Individuals with REM sleep behaviour disorder (RBD) do not exhibit the lack of muscle tone — called atonia — that usually occurs during sleep, leaving them able to move freely.
"Essentially, all of us normally are paralyzed when we go into our REM sleep," said Dr. Ron Postuma, a neurologist at McGill University who led the study, conducted at 13 sites in 10 countries. "So we're having very vivid dreams, but we don't move. Our eyes will move and we'll breathe, but that's it."
In people with RBD, the body system that keeps sleepers paralyzed stops working, so whatever a person dreams, they are capable of doing.
"It just looks like they're acting out one-half of the scenario," Postuma said from Montreal. "So, for instance, they'll be having a conversation and hold up one end of the conversation. They might gesticulate. They might talk articulately. They might laugh, they might cry, they might smile.
"If they dream they're smoking a cigarette, the hand will go up to the mouth and they'll make puffing motions. Essentially, they're pantomiming a play that's going on in their head, and that play is the dream."
But violent dreams can have nasty consequences: people with the disorder can unknowingly punch, kick and even throttle their bed partners.
In 2009, a British man strangled his wife to death while the couple were camping. Falling asleep with the sound of people in the campground "yelling and screaming," Brian Thomas dreamed he and his wife Christine were being attacked, and in his sleep state he tried to defend her by choking an imagined attacker.
"It was really very tragic," said Postuma, explaining that the man was declared not guilty because of his disorder.
With severe cases, most patients sleep alone to prevent harm to their partners, he said. But patients can also hurt themselves: they don't walk but can hurl themselves out of bed or bash the wall and break a hand, for instance.
Medications can significantly ease the disorder, which affects less than one per cent of the population and is more common in men and in older people.
In their study of almost 350 people with RBD and the same number of control subjects without the disorder, researchers found that those with the disorder were 43 per cent more likely to be smokers, with 64 per cent having smoked at one time, compared to 56 per cent of those without RBD.
The study, published in this week's issue of the journal Neurology, also found subjects with RBD were 59 per cent more likely to have had a previous head injury with loss of consciousness. Two-thirds were more likely to have worked as farmers, and were more than twice as likely to have been exposed to pesticides.
"That was the strongest one, in fact. Pesticides are very clearly associated with REM sleep disorder behaviours," Postuma said.
But the most important aspect of RBD isn't the disorder itself, but the fact it appears to be a possible precursor to Parkinson's disease and a particular type of dementia called dementia with Lewy bodies.
"What's going to happen to them is that they're going to have a 50 per cent or more chance of developing one of these very serious disorders over the next 10 to 15 years," he said.
"So this gives you a window. You can move 10 or 15 years earlier and look at the risk factors."
Dr. Brendon Boot, a neurologist at Brigham and Women's Hospital in Boston, called the study "another valuable piece in the puzzle for our understanding of ... diseases such as Parkinson's disease and dementia with Lewy bodies, the second most common form of dementia."
Boot and Dr. Brad Boeve of the Mayo Clinic published a study earlier this year that showed elderly people who act out their dreams have twice the risk of developing Parkinson's or mild cognitive impairment, considered by many an early sign of dementia, in the following four years, compared to those without the sleep disorder.
He suggested individuals who act out their dreams should consult a neurologist or sleep disorder specialist, as other disorders can produce similar symptoms. Obstructive sleep apnea, in which a person periodically stops breathing for several seconds, is the most common alternative cause.
"Not everyone who has RBD will develop one of these disorders ... not everyone will develop Parkinson's disease or dementia with Lewy bodies," Boot said Wednesday from Boston.
In fact, some patients have been acting out their dreams for decades, he said. "A full quarter of them have had it for 40 years or more and have not developed any problem."
Boot said patients diagnosed with RBD would be ideal candidates for clinical trials of potential Parkinson's drugs.
In an accompanying Neurology editorial, Drs. Shannon Sullivan and Christian Guilleminault of Stanford University's Sleep Disorders Clinic and Dr. Carlos Schenck of the Minnesota Regional Sleep Disorders Center called the McGill-led research a "novel, timely study that opens up a new branch of RBD research, and points the way to additional, larger studies."
"Without such sweeping collaborations among multiple sites, risk factors for relatively uncommon and underreported sleep disorders such as RBD may remain hidden in plain sight," they write.