The study of 2,212 African-American residents of Indianapolis, Ind., found that 23 percent had cognitive impairment that did not reach the level of dementia. Of those, 26 percent had developed Alzheimer's disease or other dementia within 18 months.
In the study, the rate of cognitive impairment increased with age. Of those ages 65 to 74, 19 percent were affected. The rate was 28 percent for those ages 75 to 84 and 38 percent for those 85 and older.
Occasionally forgetting where your glasses are doesn't mean you have cognitive impairment, according to study author Frederick Unverzagt, PhD, of Indiana University School of Medicine.
"Many people of all ages feel their memory is not what it should be, and that's not necessarily a cause for concern," he said. "Our own judgment of our memory is generally not very accurate. When a relative or someone else notices significant changes, that is an important sign that you should be seeking medical attention."
The people with cognitive impairment had fewer years of education than the people with normal cognitive functioning, but approximately the same amount as the people with dementia.
There was no medical cause for the cognitive impairment in the majority of cases. Other causes included impairment associated with a medical illness, stroke or alcohol abuse.
The study consisted of several phases. First in-home screenings were conducted with all participants and a spouse or relative who knew the participant well, and they were rated as having a high, medium or low probability of having dementia.
Next additional tests were performed on all of the people in the high probability group, half of the people in the medium probability group and five percent of the people in the low probability group. Those participants were then diagnosed as having normal cognitive functioning, cognitive impairment or dementia.
Of the 105 people diagnosed with cognitive impairment, 66 were re-examined after 18 months. Of those, 26 percent were diagnosed with Alzheimer's disease or other dementia. Fifty percent continued to have the diagnosis of cognitive impairment. Another 24 percent went from having cognitive impairment to having normal cognitive functioning.
"We're not sure what causes this phenomenon," Unverzagt said. "We looked at whether people whose cognitive impairment was caused in part by depression or a serious medical illness returned to normal cognitive functioning when their depression or illness resolved, but that was not the case. We think that, at least for some people, this is a transient state that can revert to normal."
Unverzagt said more research is needed on the risk factors for developing cognitive impairment and for its progression into dementia. "We also need to look at treatments for cognitive impairment," he said. "Can you take the therapies for Alzheimer's and apply them in this early stage? Are they still effective? Are they more effective? These are some of the questions we need to look at."
The study was supported by grants from the National Institute on Aging and the Alzheimer's Association.
The American Academy of Neurology recently released clinical practice guidelines regarding the early detection of dementia and evaluation and monitoring of people with mild cognitive impairment. The practice guidelines can be found at www.aan.com.
The American Academy of Neurology, an association of more than 17,700 neurologists and neuroscience professionals, is dedicated to improving patient care through education and research.