Effects of astigmatism on myopia development and retinal electrophysiology in chicken

Abstract

Purpose: During infancy, visual experience plays a significant role in modulating ocular growth. Although significant magnitude of astigmatism (≥1.0D) at birth has been reported and frequently co-exists with myopia, its role in myopia development remains unclear. Therefore, the main purposes of the thesis were: (1) To investigate whether and how astigmatic properties (axis and magnitude) interfere with myopia development using chicks as an animal model; and (2) to investigate whether and how experimentally induced myopic-astigmatism modulates retinal electrophysiology using an image-guided multifocal global flash stimulation. Methods: Experiment 1: One hundred ninety-three White Leghorn chicks (Gallus gallus domesticus) were randomly assigned to wear either spherical (-10D, n=14) or sphero-cylindrical lenses (low magnitude: -8DS/-4DC and high magnitude: -6DS/-8DC; n= 179) monocularly for 7 days (Post-hatch days 5-12, P5-P12). The lenses are fixed on the right eyes using Velcro mount at a vertex distance of 5mm. All lenses imposed the same magnitude of spherical-equivalent hyperopic defocus (-10D), with the two astigmatic magnitude (high: -8D or low: -4D) and 4 axes (45°, 90°, 135°, or 180°) of the sphero-cylindrical lenses altered to simulate four subtypes of clinical astigmatism. Refractive status was measured for all the chicks after 7 days of the lens treatment, whereas A-scan ultrasonography and videokeratography were performed for subsets of chicks in each of the nine treatment groups. Experiment 2: A separate set of fifty White Leghorn chicks were randomly assigned to the following four groups: (1) aged-matched normal birds (No lens treatment, normal group, n=12); (2) negative spherical lens-wear (-10D, n=12); (3) sphero-cylindrical lens wear, axis oriented vertically; with the rule, WTR (H-WTR = -6.00DS/-8.00DCx90, n = 15); and (4) sphero-cylindrical lens wear, axis oriented horizontally; against the rule, ATR (H-ATR = -6.00DS/-8.00DCx180, n = 11) for 7 days (P5 to P12). All lens treatments were imposed on the right eyes at the vertex distance of 5mm using Velcro mount. A-scan ultrasonography was performed on P11 whereas refractive status and global flash multifocal electroretinogram (MOFO mfERG) recording were measured on P12. Results Experiment 1: 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 -10D, 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 chicks 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 towards the least-hyperopic image plane. In addition, differential impacts of WTR and ATR astigmatism on effective ametropia and astigmatism were observed. Experiment 2: Multifocal electrophysiological results revealed significant regional variation in the amplitude of induced component (IC) (central greater than peripheral; both, p<0.05) in the normal and H-ATR groups, but not in the -10D and H-WTR groups. Compared to the normal and -10D groups, both hyperopic-astigmatic groups exhibited a significantly longer implicit time of the inner retinal response at the central region (both, p<0.05). In addition, the magnitude of refractive astigmatism was associated with IC amplitude for both H-WTR (r= 0.53; p<0.05) and H-ATR (r= - 0.66; p<0.05) groups. There were no significant differences in amplitudes of direct component (DC) across all groups. Conclusions (1) The presence of early astigmatism altered refractive development in chicks. Furthermore, the differential effects of WTR and ATR astigmatisms on anterior and posterior segments suggest that the eye growth control mechanism is sensitive to the optical properties of astigmatism. (2) Myopic-astigmatic eye growth was associated with a delayed central inner retinal response, suggesting an alteration at the level of pre-synaptic transmission or inner plexiform activity. Overall the effects of ATR and WTR astigmatism on eye growth (Experiment 1) and retinal electrophysiology (Experiment 2) indicate a significant role of astigmatism in myopia development.