WHAT WERE THE MAIN FINDINGS?

Dr. Roberts and colleagues¹ examined 2,719 elderly residents of Olmstead County, MN, every 15 months. The examinations showed that 534 persons had mild cognitive impairment, or MCI (see About MCI, following section). Some people had MCI when they were first seen. Others developed the condition later, so their problems were discovered during follow-up exams. The doctors knew from earlier studies that many people with MCI go on to develop dementia after a few years. But other people show little change over this time, while still others seem to recover or “revert” to cognitively normal when next examined. The doctors found that almost 40% of people with MCI “reverted” in this last way (a typical result). They wanted to know whether these “reverters” differed from other cognitively normal people in their risk of dementia. Over about 5 years of observation, they found that 65% of the “reverters” did go on to develop dementia. That was about 6 times the percentage of other cognitively normal people.

WHAT SETS THESE FINDINGS APART FROM EARLIER WORK?

There aren't many reliable studies of what happens to people who “revert” from MCI to normality. Common sense suggests that these people must be different from others who are normal. We haven't been sure, though, just how different they are. This article offers important new information to answer that question. The study is especially valuable because its subjects came from a population. They weren't necessarily people who got treatment in a clinic or doctor's office. Data from clinic samples suggest what will happen over time to people who have sought medical attention for MCI. But they say little about what happens to the others (sometimes a majority) who haven't sought help. By contrast, this study describes “real” outcomes of people with MCI in a population.

Another strength of this study is that the researchers looked at medical records of people who didn't come back for their next scheduled exam. Here, the researchers had the advantage that people in Olmstead County get almost all their medical care from the Mayo Clinic. We know from other work that people who don't return for follow-up exams have more dementia than others.² So not tracking people who don't return could lead to underestimates in the rate of onset of dementia. The Mayo Clinic records told Dr. Roberts and colleagues much of what they needed to know about these people.

WHY ARE THE FINDINGS IMPORTANT?

Today, people routinely live into their 70s, 80s, or even 90s. At those ages, problems with memory and other cognitive abilities are so common they are often considered “normal for age.” Unfortunately, many of these people go on to develop dementia. Between “benign” memory problems and dementia there are “in-between” conditions that appear to be the first signs of what may become dementia. This means that many people with MCI are actually in transition to dementia. But other people may have MCI caused by different underlying illnesses. If people with MCI have different underlying illnesses, we may want to treat them differently. We would want to know especially which people do progress to dementia but instead appear normal at their next exam. As Dr. Roberts and colleagues found, this outcome of MCI is quite common. In their study, it happened about 40% of the time. When doctors see this, a natural reaction is to think that they were “fooled” earlier into thinking the person had a serious illness. But this new article suggests they weren't fooled. Instead, they observed that cognition can be variable. This should surprise no one. All people, including those with MCI, have good days and bad days. What this article shows is that people with MCI also experience this sort of fluctuation. So it seems that those who “revert” to normal are often more like people whose MCI progresses to dementia than they are like people who didn't have cognitive problems in the first place.