Objectives: To examine familial relationships for myopia among siblings and the distribution of myopia in an adult population aged 23 to 78 years.
Methods and participants: Eye examinations were conducted in a cross-sectional study on the offspring of the Framingham Eye Study cohort. Between 1989 and 1991, 1585 persons, the offspring of 1319 parents, underwent refraction and were tested for best corrected visual acuity. A person was considered myopic if at least one eye had a spherical equivalent refractive error of at least -1.00 diopter.
Results: For both sexes, prevalence of myopia decreased with increasing age, from about 60% for ages 23 to 34 years to about 20% for ages 65 years and older. Among men, both age and years of education were related to myopia. Among women, age was related to myopia, but years of education were not. Age-, sex-, and education-adjusted analyses, using the second-order generalized estimating equations approach, showed a strong association of myopia between siblings. The association between siblings decreased with increasing between-siblings age difference: the estimated odds ratio was about 5 for an age difference of 2 years and half that for an age difference of 10 years.
Conclusions: The relative importance of genetic and environmental factors in explaining familial relationships for myopia remains unclear. Nevertheless, the large increase in myopia prevalence for more recent birth cohorts and the decrease in the strength of the between-siblings association for myopia with increasing between-siblings age difference suggest an important role for environmental factors in the cause of myopia.