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. 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."
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.
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.
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."