A rare sleep disorder that makes people act out their dreams may be an early warning of a deadly neurological illness, a new review of previous research suggests.

About half of people who have a condition known as rapid eye movement sleep behavior disorder will develop Parkinson's disease or a related disorder within a decade of being diagnosed with RBD.

Eventually, nearly everyone with RBD will ultimately develop a neurological disorder, the study found. [Top 10 Spooky Sleep Disorders]

"If you get this disorder and live long enough, you will almost certainly get Parkinson's disease or a condition similar to it — it's an early warning sign," said Dr. Michael Howell, a professor of neurology at the University of Minnesota in Minneapolis and co-author of the study, published today (April 13) in the journal JAMA Neurology.

The main symptom of RBD is moving around during the rapid eye movement (REM) period of sleep, when most dreaming occurs and the muscles are usually paralyzed by the brain stem. People with RBD are thought to have a brain-stem malfunction that allows them to move during REM sleep, and thus act out their dreams, according to the study.

People with RBD describe having vivid dreams, and their enactments range from small hand movements to violent actions such as punching, kicking or leaping out of bed. The disorder poses a risk of injury to the patient or their bed partner, Howell said. Scientists first described the disorder in the 1980s. It is distinct from sleepwalking, and affects about 0.5 percent of the population, or 35 million people worldwide, he said.

To find out whether RBD was, in fact, an early sign of Parkinson's disease and similar brain disorders, Howell and his colleagues sifted through more than 500 studies on the subject published between 1986 and 2014.

Strikingly, they found that between 81 and 90 percent of patients with RBD developed a degenerative brain disorder during their lifetimes, the studies showed.

Parkinson's disease is caused by the breakdown of certain proteins, called alpha-synuclein proteins, in neurons in the brain that produce dopamine, a chemical that produces pleasurable feelings in response to rewarding activities. It could be that RBD results from the early stages of alpha-synuclein breakdown in the brain, so it could be a useful warning sign of Parkinson's, Howell said. Not everyone who develops Parkinson's disease will have RBD first, however.

The findings could help doctors find a way to treat Parkinson's while it's still in its early stages, Howell said.

RBD is not curable, but it can be treated with high doses of the sleep aid melatonin or low doses of the anti-anxiety drug clonazepam. Patients with RBD should also take steps to prevent possible sources of injury.

"It's very important to make the bedroom environment [as] safe as possible" by removing objects that can be picked up or used as a weapon, such as guns, Howell said.

Parkinson's disease is not curable, either, but it can be managed with drugs. In addition, an experimental therapy known as deep-brain stimulation has shown promise in some patients.

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