PARIS—The prevalence of mild cognitive impairment (MCI) among older adults in the US and Western Europe ranges from about 10 percent to 20 percent, new research suggests.
That's just one finding from a special session on MCI incidence and prevalence as well as risk factors for progression that was held here at the Alzheimer's Association International Conference 2011 in July.
The global perspective — offered for the first time at the conference — includes data from six countries: the United States, Australia, Germany, the United Kingdom, Sweden, and France. [For more on the data from the individual countries, see “MCI Around the Globe.”]
The prevalence of MCI — at 35 percent — in Australia was higher than in the other countries studied. But that's probably because the researchers used a less stringent definition of the condition, said Ronald C. Petersen, MD, PhD, professor of neurology, Cora Kanow Professor of Alzheimer's Disease Research, and director of the Mayo Alzheimer's Disease Research Center at Mayo Clinic College of Medicine in Rochester, MN. Dr. Petersen worked on the US study and was a co-moderator of the session.
“In the US, we used the Mayo definition, which looks for abnormalities on two or three tests in a cognitive domain — memory, language, executive function, and visual skills; there are two tests in each domain, except memory, which has three tests. The Australian definition employed an abnormality on a single cognitive test,” he said.
If the Mayo definition had been applied in Australia, the prevalence would be closer to 10 percent, he said.
The other four countries all used a version of the Mayo definition, although there was some variation in which tests and how many tests were employed, Dr. Petersen added.
There was less agreement on the rates of progression from MCI to dementia among the countries studied, with incidence rates ranging from just over 10 per 1,000 patient-years in Sweden to over 100 per 1,000 patient-years in Australia.
Noting that in the US study, the median follow-up time was 3.5 years, Dr. Petersen said, “Based on other longitudinal data, we would expect about 80 percent to convert from MCI to dementia if they were followed longer, for 10 years.”
Laura Fratiglioni, MD, PhD, professor in the Aging Research Center at the Karolinska Institute in Stockholm, Sweden and presenter of the Swedish study, said: “When we present data on cognitive impairment, we always think it is an underestimate because people die and are not captured at follow-up.”
There was more heterogeneity among countries when it came to risk factors, said session co-moderator Henry Brodaty, MD, DSc, professor of aging and mental health and director of the Dementia Collaborative Research Centre at the University of New South Wales in Sydney, and director of Aged Care Psychiatry and head of the Memory Disorders Clinic at Prince of Wales Hospital.
While the studies didn't look at all the same measures, risk factors for incident MCI included older age, vascular conditions, depression, diabetes, stroke, higher body-mass index (BMI), comorbidities, and the presence of the apolipoprotein e4 (APOE4) allele.
The risk factors for progressing from MCI to Alzheimer disease were similar, Dr. Petersen said.
“There were about 30,000 people across the studies and yet there were more similarities than differences. It could have been pure chaos,” he said.
Importantly, many of the risk factors are amenable to modification, Dr. Brodaty said.