Shouting during a nightmare. Struggling to balance a checkbook. A weakened sense of smell. Hallucinations. Chronic constipation. This bizarre mix of symptoms often stumps doctors, but they are some of the telltale signs of Lewy body dementia—the second most common type (after Alzheimer's disease), affecting an estimated 1.4 million Americans.

Lewy bodies are protein clumps that kill neurons. Depending on where they cluster in the brain, they can cause either Parkinson's disease or Lewy body dementia, although the two conditions tend to overlap as they progress. Lewy body dementia is more difficult to diagnose and treat, in part because the earliest warning signs have remained unknown. Now a new study finds that certain sensory and motor symptoms can help predict who will acquire the disease, paving the way for targeted studies.

Researchers at the Center for Advanced Research in Sleep Medicine (which is associated with the University of Montreal) and at McGill University followed 89 patients with a history of acting out their dreams—not sleepwalking but moving or vocalizing in bed during rapid eye movement (REM) sleep. The failure to suppress such nighttime activity can be an early sign that something is going wrong in the brain; past studies have shown that up to 80 percent of patients who act out their dreams will eventually develop some form of neurodegeneration.

Over 10 years the McGill researchers carefully tracked the patients' other potential symptoms of neural disease, such as mild cognitive impairment, depression and movement problems. They found a cluster of symptoms—abnormal color vision, loss of smell and motor dysfunction—that doubled the chance that a person with the REM sleep disorder would develop Parkinson's or Lewy body dementia within three years, according to the study published in February in Neurology.

People with this cluster of symptoms have a three-year risk of 65 percent, which is high enough to justify enrolling them in studies of early warning signs and treatments. Such studies are sorely needed because doctors are currently often stumped by what looks like a mix of Alzheimer's, Parkinson's and schizophrenia—and an incorrect diagnosis can cause harm. For instance, if a patient's most troubling symptom is hallucinations, doctors may try an antipsychotic—but such drugs rapidly worsen the condition of a person with Lewy body dementia. Even if the disease is correctly identified, doctors can do little more than treat each symptom that arises. The McGill researchers hope that their findings will quickly lead to more focused studies to determine how Lewy body dementia progresses over time. A better understanding of the disease could one day lead to more effective therapies or even preventive measures.