26 May 2012. Dementia is thought of as a scourge of the developed world, with its aging population, but it actually may occur at an even higher rate in developing countries, or so, at least, report researchers led by Martin Prince at King’s College London, U.K., in the May 23 Lancet. A shift in these countries’ populations toward aging, combined with low education and undercounting, is blamed for the finding.
The authors are part of the 10/66 Dementia Research Group, which conducts aging studies in 20 low- or middle-income countries in Latin America, Asia, and the Caribbean. The group’s name refers to the fact that two-thirds of dementia cases occur in the developing world, but only 10 percent of the research is done there. Previously, the group reported that the prevalence of dementia in elderly people in seven middle-income countries approached Europe’s dementia numbers (see ARF related news story). The new study now adds incidence data from the same populations, reporting higher rates than those seen in Europe. The findings imply that dementia may be an even bigger problem worldwide than researchers have thought. Intriguingly, the authors also find that socioeconomic factors such as a good education and literacy lower dementia risk in middle-income countries just as they do in high-income countries. This supports the idea that cognitive reserve protects the brain in diverse cultures.
In contrast to the new data, several earlier, smaller studies reported lower prevalence and incidence of dementia in less developed countries than in the U.S. and Europe (see, e.g., ARF related news story; Hendrie et al., 2001; Chandra et al., 2001; Li et al., 1991). The 10/66 group contends, however, that the commonly used diagnostic criteria from the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV) are too restrictive to recognize all dementia cases, particularly in rural sites. DSM-IV dementia cases must meet four qualifying criteria, including a decline in social or occupational functioning, which in some cultures is masked by high levels of social support provided to the elderly, or less demands placed upon them. The 10/66 group developed its own diagnostic algorithm, which relies on cognitive test scores and informant reports as well as clinical interviews and neurological examinations. The 10/66 algorithm detects dementia with 94 percent sensitivity (see Prince et al., 2003), and has been validated cross-culturally and prospectively (see Jotheeswaran et al., 2010 and Prince et al., 2008), the authors claim.
Prince and colleagues used the 10/66 algorithm in their previous prevalence study. They interviewed all people over age 65 in specific urban areas in Cuba, the Dominican Republic, and Venezuela, and in both an urban and a rural area in China, Mexico, Peru, and India, for a total of more than 12,000 participants (for protocol, see Prince et al., 2007). The dementia prevalence numbers were not far below those seen in the EURODEM study of dementia in Europe (see Hofman et al., 1991). In many parts of the world, cultural issues can lead to under-recognition of dementia, particularly in lower socioeconomic strata, noted Denis Evans at Rush University, Chicago, Illinois. He was not involved in the study. For example, people in lower strata may have less access to healthcare and therefore go undiagnosed. They may also be in occupations that are less cognitively demanding, allowing mild dementia to pass unnoticed. By going door-to-door and interviewing everyone within a given community to make an independent ascertainment of dementia, the 10/66 authors avoided biases that might be present in healthcare records, Evans said.
Because prevalence data reflect a snapshot in time, variables such as survival affect the numbers, i.e., dementia may kill people too quickly for them to be counted in a prevalence study. An incidence study instead looks at how many healthy people develop a disease over a given period of time, usually a year, and provides a clearer picture of the true frequency of the disease. To determine incidence, the authors re-interviewed more than 8,000 of the participants who did not have dementia at baseline, from all areas except India, about four years later. They assessed for dementia using both the 10/66 and DSM-IV criteria. With the former, dementia incidence varied from 1.8 to 3 percent of the over-65 population per year; this is higher than the 1.8 percent seen in the EURODEM study using DSM-III criteria (see Launer et al., 1999). When adjusted for the different age distribution of the European population, the incidence in these developing countries was even higher—2 to 5 percent per year.
Notably, only half of the 10/66 dementia cases in this new study met DSM-IV criteria. The 10/66 criteria may pick up milder cases that the DSM-IV misses, the authors suggest. They point out that people who met 10/66 criteria for dementia at baseline developed DSM-IV dementia at high rates, as much as 25 percent per year in some areas.
Consistent with studies in the developed world, the incidence of dementia increased with age and was higher in women. Higher levels of education were consistently associated with lower incidence. Likewise, high verbal fluency, performance on a motor-sequencing task, and a high number of household assets—a surrogate marker for wealth—correlated with lower rates. The researchers conclude that their findings support the hypothesis of cognitive reserve, which proposes that early development of complex neural networks helps the brain resist the effects of pathology later in life (see, e.g., ARF related news story and ARF news story).
The protective effect of education and other socioeconomic factors—which is established in developed countries (see, e.g., ARF related news story and ARF news story)—is one of the most interesting findings of the study and emphasizes environmental influences on dementia, said Eric Larson at Group Health Research Institute, Seattle, Washington. “The brain is subject to early developmental forces that show themselves in late life,” he noted. A study on the rural Chinese island of Kinmen, where the median level of education is one year, showed dementia rates rising as people turned 60. This is earlier than in developed nations (see Liu et al., 1998), and implies that illiteracy and lack of education can bring on dementia sooner, Larson said. He added that the next question for science to answer is how the protective effect of education works.
It could be education per se or other factors that correlate with it. In an accompanying commentary, Kathleen Hall and Hugh Hendrie at the Indiana University School of Medicine, Indianapolis, write: “Low education might simply be a marker for socioeconomic deprivation including poverty, malnutrition, toxic environmental exposures, and institutionalized racial segregation and its consequences…. Low education has been reported as a risk factor for poor health outcomes in many illnesses, not only brain disease, throughout the world.” Evans agreed, telling Alzforum, “Education is probably the best measure of social stratification that we have.”
The study implies that dementia may be an even bigger problem worldwide than has been thought. In many developing countries, too, not just developed ones, the population pyramid is shifting toward an aging society, and this trend is projected to continue. “Age is the biggest known driver of dementia,” Evans pointed out. “That has enormous implications for the overall burden of dementia in the world. I think the growth of dementia in developing countries is going to be a very large issue in the future.”—Madolyn Bowman Rogers.
Prince M, Acosta D, Ferri CP, Guerra M, Huang Y, Llibre Rodriguez JJ, Salas A, Sosa AL, Williams JD, Dewey ME, Acosta I, Jotheeswaran AT, Liu Z. Dementia incidence and mortality in middle-income countries, and associations with indicators of cognitive reserve: a 10/66 Dementia Research Group population-based cohort study. Lancet. 2012 May 23. Abstract
Hall KS, Hendrie HC. Early childhood environment and dementia. Lancet. 2012 May 23. Abstract