In chicks wearing high powered negative lenses, spherical refraction is compensated and oblique astigmatism is induced

Abstract

Chicks develop refractive errors in response to defocusing lenses. Mean ocular refractions (MOR) and astigmatism were measured in chicks that developed myopia in response to unilaterally applied negative lenses (-30 D, n = 19; -15 D, n = 16). The MOR of the control eyes emmetropized with age and both goggle powers were fully compensated given enough time. The initial slopes of plots of MOR with age are not significantly different between inducing lens powers. Astigmatism in control eyes decreased significantly with age. In the presence of -30 D lenses, astigmatism significantly increased with age due to an exponential increase in JCC45 (oblique Jackson crossed cylinder). Although not statistically significant, a similar but smaller trend was observed for the -15 D lenses. Thus, oblique astigmatism results during myopia induction by high-powered lenses. This induced astigmatism is correlated with MOR. These results are consistent with emmetropization of astigmatism, which is disrupted by high-powered spherical lenses.

Keywords:

• myopia
• refractive error
• astigmatism
• chick
• emmetropization

Acknowledgements

The authors thank J. Bueno, A. Casey, C. Cookson and D. King for their assistance with software development. All authors were supported by NSERC Canada and CFI. ELI was also supported by PREA and CRC. JJH was supported by an OGS scholarship.