We’ve all had the experience of acting out our dreams, but this may be a harbinger of dreaded neurological diseases.  Actor Alan Alda, best known for his role on the comedy-drama television series M*A*S*H* thought he was being threatened as he slept and threw a bag of potatoes at the attacker. When he awoke, he was in his bedroom and the bag of potatoes turned out to be a pillow he’d thrown at his wife. A frightening experience like this could signal a brain-related disorder, such as Parkinson’s disease, which turned out to be the case for Alda.

According to Scientific American, acting out dreams marks a disorder that occurs during the rapid eye movement (REM) phase of sleep. The disorder, called REM sleep behavior disorder (RBD), affects an estimated 0.5 to 1.25% of the population. It is more common in men and in older adults, and may signal neurodegenerative disease, most frequently a condition in which the protein alpha-synuclein forms toxic clumps on the brain. This is called synucleinopathy.

RBD may also be triggered by certain drugs, such as antidepressants, or caused by other underlying conditions, such as narcolepsy or a brain stem tumor. Sleep walking and sleep talking are not behaviors that are associated with RBD.

When RBD occurs in the absence of these alterative explanations, the chance of having a future brain disease is high, says Scientific American. Some experts say that when dreams are enacted, there is more than an 80% chance of developing a neurodegenerative disease, particularly Parkinson’s, which is characterized by progressive loss of motor control. RBD may be the first sign of other degenerative diseases as well, including Lewy body dementia and multiple system atrophy.

But many clinicians are not familiar with the connection between dreams and disease. Alda had to convince his neurologist to do a brain scan for Parkinson’s after he read about the link in a 2015 news article. His scans confirmed his suspicion, and the actor shared his experience with the public to alert others.

“I thought anybody who has any symptoms, even if it’s not one of the usual ones, could get a head start on dealing with the progressive nature of the disease,” he says. “The sooner you attack it, I think, the better chance you have to hold off the symptoms.”

Dr. Daniela Berg, a neurologist at Christian-Albrechts-University in Germany says that RBD is “one of the strongest clinical prodromal markers we have” to predict Parkinson’s disease.  Scientists say that insight into RBD can help them trace the ways the alpha-synuclein spreads through the body and brain. In some patients, there is evidence that pathology begins in the gut and spreads up through lower brain structures such as the brain stem to the higher regions that govern movement and cognition. The most likely pathway is through the vagus nerve and at least one study has shown that cutting the vagus, a treatment used for stomach ulcers, may reduce the risk of Parkinson’s disease later in life, says Scientific American.

Alda, 86, says he is “doing everything I can to slow the progression of Parkinson’s.”  He works out, plays chess with his wife and binges on his favorite TV series, according to People. “I’m more convinced than ever that life is adapting, adjusting and revising,” he says.