Early Astigmatism Can Alter Myopia Development in Chickens

Abstract

Purpose To determine the effects of optically imposed astigmatism on myopia development in chickens. Methods Chicks were randomly assigned to wear either spherical (−10D, “LIM”, n = 14) or sphero-cylindrical lenses ( n ≥ 19 in each group) monocularly for a week from 5 days of age. All lenses imposed the same magnitude of spherical-equivalent hyperopic defocus (−10D), with the two astigmatic magnitudes (−8D or −4D) and four axes (45°, 90°, 135°, or 180°) altered to simulate four subtypes of clinical astigmatism. At the end of the treatment, refractive state was measured for all birds, whereas ocular axial dimensions and corneal curvature were measured for subsets of birds. Results Sphero-cylindrical lens wear produced significant impacts on nearly all refractive parameters ( P < 0.001), resulting in myopic-astigmatic errors in the treated eyes. Compared to LIM, the presence of astigmatic blur induced lower myopic error (all except L180 group, P < 0.001) but with higher refractive astigmatism (all P < 0.001) in birds treated with sphero-cylindrical lenses. Distributions of the refractive, axial, and corneal shape parameters in the sphero-cylindrical lens-wear groups indicated that the astigmatic blur had directed the eye growth toward the least hyperopic image plane, with against-the-rule (ATR) and with-the-rule (WTR) astigmatisms typically inducing differential biometric changes. Conclusions The presence of early astigmatism predictably altered myopia development in chicks. Furthermore, the differential effects of WTR and ATR astigmatisms on anterior and posterior segment changes suggest that the eye growth mechanism is sensitive to the optical properties of astigmatism.

Experimentally induced myopia and myopic astigmatism alter retinal electrophysiology in chickens

“…When mfERG responses were compared across three time points in a group of chicks made myopic by full-field form deprivation, noticeable timing differences in mfERG waveform were also noted 16 . It should be noted that in addition to an elongated eyeball in our sphero-cylindrical lens treated chicks, they also developed a significantly higher magnitude of astigmatism compared to -10D lens (in particular the WTR treated group, Table 1 ), in agreement with the refractive and axial changes using sphero-cylindrical lenses with different magnitudes and axes in a separate study 17 . Because earlier studies usually used spherical equivalent to represent refractive change and changes in J0 and J45 astigmatic components were not available, it is difficult to directly compare the results between Schmid et al (2013) with the current study.…”

“…Using sphero-cylindrical lenses to impose hyperopic astigmatism, which is commonly found in the clinic, we recently reported the differential effects of astigmatic subtypes on myopia progression 17 . In the current study, it was investigated whether and how induced myopic-astigmatism influenced retinal functions, using a global flash mfERG protocol, assisted by a confocal scanning laser ophthalmoscope (cSLO).…”

Bi-directional Refractive Compensation for With-the-Rule and Against-the-Rule Astigmatism in Young Adults

“…As shown in animal studies, the orientation of the imposed astigmatism is a factor influencing the magnitude of compensation, but the induced astigmatism in chicks was higher when rearing with WTR than with ATR astigmatic defocus, which is opposite to the trend observed in the current study. 25 – 27 However, it should be noted that the J0 astigmatism compensation, in terms of magnitude, was not significantly different between the two astigmatism conditions (Paired t -test, paired t (18) = 0.720, P = 0.481), so the effect of orientation on the rate of compensation cannot be concluded in the current study. To date, only one study has reported short-term, posterior anatomic change in human eyes after participants were exposed to monocular astigmatic blurs for 60 minutes.…”

“…Nevertheless, the contribution of internal astigmatism, mainly originating from the crystalline lenses and partially from the posterior corneal toricity, should not be neglected. The disappearance of infantile astigmatism in human eyes 8 and refractive astigmatism development reported in animal eyes 26 , 27 were both shown to be accompanied by changes in corneal and internal astigmatism, suggesting the involvement of the cornea and crystalline lenses in astigmatism compensation.…”

“… 23 Specifically, by optically imposing astigmatism of different orientations using cylindrical (plano- or crossed-cylinders) or sphero-cylindrical lenses, investigators have induced different degrees of compensatory refractive astigmatism with a specific axis counterbalancing the imposed astigmatism from various settings. 24 – 27 This cylindrical-lens induced refractive compensation is primarily attributed to corneal changes, 26 although the contributions of lenticular change and the relative alignment of refractive components with respect to the visual axis cannot be excluded. However, other studies using similar toric lenses failed to replicate the results in chicks 28 , 29 and monkeys, 30 indicating either methodological differences or idiosyncratic operational characteristics of the vision-dependent eye growth mechanism.…”