What's Behind the Decline?
The US population is aging, with the proportion of Americans older than 65 years projected to increase substantially over the coming decades. In turn, these trends have led to expectations that the disease burden of AMD will see a corresponding increase, a prediction that has important implications for public health policy and planning.[14,15,16]
Of note, many of these estimates have assumed that the population-level risk for AMD will remain constant over time. However, some data indicate that this assumption may not be accurate.
In the Beaver Dam Eye Study (BDES), a longitudinal cohort study of the residents of the city of Beaver Dam, Wisconsin, patients born in later birth cohorts had a reduced 5-year incidence of early AMD compared with those born in earlier birth cohorts. Similarly, the prevalence of AMD in US adults older than 40 years in the 2005-2008 National Health and Nutrition Examination Survey (NHANES) was found to be only 6.5% compared with a prevalence of 9.4% reported in the 1988-1994 NHANES.
A 2017 study pooling data from the BDES and the related Beaver Dam Offspring Study (BOSS) further explored these trends, with a particular interest in the Baby Boomer Generation that is now rapidly entering the risk period for AMD. A total of 4819 participants without AMD at baseline examinations occurring between either 1988-1990 or 2005-2008 were subdivided into the Greatest Generation (born 1901-1924), the Silent Generation (born 1925-1945), the Baby Boomer Generation (born 1946-1964), and Generation X (born 1965-1984). The age- and sex-adjusted 5-year incidences of AMD in each of these generations, respectively, were 8.8%, 3.0%, 1.0%, and 0.3%; each generation was estimated to be 60% less likely to develop AMD than the preceding generation. These relationships were preserved even after adjusting for various environmental factors such as smoking, educational attainment, cholesterol levels, and C-reactive protein.
The study was well designed and conducted, though some limitations are notable. For example, the BDES used film-based systems for color fundus photography, while the BOSS used more modern digital systems. This difference introduces a possible confounding factor, though utilization of centralized image grading by a reading center likely mitigated most of these effects.
Additionally, this study included predominantly non-Hispanic white participants, meaning that the results may not apply to other racial or ethnic groups.
As the authors noted, if the incidence of AMD truly is decreasing over time, the cause is unlikely to be genetic given the relatively short timeframe over which these changes have occurred.
Considering the persistence of the generational effect, even with adjustment for a number of "traditional" lifestyle factors, it may very well be that yet-unidentified environmental factors are at play. These could include advancements in medicine, improvements in air quality, changing patterns of healthcare access, and other changes that have precipitated over the years.
Furthermore, the differential effects of events such as the World Wars, the Great Depression, and the Cold War on stress and emotional health, particularly for the older generations, should not be overlooked. Alas, as with similarly observed declines in cardiovascular disease risk over time, the precise drivers of this decline may never be fully untangled.
Causality aside, it is also important to consider how this trend should be interpreted. Even if absolute risk for AMD is declining, populations will continue to age both in the United States and worldwide. In Europe, even when accounting for an observed decline in AMD prevalence, the projected number of individuals with AMD is still expected to grow. Additionally, it remains unclear how rising rates of obesity and diabetes will affect these trends, and it may take years for the impact of these patterns to fully manifest at the population level. Thus, efforts to improve detection, management, and outcomes in AMD should remain a priority.
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Cite this: Is the Incidence of Age-Related Macular Degeneration Declining? - Medscape - Jan 18, 2018.