Prevalence of refractive errors and incidence of myopia in Swedish schoolchildren

Abstract: PurposeThe aim of this thesis was to investigate the prevalence, incidence, and risk factors for myopia development in a cohort of Swedish schoolchildren. The specific goals were: - to recruit a cohort of children aged 8 to 16 years and to follow the cohort over a period of 24 months with regular study visits; - to investigate the predictive value of relative peripheral error and other optical parameters for the prevalence and incidence of myopia; - to determine and investigate the predictive value of genetic and environmental factors, and structural characteristics of the eye to the incidence of myopia.MethodsThis was a longitudinal study with a follow-up period of 2-years conducted at Linnaeus University in Kalmar. All participants underwent eye examinations and completed questionnaires at regular intervals during the follow-up period. Data analysis was performed assuming hypotheses such as: - myopia development is associated with genetic factors and environmental factors (Papers I and IV); - changes in refractive error over time are explained by multiple genetic and environmental factors (Paper IV) - instruments with different measurement principles can lead to different refraction results (Paper II); - there is a relationship between refractive error and the characteristics of the choroid (Exploratory study, not published) and the characteristics of the microvasculature of the retina (Paper III).ResultsA total of 128 children (70 females and 58 males) participated in this study with mean age of 12.0 years (SD=2.4). Paper I: based on cycloplegic SER of the right eye, the distribution of refractive errors was: hyperopia 48.0% (CI95=38.8-56.7), emmetropia 42.0% (CI95=33.5-51.2) and myopia 10.0%. (CI95=4.4-14.9). Participants with two myopic parents had higher myopia and increased axial length than those with one or no myopic parents. Paper II: The Shin-Nippon was 0.30 D more hyperopic than COAS-HD VR at 2.5-mm pupil and 0.50 D more hyperopic than COAS-HD VR at 5-mm pupil for central refraction. Paper III: The sample included 86 out of the 128 participants, 51 (59%) females. The area of the foveal avascular zone (AFAZ) was correlated with central vessel density, perfusion, foveal thickness and with fovea-to-macula thickness ratio. Paper IV: The cumulative incidence of myopia during the two-years was 5.5%, incidence rate of myopia was 3.2 cases per 100 person-years. Cox regression revealed that the probability of myopic shift reduced with “age” and increased with “axial length/corneal-curvature ratio”. Myopic children at the baseline and children with two myopic parents showed a significant faster-paced SER change over time.ConclusionThe prevalence and incidence of myopia in Sweden was lower than expected when compared with countries in East Asia. Genetic factors such as parental myopia remains a critical factor to consider when predicting myopia onset and progression. Children born full-term and visual acuity within the normal range can have very different retinal microvasculature at the centre of the macula that may influence refractive error development. Future studies are necessary to find out possible relationships between vasculature, structural changes and refractive error development. In addition, more studies involving children from different ethnicities and incorporating longer follow-up period are necessary to increase our understanding of the incidence of myopia in Swedish schoolchildren.