Lifetime risks of developing Alzheimer's disease dementia vary considerably by age, gender and whether any signs or symptoms of dementia are present, according to a new study published online by Alzheimer's & Dementia.

According to the authors, these are the first-lifetime risk estimates for Alzheimer's that take into account what are believed to be biological changes in the brain that occur 10 to 20 years before the well-known memory and thinking symptoms appear.

These early changes, before overt clinical symptoms, are referred to as preclinical Alzheimer's disease. This designation is currently only for research use until more scientific evidence is produced to determine if it can accurately predict the progression of symptoms.

The prevalence of this research-only stage of the Alzheimer's continuum, known as preclinical Alzheimer's disease, in the U.S. has been estimated at nearly 47 million people in a previous study.

An example from this newly published report is of a 70-year-old male who has just amyloid, but no signs of neurodegeneration and no memory loss has a lifetime risk of 19.9%. But, if he also had neurodegeneration in addition to amyloid, the lifetime risk rises to 31.3%. In case of mild cognitive impairment (MCI) plus amyloid plus neurodegeneration, the risk increases to 86%.

Symptoms of Alzheimer's disease.

"What we found in this research is that people with preclinical Alzheimer's disease dementia may never experience any clinical symptoms during their lifetimes because of its long and variable preclinical period," said Ron Brookmeyer, Ph.D., from the UCLA School of Public Health, Los Angeles. "The high mortality rates in elderly populations are also an important factor as individuals are likely to die of other causes."

That same 65-year-old female with amyloid plaques has a 10-year risk of Alzheimer's disease dementia of 2.5%. Lifetime risks for females are generally higher than males because they live longer.

Brookmeyer and his co-author Nada Abdalla, M.S., also of UCLA, state that the lifetime and 10-year risks provide an indication of the potential that someone will develop Alzheimer's disease dementia based on their age and screenings for amyloid deposits, neurodegeneration, and the presence or absence of MCI or any combination of those three.

"Just as there are risk predictors for whether you might have a heart attack, it will be important in the future to measure the likelihood that someone will develop Alzheimer's disease," said Maria Carrillo, Ph.D., Alzheimer's Association chief science officer.

"In the future, when treatments are available, this would be helpful, especially for people in the stages before the clinical symptoms appear. For example, those people with the highest 10-year risk of getting Alzheimer's dementia would be a high priority to volunteer for clinical trials evaluating Alzheimer's medications or other therapies" said Maria Carrillo.