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J. F. Leone, K. A. Rose, A. Kifley, M. Cosstick, P. Mitchell, Sydney Childhood Eye Study, Sydney Myopia Study; Prevalence of Heterophoria and Its Association With Refractive Error in Australian School Children. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4841.
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To examine the prevalence of heterophoria and its relationship with refractive error in population-based random samples of 6- and 12-year old school children.
The Sydney Myopia Study randomly selected 55 primary and secondary schools, stratified by socio-economic status. All Year 1 and Year 7 students were invited to participate. Cycloplegic autorefraction, cover/uncover, alternate cover test and prism bar cover tests at near (33cm) and distance (6m) fixation were performed. Myopia was defined as spherical equivalent ≤-0.5D; emmetropia >-0.5 to <+0.5; mild hypermetropia ≥+0.5 to <+2.00; and significant hypermetropia as ≥+2.00 in at least one eye.
Of the 4107 students who participated, 1692 (mean age 6.7 years) and 2289 (mean age 12.7 years) who had no strabismus or vertical phoria were included in this analysis. For near, exophoria was highly prevalent (Year 1: 58.3% vs. 9.2% for esophoria; Year 7: 52.2% vs. 10.4% for esophoria). For distance, there was a trend of reducing exophoria prevalence with increasing age (Year 1: 13.5%, CI 10.8-16.2%; Year 7: 7.8%, CI 4.3-11.2%) with very low rates of esophoria (Year 1: 1.0%; Year 7: 1.3%) and orthophoria the norm (Year 1: 85.4%; Year 7: 90.9%). There was a significant association between near phoria and refractive error in both the Year 1 (p=0.0296) and Year 7 students (p<0.0001). Children with hyperopia were more likely than children without significant refractive error to be esophoric at near (Year 1: OR 1.7, CI 1.1-2.8; Year 7: OR 2.9, CI 1.7-4.8). At age 12, children with myopia were more likely than children without significant refractive error to be exophoric (near: OR 2.1, CI 1.5-2.7; distance: OR 3.1, CI 2.1-4.4). It was rare for children to have both myopia and esophoria at near (Year 1: 0.06%; Year 7: 0.6%).
Consistent with other studies, we found that esophoria for near was rare. This finding has implications for anti-myopia therapy using progressive addition lenses. While orthophoria has more typically been found at near in studies of comparable samples (aged 5-13 years), we found a high prevalence of exophoria at near. Differences in detailed methodology may provide a partial explanation.
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