Kids who play "choking games" to achieve a euphoric high are more likely to be suicidal and face a greater risk of injury and death when they play alone, a U.S. study suggests.

It's dangerous enough in groups, but without others present to interrupt the asphyxiation children face an increased risk of loss of consciousness and inability to stop strangulation when a noose, belt or other ligature is being used. Nearly all deaths from the game occur when youngsters play alone.

Solo players are also more than four times as likely to have suicidal thoughts and more than twice as likely to suffer from serious mental health problems compared to kids who try the choking game in groups, researchers report in Pediatrics.

"Many kids don't understand how dangerous this activity is," said study co-author Sarah Knipper, a researcher with the Oregon Health Authority in Portland.

"A lot of kids who participate in the choking game, particularly alone, also have depression and/or use alcohol or drugs," Knipper added by email.

Players typically use their hands, a belt or a tie to put pressure on the carotid artery in the neck, temporarily limiting the flow of oxygen and blood to the brain. The goal is to achieve a euphoric feeling when the flow of blood and oxygen rushes back to the brain.

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To understand what separates group participants from youth who play alone, researchers examined survey data collected from almost 21,000 Oregon teens in 2011 and 2013. Participants were in eighth grade and about 14 years old on average.

Among other things, the survey asked about physical and mental health, nutrition, school absenteeism, sexual activity, substance use and community characteristics. Questions on the choking game focused on whether participants had heard of it, tried it, or done it alone.

Overall, just 3.7 percent of teens said they had played the game.

Among those who had tried the choking game, however, roughly 18 percent said they had played alone.

Slightly less than 1 percent of teens said they had helped someone else play the game.

About 77 percent of the survey participants had never heard of the choking game.

One limitation of the study is that it relied on teens to accurately recall and report if they knew about the choking game or had played it, the authors note. In addition, because the survey is administered only in public schools, researchers lacked data on youth who were in juvenile detention or who had dropped out of school.

The findings, while distressing, aren't surprising given the popularity of self-harm and substance use among teens, particularly for youth with mental health problems, said Dr. Benjamin Shain, head of child and adolescent psychiatry at NorthShore University HealthSystem in Chicago.

"A possible explanation for the 18 percent who tried it alone is that suicidal teens often engage in behavior that is not intended for suicide but carries a risk of death, partly hoping to die by accident," Shain, who wasn't involved in the study, said by email.

It can be hard to identify kids who play the game because there may not be overt signs of participation, said Knipper.

However, parents should watch for physical signs like bloodshot eyes, unexplained marks on the neck, small dots around the face, eyelids or eyes, and frequent severe headaches, Knipper added. Participants may also seem disoriented after spending time alone, or leave evidence such as dog leashes, bungee cords, belts or scarves tied to bedroom furniture or doorknobs or knotted on the floor.

"We do recommend that parents talk to their kids about it using simple factual words," Knipper said.

"They can ask their kids if they know what it is, if they know anyone who has done it (or if they've done it themselves), and make sure they know that it is not safe and someone could get really hurt even if they are doing it with a group of friends," Knipper added. "They should stop and tell a safe adult."