June 5, 2009 -- The mysterious death of actor David Carradine -- perhaps by auto-erotic asphyxia -- focused renewed attention on a practice that is one of the greatest and most dangerous sexual taboos.
The 72-year-old actor was found dead in a Thai hotel room closet in an intricate web of ropes -- one around his neck, another around his genitals and the two tied together, according to Thai authorities.
Sex experts say that Carradine's advanced age suggests that he may have been a lifelong practitioner of the secretive and dangerous practice, one that can go fatally awry.
Los Angeles Superior Court documents of Carradine's divorce put online by The Smoking Gun show that his most recent ex-wife, Marina Anderson, accused the actor of "deviant sexual behavior which was potentially deadly." The alleged behavior wasn't described in the court documents.
Also known as hypoxyphilia, the practice is a sub-category of sexual masochism that involves reducing the oxygen supply to the brain while masturbating to achieve a heightened orgasm.
'Fine Line' Between Euphoria and Death
"There's a fine line between the state of hypoxia [lack of oxygen in the brain] and death, and it's in that state that a person becomes highly aroused and it's what allows them to orgasm," said Eli Coleman, chair of the sexual health department at the University of Minnesota.
"People usually have safety nets and no intention to die, but something often goes wrong in their calculation," he told ABCNews.com. "Maybe a stool or something they are standing on somehow slips away."
The Federal Bureau of Investigation estimates there are between 500 and 1,000 such deaths in the United States annually, mostly among young men. Many more may be falsely ruled as homicides or suicides.
Auto-erotic asphyxia often starts in adolescence, which is why many of the deaths associated with the practice are among young adults. But, the practice is not unknown among girls, according to Coleman.
Because it is practiced alone, AEA is particularly dangerous. It is a compulsive activity that can also escalate, as may have been the case with Carradine, said Coleman.
"We know very little about the people who practice this," he said. "Most of what we know is from those have accidentally died."
Auto Erotic Asphyxia Often Ruled Suicide
Forensic experts often rule these deaths as suicides, but the evidence often suggests otherwise: the body is naked, there are pornographic materials and perhaps even semen present.
"It's tragic for families because no one has any suspicion of what is going on," said Coleman. "Usually it's a carefully guarded secret."
Such was the case with Michael Hutchence, the late lead singer of the superstar Australian band INXS, whose death in 1997 was ruled a suicide.
The singer was found naked, hanging from his leather belt in a Sydney hotel room with pornographic literature at his feet and no suicide note.
Hutchence's wife spoke publicly of their kinky sex life and his desire to try AEA, and a British documentary eventually concluded that was the case.
According to research by Stephen Hucker, forensic psychiatrist from the University of Toronto who is one of the foremost experts in the field, the practice is not entirely modern.
Sexual Practice Dates Back Centuries
Stone statues dating back to 1,000 AD suggest it was known to the ancient Mayans in Mexico. The Marquis de Sade, whose name was given to the practice of sado-masochism, described it in his 1791 novel, "Justine."
The more common ways of carrying out the practice are self-hanging, strangulation, choking, suffocation and techniques to restrict breathing movements. Self-hanging is the most common method observed among fatal cases, according to Hucker's Web site, Forensic Psychiatry
"Fatalities resulting from AEA practices occur with a frequency of about one per million of the population per year in North America though it is important to note that this figure is based on studies of cases that have been recognized as AEA or Hypoxyphilia," said Hucker.
Usually a rope or ligature is the method used to strangulate the oxygen supply, but others may be present as part of the person's own sexual ritual. Plastic bags and more complicated apparatuses may also be used.
Sex therapists regard this as a paraphilias -- or a socially unacceptable sexual practice. It is often associated with forms of anxiety and depression, though not suicidal thoughts.
Auto Erotic Asphyxia Evokes Shame
Few who seek help because of the shame, which fuels a cycle of guilt, anxiety and then release.
"Many look pretty normal underneath but when they are accelerating this need for a very intense high, what they are doing in medicating some state of dysphoria or unease," said Coleman. "And because it is rather unusual and there is a lot of stigma around masturbation to begin with, the guilt and shame become part of a repetitive cycle of obsession and compulsion."
Teens often discover the practice on their own, rather than learning it from others. Its sensations are similar to the "choking game" played by young children, who get a thrill from the dizziness created by holding their breath.
Divers have also reported a "high" from oxygen deprivation, according to Coleman. "For some people that may be pleasant or something associated with sexual arousal."
"This has been around long before the Internet.We have no idea how they would discover this," said Coleman. "It's not something they learned. It was a puzzlement and we concluded they were learning it on their own."
"We can only start to piece this together from people who have actually survived," he said.
Carradine May Have Practiced for Years
Coleman, who did not consult in the Carradine case, speculates that many, like the actor, have practiced AEA for years.
"It becomes for them a fetish kind of thing and that is what it really takes for them," he said. But often their activities escalate in risk for the thrill, resulting in accidents.
But in the case of teens, accidental death often happens because they are inexperienced in the practice. "For those who have done it a long time, they know the limits and are pretty careful."
Coleman said people, particularly young teens, need to learn about the dangers of AEA.
"There are a lot of things on the Internet and we have no idea how this is impacting people," he said. "We are certainly concerned that [the Carradine case] might lead to an increase."
"We need to educate people about the risks rather than shield people from the information," he said. "A lot of people are engaged in this activity and they don't know the risk."
Carradine's ex-wife Anderson also said in divorce documents that her "pleas for him to get counseling" were ignored.
Attempts by ABC News to reach Anderson for comment were unsuccessful.
ABC senior researcher Sheelagh A. McNeill contributed to this report.