People who kick and lash out while fast asleep in bed face a high risk of developing Parkinson’s disease and certain forms of dementia, scientists report online December 24 in Neurology.

The condition, called rapid-eye-movement sleep behavior disorder, results when a person’s muscles fail to relax during sleep. “During REM sleep, with the most vivid dreaming, mostly we’re paralyzed,” says neurologist Ronald Postuma of McGill University in Montreal. “The brain shuts off muscle tone. We want to run but we can’t.”

But in people with REM sleep behavior disorder, muscle tone isn’t shut down. “As a consequence, you act out your dreams,” he says. People with the condition have been known to break a hand on a wall, hurt a spouse or fall out of bed, he says.

Postuma and his colleagues have monitored the progress of 93 people who were diagnosed with REM sleep behavior disorder between 1989 and 2006 at Sacré Coeur Hospital, also in Montreal. The team followed some patients for 15 years or more. Roughly 80 percent are men, and most were enrolled while in their 60s.

Of the 93 participants, 26 have developed a neurodegenerative disease during the study years. Of these, 14 developed Parkinson’s disease, and seven developed Lewy body dementia, which is marked by the appearance of Lewy bodies — abnormal protein deposits — in the brain. Four other study participants were diagnosed with Alzheimer’s disease, but the researchers suspect that these patients might actually have Lewy body dementia. One person developed a less common neurodegenerative condition called multiple system atrophy.

Among the entire group, the average risk of developing one of these diseases within five years of being diagnosed with the sleep disorder was 18 percent, the scientists calculated. For those monitored for 10 years, the risk was 41 percent, and by 12 years it was 52 percent.

By comparison, in the general population the average lifetime risk of developing Parkinson’s disease is only 1 or 2 percent, Postuma says. For developing Lewy body disease, the second-most common form of dementia after Alzheimer’s disease, the lifetime risk is roughly 1 to 3 percent, he says.

Researchers at the University of Minnesota in Minneapolis first identified the REM sleep behavior disorder in 1986. “We thought it was a cute clinical observation,” says Mark Mahowald, a neurologist at the university. But what started out as an academic curiosity now has been shown to be a serious condition and a harbinger of trouble, he says.

Based on past studies and the new report, he says, “there’s now just overwhelming evidence that the majority of people who develop REM behavior sleep disorder ... will eventually go on to develop a neurodegenerative disease.”

The sleep disorder is treatable with drugs, such as muscle relaxers, sedatives, anticonvulsants and other psychoactive drugs. But these address only the symptoms and not the underlying problem.

In normal REM sleep, the brain stem — where the brain meets the spinal cord — blocks motor neuron communication. The resulting paralysis keeps people from physically acting out their dreams.

This safeguard is disabled in the sleep disorder, but scientists have yet to sort out how. A key suspect is a protein called alpha-synuclein, which is a component of Lewy bodies. But the precise role of Lewy bodies and alpha-synuclein in these conditions remains unclear, Mahowald says.

Earlier work suggested that REM sleep behavior disorder may arise from damage in the brain stem that alpha-synuclein orchestrates. The protein is also implicated in Parkinson’s disease.

“Right now we don’t have any medications that would be termed neuro-protective for Parkinson’s,” he says. “However, when such a drug is identified — and it’s just a matter of time before we find one — just about everyone with REM sleep behavior disorder will be placed on that medication.”

Meanwhile, Postuma says, people with the sleep disorder should see a neurologist at least once a year to make sure they aren’t developing other problems.

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