Thrombosis, the silent danger - also in sports

Whether French basketball superstar Victor Wembanyama will be able to compete in the European Championship at the end of August 2025 remains to be seen. The 21-year-old, who plays in the North American professional league NBA, has been sidelined since the end of February due to a venous thrombosis in his right shoulder. He underwent shoulder surgery in March. "I'd like to do more, but I have to listen to the doctors," says Wembanyama. Thrombosis is not something to be taken lightly.

Former NBA star Chris Bosh, a member of the Basketball Hall of Fame, was forced to end his career due to thrombosis. Tennis icon Serena Williams also suffered from multiple blood clots during her playing days. In 2011, she had to undergo emergency surgery for a pulmonary embolism. Three-time World Handball Championship winners Mikkel Hansen of Denmark and Nikola Karabatic of France also had to be treated for thrombosis. And in the 2016 Bundesliga, then-Borussia Dortmund player Neven Subotic was sidelined for the second half of the season due to a venous thrombosis in his arm.

Even competitive athletes are not immune to thrombosis. "While plenty of exercise is an important component in preventing thrombosis, there are also other risk factors," says sports physician Pascal Bauer. "Athletes are only human and can therefore get all the same diseases that affect others." Bauer heads the departments of sports cardiology, cardiovascular prevention, and vascular diseases at the University of Giessen . Last year, he supervised and advised nearly 400 professional athletes, including the HSG Wetzlar handball Bundesliga players.

What happens during thrombosis?

Thrombosis is the Greek word for clot or lump. Thrombosis occurs when a blood clot forms in a blood vessel, such as a vein or artery, blocking the vessel and thus impeding blood flow. Every thrombosis is an emergency. Blockages in the deep veins of the legs and pelvis are considered particularly dangerous. If part of the blood clot breaks away, it can travel through the heart to the lungs and block arteries vital for breathing. This is called a pulmonary embolism.

"It can vary in severity – from 'I feel a little shortness of breath or nothing at all' to death," says vascular expert Bauer. The larger the dislodged blood clot, the more dangerous it becomes. The Thrombosis Action Alliance estimates that between 40,000 and 100,000 people die of pulmonary embolisms in Germany each year. According to United Nations estimates, one in four deaths worldwide is associated with thrombosis.

Are there any sports that are particularly at risk?

"There's a slightly increased risk in sports that emphasize the upper body and involve repeated arm movements," explains Bauer, citing ice hockey, handball, basketball, volleyball, baseball, and tennis as examples. "The athletes develop a great deal of muscle mass, which can lead to even more cramped space in the already narrow shoulder area, where arteries, veins, and nerves snake through. The weakest link is the veins. If they are repeatedly compressed, endothelial damage [cell injuries on the inside of the vein - editor's note] can occur, resulting in thrombosis." Thrombosis can also occur in contact sports such as soccer, where the risk of leg injuries, including fouls, is higher. The same applies to long-distance running, where there is a risk of micro-injuries to the legs.

According to the scientist, professionals in all sports are at greater risk if they frequently travel long distances to their competitions and therefore spend a lot of time on buses or planes. This poses a risk of what's known as stasis: the blood clots, increasing the risk of developing thrombosis.

What warning signs indicate thrombosis?

"Classic deep vein thrombosis begins with unilateral swelling of the lower leg, accompanied by pain, a feeling of heat, and often a bluish-purple discoloration, as well as visible veins in the shin area," says sports physician Bauer. Swelling also develops with brachial vein thrombosis. The athlete initially feels a feeling of tension, followed by a more pressing pain, "as if the arm were bursting from within," says Bauer. Another characteristic is a network of veins that often forms on the surface of the skin in the shoulder area, "a bypass circulation because the deeper vein is blocked."

Especially in contact sports, there's a risk that athletes may mistake the warning signs of thrombosis for normal symptoms of injuries, or even confuse the pain with muscle soreness.

What can active people do to prevent possible thrombosis?

"The most important thing is sports screenings and good training hygiene," answers vascular specialist Bauer. "Athletes should make sure they drink enough. What they sweat out, they must also replace in their bodies so that the blood doesn't thicken. Secondly, they have to remember to move their legs while traveling." It may also be advisable to wear compression stockings on long flights or bus rides. "And athletes should always pay attention to and listen to their own bodies," recommends Bauer.

Doping —for example, with anabolic steroids, EPO, or autologous blood—also increases the risk of thrombosis. In bodybuilding, many deaths have been observed after thrombosis in athletes between the ages of 30 and 50, says the physician from the University of Giessen. "But I don't want to accuse anyone of doping just because they have a thrombosis."

How is thrombosis diagnosed and treated?

It is diagnosed using a combination of blood analysis and imaging techniques. A special blood test determines whether a significant blood clot has occurred in the body. Ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI) can easily detect thrombosis in blood vessels and organs, as well as even the smallest blood clots. The patient is usually then given a blood-thinning medication for a period of three to six months until the thrombosis disappears.

After a while, athletes are allowed to train cautiously again, but not in competition mode. "The risk of bleeding from injuries is significantly increased by the medication," explains vascular expert Pascal Bauer. "If, for example, a handball goalkeeper gets hit in the head by a ball and suffers a hemorrhage, this can have serious consequences. Therefore, the need for blood-thinning therapy can prevent the athlete from returning to competition in contact sports, even if they are already feeling well."

After brachial vein thrombosis, some athletes undergo shoulder surgery. The top rib is shortened to give the vein more space and prevent it from constantly colliding and being compressed against the rib during overhead movements.