season while undergoing treatment. He was sidelined again in 2016 reportedly after a deep vein thrombosis (DVT) in the leg occurred and is missing the NBA Playoffs. National Hockey League players have also been affected. Star forward Steven Stampkos from the Tampa Bay Lightning had a DVT in the arm veins and underwent surgery in April 2016 to remove a rib that was reducing blood flow.

Why would these super-healthy people get blood clots?

Here’s how: Abnormal blood clots can occur in the leg veins. This is called deep vein thrombosis, or DVT. It causes leg pain, swelling and sometimes color change like red streaks. DVT can also be present without any symptoms. Sometimes when there is a DVT, pieces of the blood clot break free and travel to the lungs. This can be deadly and is called pulmonary embolism or PE.

These types of blood clots are treated with blood-thinning medications, also called anticoagulants. This problem is serious. These clots occur in up to 900,000 people each year in the United States and about 100,000 people die from them. This is more people than die from breast cancer, AIDS and car accidents combined.

About one or two middle-aged person in every 1,000 gets blood clots every year. The rate is lower in younger people – about one to two in every 10,000. So, just by chance athletes could get clots.

Risk Factors for Blood Clots in Athletes

• Obesity is a risk factor for these abnormal clots. Most pro athletes aren’t obese, but research suggests that larger people, like those who are taller or have bigger legs, are at risk. This is because the ability of blood to flow up the legs is more problematic for bigger people. So for some athletes, especially basketball players who tend to be very tall, risk might be higher. It’s interesting that we don’t hear many reports of NFL players with clots, as they are more likely to have obesity. It may be that it is just less often reported; no research is available.
• Athletes who use their arms heavily, like tennis players, baseball pitchers and basketball players, are at risk of clots involving the arm veins. These clots can occur when a structure at the base of the neck next to the shoulder called the thoracic outlet is narrowed. The vein, artery and nerve supplying the arm has to pass through this outlet, which is lined by muscles and bony structures. For athletes, especially those using their arms, the muscles can get large and contribute to the narrowing of the outlet. In this situation, repetitive injury to the vein going through that narrowed outlet sets the stage for a clot forming in the vein. These clots can cause swelling and pain of the arm, and sometimes numbness. Like leg clots, they can travel to the lungs causing pulmonary embolism. Treatment is with anticoagulant medications and often surgery is used to open up the outlet so that clots might not form again. 
• Injuries are a major source of risk for clots, especially leg injuries that result in leg immobility afterwards. The worst example of this is a leg fracture with casting. However, minor injuries also can increase the risk somewhat; this is likely a factor for hockey players as they often get hit by the puck.
• Surgery greatly increases the risk of blood clots for a period of several weeks. The risk is greater for orthopedic than general surgery, so it is a factor for athletes. In 2013, NBA Star Anderson Varejao's great season with the Cavaliers was cut short when he developed back and chest pain from a PE shortly after surgery on his quadriceps. His case proved that this condition doesn’t have to end a sports career; Varejao apparently had a finite treatment with anticoagulation and is currently playing for Golden State in the NBA playoffs.
• Like anyone else, genetics plays a role for athletes. Kimmo Timonen, four-time NHL All Star with the Philadelphia Flyers developed a leg DVT and PE in 2014. He had had less serious superficial clots (in veins just under the skin) previously, which we know is a risk factor for DVT and PE. He was found to have protein C deficiency - a genetic condition that contributes to blood clot risk by lowering the body’s ability to stop formation of clots.
• Travel: We know that long travel slightly increases the risk of clots, and pro athletes travel a lot.
• Birth control pills increase the risk of clots in women. This risk is highest in the first year of use and in women who are older when using them: risk continues until the pills are stopped.

Can athletes with clots continue to play their sport?
There are no medical guidelines on the best treatment for athletes so we customize treatment recommendations the same way we do for non-athletes. If the blood clot was limited to the arm veins or triggered by surgery, trauma or immobilization, usually 3 months of anticoagulation is adequate treatment.

If the clot occurs spontaneously, even one episode of clotting can mean a recommendation for long-term treatment with blood thinning medication. Long-term treatment is also common for people who experience clots on more than one occasion, especially if they have occurred spontaneously.

For professional athletes, this might mean the end of their career, although every sport is different. Golf, for example, doesn’t have the risk of injury that hockey does, so being on a blood thinner would not be a problem. For those playing sports with more contact, anticoagulation medication can be a problem due to risks of bleeding with injury. In this case other treatments are sometimes appropriate, like low dose aspirin and low dose blood thinning, so depending on the potential for injury with the sport, advice to the athlete can be customized. There are also some blood thinning medications that don’t build up over time in the body, so taking them at certain times of the day might allow sports participation; Jed Ortmeyer played in the NHL for 6 years doing this.

In the end, venous blood clots are the third leading vascular diagnosis in adults. So all of us need to be more aware. Key issues for athletes require custom advice from experts in treating these clots.

Link to the UVM Medical Center blog post here.