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Kathryn Saunders, Sara Jayne McCullough, Daniel Ian Flitcroft; What can anisometropia tell us about eye growth?. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5856.
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© ARVO (1962-2015); The Authors (2016-present)
Since the two eyes of one individual share the same environment and genes, we examined interocular differences in biometry to determine the potential role of stochastic growth processes in refractive development.
Data were derived from the 6-7 year old (n=391) cohort of The Northern Ireland Childhood Errors of Refraction (NICER) Study. Measures of cycloplegic autorefraction (Shin-Nippon open-field autorefractor, SRW-5000) axial length, corneal curvature, anterior chamber depth (Zeiss IOLMaster) and presenting monocular visual acuity (VA) (Crowded LogMAR) were assessed. Refraction is reported as the spherical equivalent.
389 subjects had data available from both eyes. 267 (ISO group) showed an interocular difference of <0.50D and 122 (ANISO group) had a difference of ≥0.50D (31.20%). 14 (3.58%) had anisometropia ≥1.50D. The two groups showed different refractive distributions (Kolmogorov-Smirnov test, p=0.001) with the ISO group showing a nearly Gaussian distribution (Fig 1) and the ANISO group showing positive skew, a hyperopic shift and a bi-Gaussian distribution (Fig 2). The difference could not be attributed to the amblyogenic potential of anisometropia as the pattern remained whether the most hyperopic or least hyperopic eye was examined, and when subjects with VA >0.2logMAR in either eye were excluded. The hypothesis that anisometropes show poorer emmetropisation is supported by the double Gaussian model (Figs 1&2), with the ANISO group showing a substantial subpopulation (33.3%) with mean +3.61D (SD 2.08D). The ISO group also contained a similar subpopulation but this comprised only 6.4% with a similar mean and SD (+4.05±2.28D). A marker of emmetropisation is the poor correlation of refraction in older children with corneal curvature, due to regulated growth of axial length after corneal curvature stabilizes at around 18 months of age. In the ISO group there is no significant correlation between refraction and corneal curvature (r=0.087, p=0.156) but in the ANISO group there is a significant correlation (r=0.35, p=0.00024).
In six-year-old children, the presence of small degrees of anisometropia (≥0.50D) is associated with impaired emmetropisation. This suggests that anisometropia of this degree is a marker for poorly regulated eye growth, indicating that, in addition to environmental and genetic influences on eye growth, stochastic processes contribute to refraction.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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