Epidemiology of age-related macular degeneration

Prevalence and incidence

Numerous population-based studies of age-related macular degeneration (AMD) have been reported around the world. AMD is the leading cause of irreversible blindness of the elderly worldwide. The reported prevalence of AMD, which varies regions, populations, and disease definitions, is constantly updating. In 2014, a systemic review and meta-analysis established worldwide prevalence and projected the number of people with AMD from 2020 to 2040. The pooled global prevalence of early and late-stage disease in adult populations was 8.01% (3.95%–15.49%) and 0.37% (0.18%–0.77%), respectively. The overall prevalence of any AMD was 8.69% (4.26%–17.40%). It has been realized that the prevalence of AMD varies in different ethnicities. Any AMD was more prevalent in populations of European ancestry than Asian (12.3% vs. 7.4%; Bayes factor 4.3, suggesting moderate evidence); and European ancestry had a higher prevalence than African ancestry for early, late, or any AMD. Particularly, the comparison of prevalence of late AMD between the people with European and African ancestry was 12.3% versus 7.5%; Bayes factor 31.3, suggesting very strong evidence. Subgroup analysis (8 of the 39 studies with information on geographic atrophy (GA) and neovascular subtypes) showed that Europeans had a higher prevalence of GA (1.11%, 0.53%–2.08%) than Africans (0.14%, 0.04%–0.45%), Asians (0.21%, 0.04%–0.87%), and Hispanics (0.16%, 0.05%–0.46%). There was no significant difference in the prevalence of neovascular AMD between ethnicities. In 2013, Bourne et al. reported that the proportion of blindness caused by AMD worldwide was higher in women than in men. Contrary to this previous report, the recent meta-analysis did not show evidence of gender difference in both early and late AMD prevalence. This finding is consistent with other reviews in people of European ancestry, where no significant gender difference was found in the prevalence of neovascular AMD and GA. Wong et al. by using UN World Population Prospects, assessed differences by ethnicity, region, and sex, and projected the individuals affected worldwide by the condition in 2020 and 2040. The projected number of people with AMD is around 196 million in 2020, and increasing to 288 million in 2040 ( Fig. 2.1 ). The prevalence of AMD at these given time points helps assess the socioeconomic burden and distribute the resource for management.

On the other hand, the incidence of AMD at a certain period helps examine risk factors for AMD and genetic and environmental interactions in the etiology of AMD. It may be exemplified by the Beaver Dam Eye Study, which is a population-based longitudinal cohort study of residents of Beaver Dam, Wisconsin, aged 43–84 years in 1987–1988. A 15-year cumulative incidence of both early and late AMD was reported in the Beaver Dam Eye Study population with different age groups. The high incidence of early AMD (24%) and late AMD (8%), respectively, was observed in those who were >75 years age at baseline, indicating advanced age is a key risk factor for both early and late AMD ( Fig. 2.2 ).

Based on the data from the longitudinal cohort Beaver Dam Eye Study, the change of incidence of every 5-year interval was studied. This cohort study was designed to have four examination visits, 5 years apart in 1988–90, 1993–95, 1998–2000, and 2003–05. While controlling for age, smoking, blood pressure, and other related factors, the 5-year incidence of AMD was 60% declined for each successive generation. The decline in the incidence across three generations suggests that environmental and/or behavioral factors are significant risk factors in the etiology of AMD because rapid genetic changes are unlikely.