David Thompson lost sensation in his right hand.

Then his triceps swelled, and his arm turned purple.

The diagnosis was daunting: a blood clot related to thoracic outlet syndrome. When he heard the words come out of the doctor's mouth, Thompson knew his sophomore season at Florida's University of Miami was going to be interrupted. He knew little else.

"I was super scared because you hear 'blood clot' and people can die from blood clots," said Thompson, the Mets' fourth-round draft pick in 2015, who currently plays third base for the Brooklyn Cyclones. "It was really scary. I didn't know if I'd be able to play baseball again."

Venous thoracic outlet syndrome occurs once in every 100,000 people, according to the National Center for Biotechnology Information's website. "Strenuous use of the arm and shoulder" compounded by a "congenital narrowing of the space through which the major arm vein passes from the shoulder area into the heart" is a common cause, according to vascularweb.org.

The injury isn't that uncommon among baseball players. Chris Carpenter, Josh Beckett, Kip Wells, Shaun Marcum and Jeremy Bonderman are among those who underwent surgery for symptoms related to thoracic outlet syndrome in recent years.

Thompson's symptoms emerged in March 2014, he said, and he underwent surgery to remove the clot as well as his first rib to prevent future impingement.

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"It was very serious," he said.

The most serious thing about Thompson these days is his reputation.

The 6-2, 220-pounder, who signed with the Mets in mid-June after Miami was ousted from the College World Series, was the NCAA season leader in RBIs (90) and runner-up in home runs with 19. Six outlets named him to the All-America First Team.

All after battling back from a blood clot that could have ended his life.

"He's a gamer," said Cyclones manager Tom Gamboa. "I love his makeup."

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Thompson, 21, was hitting .254 going into Wednesday's game at Staten Island. He has not homered, but Gamboa thinks Thompson has been pressuring himself to live up to his reputation.

"My only comment to him to try to take the pressure off him was to let him know that he was going to play virtually every day," Gamboa said. "But he needed to calm down at the plate and get better pitches to hit because David had some games where he was going 0-for-4 and literally not swinging at a single strike. He was so anxious to do something that he was committing to pitches that nobody could hit."

Gamboa noticed a change in Thompson's approach during a July 8 win over Hudson Valley. Thompson, the manager said, was patient at the plate. He waited for the right pitch before uncorking his powerful swing. That resulted in a 4-for-5 day.

"I was getting hits, but I just didn't feel like I was hitting the ball as hard as I should be," Thompson said of his performance leading up to that four-hit day. "[July 8] I felt like I was squaring up the baseball and putting good swings on good pitches."

Thompson's power has yet to manifest itself in the tangible result of a home run. His track record indicates that will change. Thompson is a home-run hitter. He homered 55 times at Westminster Christian School, shattering Prince Fielder's Florida prep record of 42 home runs. He hit cleanup in his first game at Miami, a national powerhouse, and led the team with six home runs as a freshman.

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"I always tell people it takes 50 at-bats to get your feet under you and get acclimated to a new team and pro ball," Gamboa said. "It's obvious he's got talent."

Thompson is confident the power will come. It always has.

For now, he's just happy to be healthy and playing professional baseball. He's grateful for his speedy, full recovery.

Said Thompson, "It was definitely a big accomplishment and a huge blessing."