The 35-year-old Alderweireld, who still plays for Royal Antwerp in the Belgian league, quit international soccer in March 2023 after winning 127 caps.
He was part of the so-called "Golden Generation" of players who finished third at the 2018 World Cup. He also took part in the 2014 and 2022 editions.
Alderweireld said a first serious warning about his health came after a defeat in a Belgian Cup match against Union Saint-Gilloise.
"That's why I quit the Devils," he told Belgian media VRT.
"I couldn't sleep that night and went to the club early the next morning to do a strength session," he recalled. "Before I left, I took a caffeine pill because I don't like coffee. When I was in the car, my heart suddenly started beating at a rate of a thousand an hour. I thought: I'm going to have a heart attack. I'm done for, I'm never going to see my children again. I pulled over, walked into a furniture store and asked if they could call 911."
Some time later, Alderweireld said panic took over him again, this time at night.
"It turned out that all the stress made my heart pound," he told the broadcaster. "That gave me a panic attack, which made my heart beat even faster. You actually drive yourself crazy. And at a certain point you think you're going to have a heart attack and die."
Alderweireld later underwent medical examinations.
"We did extreme tests, but everything was okay," he said. "Then I started talking to people and they said it was due to too much stress. Purely a panic attack. Now I still suffer from that sometimes, but I can accept that because I know it's nothing."
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