What is commotio cordis, which NFL player Damar Hamlin says stopped his heart?

Damar Hamlin has confirmed the cause of his near-fatal collapse on "Monday Night Football" as commotio cordis, a rare event caused by a blow to the chest.

"This event was life-changing, but it's not the end of my story," Hamlin said Tuesday.

Hamlin, 25, a safety for the Buffalo Bills, spoke at a news conference after working out with the team. Earlier, general manager Brandon Beane said that multiple specialists had seen Hamlin and that he had been "fully cleared" to return to football.

"I got a long journey to go, but I'm committed to it each day, day in day out," Hamlin said. "So I thank everybody for being on this journey with me."

Hamlin's collapse after being hit in the chest while making a tackle on Jan. 2 left millions wondering how a run-of-the-mill play could stop an athlete's heart.

Commotio cordis requires several things to go exactly right – or exactly wrong – said Dr. Gordon F. Tomaselli, the Marilyn and Stanley M. Katz Dean of the Albert Einstein College of Medicine in the Bronx, New York. He also is executive vice president and chief academic officer of Montefiore Medicine.

Unless witnesses recognize it, start CPR, alert emergency medical services and use a defibrillator, "patients will not survive," said Tomaselli, a former president of the American Heart Association who specializes in sudden cardiac death and heart rhythm disturbances.

Here are some basics about what commotio cordis looks like and what people should know about protecting themselves and their families.

What is commotio cordis?

Commotio cordis (pronounced ke-MO-she-o-KORD-is) comes from the Latin for "agitation of the heart." It was first described in the 1700s, but most of what is known about it has been learned since the 1990s.

Put simply, it's a rare cardiac arrest immediately following a blow to the chest. The impact induces a potentially lethal heart rhythm disturbance, or arrhythmia, called ventricular fibrillation, Tomaselli said.

What does it look like?

What viewers witnessed when Hamlin collapsed was typical, said Dr. Barry J. Maron, a cardiologist at Lahey Hospital and Medical Center in Burlington, Massachusetts, who was watching the game.

"I saw it in real time," said Maron, as Hamlin took what seemed to be a routine blow, then staggered, and collapsed. "That was a commotio cordis event."

Maron, who has spent decades doing pioneering research on commotio cordis, was not aware of Hamlin's official diagnosis prior to Tuesday's announcement. But he said last week that after a blow to the chest, the person can tolerate the arrhythmia for a few seconds before collapsing. Maron said in one memorable case, a second baseman playing softball was struck by a line drive in the chest. "The ball dropped right in front of him. He reached down, picked up the ball and threw the runner out. And then collapsed. And died."

How rare is it?

There's no way to know exactly how common commotio cordis events are, said Maron, who helped found the U.S. Commotio Cordis Registry in 1995. Reporting cases to the registry is voluntary. "But they are for sure very, very rare."

A scientific review just published last month in JACC: Clinical Electrophysiology identified only 334 cases dating back to 1980.

Who is most often affected?

In the recent scientific review, 94% of victims were male, and their average age was 19. A little less than two-thirds of the cases involved sports, and most of those involved a projectile such as a baseball or softball.

Among the non-sports cases, about three-fourths involved assault, and 7% were from motor vehicle accidents.

Commotio cordis has been caused by snowballs, sibling scuffles and falling off bikes. Children have died after being struck by toy bats, T-balls or plastic sleds.

Younger people are more susceptible to commotio cordis because their chests are less developed and more pliable, Tomaselli said, and they're more likely to be doing something – such as playing sports – that could put them at risk.

There's no data on whether physical differences might protect females, he said, and their lower rates are probably related to the fact that boys and young men, in the past, have been more likely to participate in sports.

Overwhelmingly, Tomaselli said, commotio cordis occurs in people who have no underlying heart disease.

What exactly has to go wrong for a seemingly simple blow to the chest to stop a heart?

First, Maron said, "the blow has to be over the heart. If it's, for example, on the right side of the chest, it's not going to trigger the arrhythmia."

Second, it has to occur during a precise moment in a heartbeat. "It's a very narrow time window," Tomaselli said. "Anywhere between 20 and 40 milliseconds." For comparison, an eyeblink takes as much as 300 milliseconds.

Finally, the blow must be not too hard and not too soft, Tomaselli said. "It's kind of a Goldilocks phenomenon." If the blow is too intense, the heart gets bruised, and commotio cordis generally does not occur. A less-intense blow – even a soft one, Maron said – can trigger the problem.

What sorts of safety measures are possible?

"It would be very hard to protect oneself from commotio cordis," Maron said. People have died despite wearing chest protection; children have died from gently tossed balls that bounced off a glove. "It's a bizarre occurrence."

But proper training in how to respond can make a huge difference. A 2015 AHA and American College of Cardiology scientific statement that Maron helped write showed that survival rates had increased in recent decades to nearly 60% by 2012.

Tomaselli agreed that the best preventive measure would be to have more people able to recognize and respond to a cardiac arrest by calling 911, knowing how to perform effective CPR and making sure automated external defibrillators, or AEDs, are available for all sporting events.

"It's not so much prevention as it is prompt treatment," Tomaselli said.

What went right in Damar Hamlin's case?

With any cardiac arrest, survival depends on people nearby acting to correct the problem within seconds to minutes, Tomaselli said. On Jan. 2, the chain of survival began with immediately activating emergency medical personnel, beginning CPR and using a defibrillator.

"The reason he did so well was because he was in a situation where there was a prompt and effective response," Tomaselli said.

He added that when there is no underlying heart condition, someone who has experienced commotio cordis should be able to return to their usual activities.

Is there any overarching lesson we can learn from this?

"It's reversible," Maron said. In theory, virtually any case of commotio cordis could be undone if people are trained to recognize it, have access to AEDs, and act.

"Anybody can save a life," Tomaselli said. "And you can save a life by knowing how to do basic cardiac life support and CPR."