Moving around, walking, talking or hitting out while asleep is the strongest predictor that a man will develop dementia with Lewy bodies – the second most common form of dementia in the elderly after Alzheimer's.
Another example would be unconsciously mimicking the action of holding a steering wheel while dreaming about driving a car.
Physically acting out dreams is a condition known as rapid eye movement (REM) sleep behaviour disorder.
Middle-aged men who acts out their dreams while asleep are five times more likely to develop dementia in old ageResearchers at the Mayo Clinic, in the U.S., found that men with the condition are five times more likely to develop this form of dementia than those who display other risk factors currently used to make the diagnosis, such as hallucinations.
REM sleep is the phase of sleep during which the most vivid dreams occur, while REM sleep behaviour disorder is the name given to the condition that causes the loss of the normal muscle paralysis that occurs during REM sleep.
This precursor can appear as much as three decades or more before a diagnosis of dementia with Lewy bodies is made, the researchers said.
This type of dementia accounts for ten per cent of all cases in the elderly.
WHAT IS REM SLEEP BEHAVIOUR DISORDER?
For most people, dreams occur purely in the mind while the body is resting.
However, people who suffer from REM sleep behaviour disorder physically act out their dreams while they are asleep.
They physically move around, walk, talk or hit out in their sleep.
It is often not diagnosed until it starts to prove a danger to the sufferer's bed partner.
Studies of animals suggest that the symptoms of REM sleep behaviour disorder may be caused by lesions on the brain stem (the area of the brain which joins the spinal cord).
The disorder can be treated quite successfully using medications such as antidepressants..
However, the link between dementia with Lewy bodies and the sleep disorder is not as strong in women, they added.
Researchers at Mayo Clinic in Minnesota and Florida examined magnetic resonance imaging (MRI) scans of the brains of 75 patients diagnosed with probable dementia with Lewy bodies.
The researchers then checked the patients' histories to see if the sleep disorder had been diagnosed in them while they were under Mayo care.
‘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 per cent of men with dementia with Lewy bodies in our Mayo database did experience REM sleep behaviour disorder,' said lead investigator Dr Melissa Murray, a neuroscientist at Mayo Clinic. 'So it is a very powerful marker for the disease.'
She added that the study's findings could improve the diagnosis of this form of dementia and that this can lead to better treatment.
‘Screening for the sleep disorder in a patient with dementia could help clinicians diagnose whether they are suffering from dementia with Lewy bodies or Alzheimer's disease,’ she says.
‘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 two to three per cent of patients with Alzheimer's disease have a history of this sleep disorder.’
Once the diagnosis of dementia with Lewy bodies is made, patients can use drugs that can treat the related cognitive issues, Dr Murray says. No cure is currently available.
Dementia with Lewy bodies affects men and women equally and is most prevalent in the over 65s.
Red areas on a brain scan are a warning of Alzheimer's disease. Green areas are healthy brain
Lewy bodies, named after the doctor who discovered them, are small spherical protein deposits found in nerve cells.
Their presence in the brain interferes with the normal functioning of the brain – it is yet to be established why they occur in the brain and how they cause damage.
Dementia with Lewy bodies is a degenerative disease with symptoms including problems with alertness, spatial disorientation, difficulty planning ahead and memory problems.
People with this form of dementia sometimes also develop symptoms including slowness, muscle stiffness, trembling of the limbs, a tendency to shuffle when walking, loss of facial expression, and changes in the strength and tone of the voice.
The research comes just after another study which found that disrupted sleep could be an early sign of Alzheimer's disease.
Researchers at Washington University found that people with early stage Alzheimer's often suffer disrupted sleep before they display other symptoms of the condition.