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Acting out dreams while sleeping is the strongest predictor of the second most common form of dementia in the elderly, researchers discovered.

Mayo Clinic researchers found that patients are five times more likely to have dementia with Lewy bodies if they experience a condition known as rapid eye movement (REM) sleep behavior disorder than if the exhibit other risk factors like fluctuating cognition or hallucinations.

Scientists presenting their study at the annual meeting of the American Academy of Neurology in San Diego say that the latest findings could improve diagnosis of dementia with Lewy bodies, and can lead to better treatment.

Researchers explain that REM sleep behavior disorder is caused by loss of normal muscle paralysis that normally occurs during REM sleep. Researchers said the sleep disorder can appear three decades or more before a diagnosis of dementia with Lewy bodies is made in men.  However, researchers noted that the link between the sleep disorder and dementia with Lewy bodies is not as strong in women.

"While it is, of course, true that not everyone who has this sleep disorder develops dementia with Lewy bodies, as many as 75 to 80 percent of men with dementia with Lewy bodies in our Mayo database did experience REM sleep behavior disorder. So it is a very powerful marker for the disease," lead researcher Melissa Murray, Ph.D., a neuroscientist at Mayo Clinic in Florida, said in a statement.

Murray said screening for the sleep disorder in dementia patients could help healthcare professionals diagnose wither dementia with Lewy bodies or Alzheimer's disease.

In the study, Murray and her team examined MRI scans of the brains of 75 patients diagnosed with probable dementia with Lewy bodies. 

They then conducted an autopsy examination of the brain to determine the likelihood of dementia.

Afterwards researchers looked at the patients; medical histories to see if the sleep disorder had been diagnosed while under patients were in care.  Using medical history data and brain scans, they matched a definitive diagnosis of the sleep disorder with a definite diagnosis of dementia with Lewy bodies five times more often than they could match current diagnostic risk factors such as loss of brain volume.  Furthermore, researchers found that low-probability dementia with Lewy bodies patients who did not have the sleep disorder had findings characteristic of Alzheimer's disease.

"It can sometimes be very difficult to tell the difference between these two dementias, especially in the early stages, but we have found that only 2 to 3 percent of patients with Alzheimer's disease have a history of this sleep disorder," she explained.

She said that once the correct dementia diagnosis is made, doctors would then be able to give their patients the right kinds of drugs to treat their condition.

"When there is greater certainty in the diagnosis, we can treat patients accordingly," Murray said.  "Dementia with Lewy bodies patients who lack Alzheimer's-like atrophy on an MRI scan are more likely to respond to therapy - certain classes of drugs - than those who have some Alzheimer's pathology."