Please use this identifier to cite or link to this item: http://hdl.handle.net/10397/92699
Title: Intrinsic ocular mechanisms underlie lens-induced astigmatism in chicks
Authors: Stell, WK
Popa, V
Kee, CS 
Issue Date: Sep-2016
Source: Investigative ophthalmology and visual science, Sept. 2016, v. 57, no. 12 (Abstract)
Abstract: Purpose : Ocular astigmatism is a refractive error due to differential meridional powers of ocular components, causing blurred vision at all viewing distances. The cause(s) remain poorly understood. Here we used a novel animal model of lens-induced astigmatism to test the hypothesis that processing of astigmatic images in retinal circuits causes the optical abnormality
Methods : We induced astigmatism by mounting +4.00/-8.00D crossed-cylinder lenses over the right (treated) eyes of 7-day-old chicks (P7), in groups of n=12, with the -8.00D axis oriented vertically (at 90°) or horizontally (180°); the left (fellow) eyes wore no lens. Net refractive errors of both eyes were measured by streak retinoscopy, before and after 1 week of lens-wear; in selected cases the corneal component was measured by keratometry. To test whether neuronal pathways between retina and brain are required, we injected tetrodotoxin (TTX; 7µL of 10-4M) or PBS (7µL) into the vitreous of the treated eyes on P7, P9 and P11; we assessed the efficacy and duration of action of TTX by the pupillary light reflex and optokinetic response (n=6 each). To confirm that retinal circuitry is required, we injected mixed excitotoxins (2µmol N-methyl-D-aspartate, 0.2µmol quisqualic acid, 0.2µmol kainic acid; in 20µL water) into the treated eyes of n=12 chicks at P7. Fellow eyes always received vehicle alone. Interocular differences (treated - fellow) were assessed by 2-tailed unpaired t-test, or 2-way ANOVA + Tukey’s post-test.
Results : Crossed-cylinder goggles reliably induced refractive astigmatism. Maximum astigmatic error was induced at 90°, by -8.00DC axis oriented vertically; this compensated for the imposed defocus. Treated eyes developed astigmatism after injecting TTX or PBS, but not after excitotoxins. Keratometry confirmed that the cornea itself was astigmatic.
Conclusions : In our chicks, crossed-cylinder lenses reliably induced compensatory astigmatism. Prevention of astigmatism by excitotoxins showed that the mechanism requires the retina, and not some other light-sensitive ocular tissue (e.g., iris); furthermore, the failure of TTX to affect astigmatism showed that extraretinal neural pathways are not required. We suggest that lens-induced astigmatism is due to local mechanisms of scleral growth-regulation by image defocus, plus mechanical deformation of the cornea by the distorted sclera.
Publisher: Association for Research in Vision and Ophthalmology
Journal: Investigative ophthalmology and visual science 
ISSN: 0146-0404
EISSN: 1552-5783
Description: This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Rights: This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (https://creativecommons.org/licenses/by-nc-nd/4.0/).
The following publication Stell, W. K., Popa, V., & Kee, C. S. (2016). Intrinsic Ocular Mechanisms Underlie Lens-Induced Astigmatism in Chicks. Investigative Ophthalmology & Visual Science, 57(12), No Pagination Specified is available at https://iovs.arvojournals.org/article.aspx?articleid=2561949
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