Alzheimer's is the most common form of dementia, affecting 5.4 million Americans and accounting for 50 to 80 percent of all cases, according to the Alzheimer's Association.
Although the majority of people with the disease are older than 65, and the average age of onset is about 75, up to 5 percent of patients have an early-onset form of the disease, according to the association.
The disease is not a normal part of aging, and it is not the only cause of memory loss. Alzheimer's is progressive and there is no known cure.
Morris advocates an emphasis on the underlying cause of the disease.
But Dr. William Thies, chief medical and scientific officer of the Alzheimer's Association, emphasizes that AD is a continuum and there are differences at the extremes.
The categorization of the continuum is "artificial," and researchers are making their "best guess" about where the worst case of mild cognitive impairment ends and the earliest onset of Alzheimer's begins.
The disease "starts long before we see symptoms outwardly," he said. The span between MCI and AD is "hugely different."
"The person lives by himself and manages his affairs, even if he's a little forgetful," Thies said. "In late state AD, the patient is incapable of doing anything for himself and needs help doing the most basic activities."
Researchers want to know where the dividing line is and to be able to support that knowledge with data. "Not all studies reflect the same answers," he said. "We need a number of studies not to tell us where the line needs to be and how wide the line. It's not a white line; it's fuzzy."
"We are trying to quantify where the line is between MCI and early dementia and where the line lies," Theis said. "It will take community-based population studies that need to be done."
The new criteria, published in 2011 in "Alzheimer's & Dementia: The Journal of the Alzheimer's Association" followed a report that brain areas affected by Alzheimer's disease start shrinking up to a decade before symptoms appear.