Adult-onset myopia: the Genes in Myopia (GEM) twin study.

PURPOSE To report the frequency of adult-onset myopia in a large cohort of Caucasian twins that were assessed as part of the Genes in Myopia (GEM) twin study and to quantify the genetic contribution in adult-onset myopia using the classic twin model. METHODS All twins aged 18 years or older were invited to participate in the GEM twin study through the Australian Twin Registry (ATR). Each twin completed a standard questionnaire and underwent a comprehensive eye assessment, including cycloplegic objective examination. Adult-onset myopia was defined as having the first spectacle/contact lens correction at the age of 18 years or older. Myopia was defined as spherical equivalent worse than or equal to -0.50 D. RESULTS A total of 1224 twins (690 monozygotic [MZ] and 534 dizygotic [DZ]) between 18 and 86 years of age were recruited into the GEM study. A total of 96 twins (96/347 = 27.7%) comprising 50 MZ and 46 DZ twins were first prescribed optical correction for myopia at the age of 18 years or older. A significantly higher MZ intrapair correlation (r = 0.61) compared with that in DZ twins (r = 0.16, P < 0.01) for spherical equivalent was found in twins with adult-onset myopia. CONCLUSIONS Adult-onset myopia is a relatively common condition, with approximately one quarter of cases occurring in adulthood. To the authors' knowledge, the GEM twin study is the first study of its kind to provide evidence to support a genetic component in adult-onset myopia.