Clinical and Epidemiologic Research Profile of Anisometropia and Aniso-Astigmatism in Children: Prevalence and Association with Age, Ocular Biometric Measures, and Refractive Status Lisa O’Donoghue, 1 Julie F. McClelland, 1 Nicola S. Logan, 2 Alicja R. Rudnicka, 3 Chris G. Owen, 3 and Kathryn J. Saunders 1 PURPOSE. We describe the profile and associations of anisome- tropia and aniso-astigmatism in a population-based sample of children. METHODS. The Northern Ireland Childhood Errors of Refraction (NICER) study used a stratified random cluster design to recruit a representative sample of children from schools in Northern Ireland. Examinations included cycloplegic (1% cyclopento- late) autorefraction, and measures of axial length, anterior chamber depth, and corneal curvature. v 2 tests were used to assess variations in the prevalence of anisometropia and aniso- astigmatism by age group, with logistic regression used to compare odds of anisometropia and aniso-astigmatism with refractive status (myopia, emmetropia, hyperopia). The Mann- Whitney U test was used to examine interocular differences in ocular biometry. RESULTS. Data from 661 white children aged 12 to 13 years (50.5% male) and 389 white children aged 6 to 7 years (49.6% male) are presented. The prevalence of anisometro- pia ‡1 diopters sphere (DS) did not differ statistically significantly between 6- to 7-year-old (8.5%; 95% confidence interval [CI], 3.9–13.1) and 12- to 13-year-old (9.4%; 95% CI, 5.9–12.9) children. The prevalence of aniso-astigmatism ‡1 diopters cylinder (DC) did not vary statistically significantly between 6- to 7-year-old (7.7%; 95% CI, 4.3–11.2) and 12- to 13-year-old (5.6%; 95% CI, 0.5–8.1) children. Anisometropia and aniso-astigmatism were more common in 12- to 13-year- old children with hyperopia ‡ þ 2 DS. Anisometropic eyes had greater axial length asymmetry than nonanisometropic eyes. Aniso-astigmatic eyes were more asymmetric in axial length and corneal astigmatism than eyes without aniso- astigmatism. CONCLUSIONS. In this population, there is a high prevalence of axial anisometropia and corneal/axial aniso-astigmatism, associated with hyperopia, but whether these relations are causal is unclear. Further work is required to clarify the developmental mechanism behind these associations. (Invest Ophthalmol Vis Sci. 2013;54:602–608) DOI:10.1167/ iovs.12-11066 A nisometropia is an interocular difference in refraction that can be associated with significant visual problems, including aniseikonia, strabismus, and reduced stereopsis. 1–3 It also is recognized widely as an amblyogenic risk factor, 4 with a greater magnitude of anisometropia being associated with more severe amblyopia. 3,5 The prevalence of anisometropia decreases typically during the first year of life. 6,7 Beyond this early period, several studies have reported that prevalence is relatively stable in early 8–10 and late childhood. 10,11 Other studies have reported an increase in prevalence throughout childhood associated with myopia 12,13 and hyperopia. 12 Describing the prevalence of anisometropia in childhood is complicated further as it may be population-specific, with reports of a prevalence (‡1.00 diopters sphere [DS]) of 1.6% in 6-year-old children in Australia, 14 6.7% in 4- to 13-year-old American Indians, 11 and 9.9% in 7- to 18-year-olds in Taiwan. 15 Whether these differences are genetic or geographic in nature, or merely reflect the different ages of the children or the differing prevalence of myopia and hyperopia in these populations is unclear. Few studies have examined interocular differences in astigmatism (aniso-astigmatism), but of those that have, the prevalence of aniso-astigmatism (‡1.00 diopters cylinder [DC]) also varies widely from 1% in 6-year-old children in Australia 14 and 3.3% in 6- to 11-year-olds in Japan 16 to 15% in 4- to 13-year- old American Indian children where there also is a high prevalence of astigmatism and hyperopia. 11 Little is known about either the prevalence of anisometro- pia or aniso-astigmatism in childhood in Northern Ireland, where there is a high prevalence of refractive errors (myopia, hyperopia, and astigmatism) compared to similarly-aged white children elsewhere. 17–20 Understanding the prevalence of anisometropia and aniso-astigmatism, and their associations with refraction and age may be useful in designing programs to prevent amblyopia, and assist in deciding when and how to implement such programs. 15 This study describes the prevalence of anisometropia and aniso-astigmatism in 6- to 7-year-old and 12- to 13-year-old white children in Northern Ireland, UK, and explores the associations with refractive error and ocular biometric parameters. The relation between binocular vision status and anisometropia is also explored. From the 1 Vision Science Research Group, School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland, United Kingdom; the 2 Ophthalmic Research Group, Aston University, Birmingham, United Kingdom; and the 3 Division of Population Health Sciences and Education, St. George’s, University of London, London, United Kingdom. Supported by a research scholarship from the College of Optometrists, London, United Kingdom (the NICER study). Submitted for publication October 2, 2012; revised November 29, 2012; accepted December 5, 2012. Disclosure: L. O’Donoghue, None; J.F. McClelland, None; N.S. Logan, None; A.R. Rudnicka, None; C.G. Owen, None; K.J. Saunders, None Corresponding author: Lisa O’Donoghue, Vision Science Re- search Group, School of Biomedical Sciences, University of Ulster, Cromore Road, Coleraine, Northern Ireland, BT52 1SA, UK; l.odonoghue@ulster.ac.uk. 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