Age-related decreases in the prevalence of myopia: Longitudinal change or cohort effect?

PURPOSE. The prevalence of myopia shows a decline with age in cross-section al studies. This pattern may represent an increase in the prevalence of myo pia in younger generations, possibly through increased exposure to near wor k, or an intrinsic age-related decline in myopia prevalence. Data were anal yzed from published studies to determine which of these two alternatives be tter explains the data: a cohort effect of changing prevalence by decade or a longitudinal effect of changing prevalence as a function of age. METHODS. Prevalence data were taken from three studies conducted in the lat e 1980s and compared with those obtained indirectly from a national survey conducted in the early 1970s. The prevalence of myopia was then plotted as a function of age and year of birth. RESULTS. The pattern of change in the prevalence of myopia as a function of age was consistent across all studies when data were scaled relative to th e prevalence of myopia at age-range midpoints from 44.5 to 49.5 years. The pattern of change was not consistent as a function of year of birth. When t he data were scaled relative to the prevalence of myopia among those with y ears of birth from 1940 to 1942 and plotted by year of birth, results from the early 1970s were offset from those of later studies by approximately 18 years. CONCLUSIONS. The decline in the prevalence of myopia in older adults betwee n the early 1970s and the late 1980s can be better explained by age than by year of birth. The prevalence of myopia appears to decrease because of an intrinsic age-related decrease in the amount of an individual's myopia rath er than because of a cohort effect of increasing prevalence over time. The hypothesis that increasing environmental exposures to near work in recent d ecades have changed the prevalence of myopia is not supported by this analy sis.