Scientists may have discovered a new subtle warning sign of dementia - and it affects one million Britons

The movement disorder typically causes involuntary shaking in the arm and hand, but it can also extend to the head, jaw and voice.

Many people confuse essential tremors with Parkinson's disease, but few have drawn a link to dementia.

Researchers sought to investigate whether there was a link between the two. They recruited 222 people with essential tremor who had an average age of 79 for the study.

The participants took thinking and memory tests to determine whether they had normal cognitive skills, mild cognitive impairment or dementia at the start of the study.

Participants then had follow-up exams every 1.5 years for an average of five years.

Of this group, 168 people had normal cognitive skills, 35 had mild cognitive impairment and 19 had dementia at the start of the study.

During the study, 59 developed mild cognitive impairment and 41 developed dementia.

Researchers then compared the rate at which people developed mild cognitive impairment and dementia, as well as the prevalence of these conditions, to the rates and prevalence for the general population. They also compared participants to the rates and prevalence for people with Parkinson’s disease.

Researchers found 19 percent of participants had or developed dementia during the study, and each year an average of 12 percent of people diagnosed with mild cognitive impairment went on to develop dementia.

These rates were three times higher than the rates in the general population. However, the rates were lower than those seen in people with Parkinson’s disease, a population for which dementia is more prevalent.

Researchers also found that 27 percent of participants had or developed mild cognitive impairment during the study, a rate almost double that of the rate of 14.5 percent for the general population, but less than the rate of 40 percent for people with Parkinson’s disease.

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“While many people living with essential tremor experience mild tremor, in some individuals, the tremor can be quite severe,” said study author Elan D. Louis, MD, MSc, at University of Texas Southwestern Medical Center in Dallas and a Fellow of the American Academy of Neurology.

“Not only do tremors affect a person’s ability to complete daily tasks such writing and eating, our study suggests that people with essential tremor also have an increased risk of developing dementia.”

Louis added: "While the majority of people with essential tremor will not develop dementia, our findings provide the basis for physicians to educate people with essential tremor and their families about the heightened risk, and any potential life changes likely to accompany this diagnosis."

A limitation of the study was that the comparison data for the general population was published prior to the start of the study.