Zack Wheeler on IL with blood clot: What it means, what to expect

The Philadelphia Phillies placed Zack Wheeler on the 15-day IL on Saturday due to a blood clot in his right arm. President of baseball operations Dave Dombrowski said Wheeler was diagnosed with a "right upper extremity blood clot," and few other details were offered.

Wheeler had pitched Friday and was limited to five innings, later reporting what Phillies head athletic trainer Paul Buchheit referred to as "heaviness." Heaviness is a description patients sometimes use when describing circulatory compromise that can occur as the result of a clot.

Blood clots in athletes can be caused by a number of factors. They can occur as the result of direct trauma, resulting in bleeding or swelling that contributes to clot formation. They can follow a period of immobilization (for instance, postsurgery when a limb is immobilized for a period, there can be an increased risk of clot formation). Genetic clotting disorders can be an origin source, but they are rare in an elite athlete. The most likely cause of clot formation in an elite athlete, particularly in the upper extremity of one who repeatedly subjects the arm to overhead stress, is thoracic outlet syndrome.

Thoracic outlet syndrome occurs when the first rib, or occasionally an extra rib, creates compression over the blood vessels and/or nerves as they exit the neck region under the clavicle (collarbone) and travel through the shoulder to the arm and hand. Overhead athletes -- most notably, baseball pitchers along with softball players, volleyball players, rowers and swimmers -- are most susceptible to clots in the shoulder area due to thoracic outlet syndrome.