- Title
- The ocular response to hyperopic and myopic defocus in the guinea pig eye
- Creator
- Leotta, Amelia
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Background and Aims: Myopia is a highly prevalent disorder of the eye that can lead to pathological outcomes including blindness. It is increasing in prevalence and has enormous public health impacts and societal costs. Genetic and environmental factors are involved in the aetiology of myopia, but the cause(s) of the increasing prevalence are unknown. Animal models have been used to determine the effects of hyperopic and myopic defocus on the refractive status and ocular growth of the eye, with defocus imposed using both spectacle lenses and diffusers. Experiments using animals suggest that multiple cues and biochemicals are likely to be used in the detection and compensation for defocus. Determining the response to defocus in the mammalian eye, which is phylogenetically similar to the human eye, will help to elucidate both how defocus alters eye growth and how it can be manipulated to assist with the recovery from myopia. The guinea pig is a useful mammalian model of the effect of myopic and hyperopic defocus on eye growth as it rapidly compensates for lenses and diffusers. In this thesis, the guinea pig will be used to determine the effects of myopic and hyperopic defocus during lens-wear (induction) and recovery from myopia, when imposed monocularly and binocularly and when imposed sequentially on the eye. Methods: In Chapter 2, different groups of guinea pigs wore lens powers from -10D to +15D on one eye for 10-13 days before being measured. Repeated measures were performed on animals wearing a -8D lens (n=5), no lens (n=4), or a +5D lens (n=6) for approximately 2 weeks. Refractive error (RE) and ocular length (OL) were measured at the end of the experiment to determine the range and features of compensation to myopic and hyperopic defocus. Chapter 3 determined the temporal properties of hyperopic defocus in isolation (without a competing signal) and competition. Guinea pigs wore monocular -4D lenses (n=111) for repeated periods interspersed with different dark intervals over 12 days, or monocular -5D lenses (n=152) for repeated periods interspersed with different length free viewing periods over 7 days. Ocular parameters were measured at the conclusion of the lens wear periods. In Chapter 4, the recovery from myopia was investigated when either one (n=28) or both (n=13) eyes were first made myopic by minus lens-wear, and then wore either negative lenses, lens mounts or positive lenses to manipulate the subsequent defocus experienced. Additionally, the fellow eyes of animals (n=33) were manipulated during the recovery from myopia to assess the yoked effects during inhibitory growth. Guinea pigs (n=21) also wore +10D lenses monocularly or binocularly to determine whether compensation was more complete for isometropic defocus. Finally, in chapter 5, guinea pigs (n=50) had myopic and hyperopic defocus imposed on their eyes sequentially for varying percentages of the day period to determine if myopic defocus preferentially guides eye growth. Results: In Chapter 2, it was found that compensation in RE in the guinea pig eye occurred for a larger range of hyperopic defocus compared to myopic defocus (-8D to +6D lens powers), but a reduction in the vitreous chamber elongation was induced with all positive lens powers tested (up to +15D). The response to myopic defocus also involved a thickening of the crystalline lens and an initial myopic refractive error. In Chapter 3, hyperopic defocus presented in isolation, led to growth signals which were resistant to decay (time constants of 22hrs for RE and 31hrs for OL). In contrast, much shorter decay times occurred when hyperopic defocus was presented in competition with a daily free viewing period (36min). In Chapter 4, recovery from myopia was suppressed by more than 2D when the fellow eye was becoming myopic. The recovery from myopia was increased by adding extra myopic defocus to a recovering eye with a +4D lens. Compared to eyes wearing a lens mount control, an extra +0.15D /day of recovery occurred over 13 days (p = 0.004). However, recovery was not enhanced when the initial myopia and additional myopic defocus was approximately equal in both eyes. When +4D lenses were worn binocularly, the recovery from myopia was +4.3D, the same as in animals with unimpeded vision wearing lens mounts only. Binocular +10D lens wear led to no overall differences in compensation for refractive error compared to monocular lens wear, although when the data from the preferred eyes were compared, the vitreous chamber was 98µm smaller in binocular lens-wearing animals. Finally, Chapter 5 showed that guinea pig eyes experiencing sequential myopic and hyperopic defocus tended to average the defocus presented, though animals wearing +5D lenses for only 2 of 8 hours became significantly more hyperopic than the weighted average of defocus predicted (+1.46D, p = 0.019). Conclusion: The results of this thesis suggest that the guinea pig eye does not change its refractive error when myopic defocus is imposed on normally growing eyes, despite compensatory changes in the vitreous chamber. However, adding myopic defocus to an eye which was already myopic enhanced its recovery. Presenting single vision myopic defocus binocularly did not cause refractive error compensation during lens-wear or recovery from myopia. The mammalian eye responds to high amounts of hyperopic defocus, and this defocus causes a long lasting acceleration in eye growth when presented in isolation, but when presented in competition with periods without lens-wear, the growth signal rapidly decays. Hyperopic defocus was also found to reduce the capacity of a fellow eye to recover from myopia. However, eye growth was biased towards compensation for myopic defocus when the signals were presented sequentially. Taken together, these results suggest eye growth and growth inhibition do not arise from a single bi-directional signal, but rather separate processes underlie the response to myopic and hyperopic defocus in the guinea pig eye.
- Subject
- myopia; hyperopia; guinea pig; induction; recovery; myopic defocus; hyperopic defocus; binocular; monocular
- Identifier
- http://hdl.handle.net/1959.13/1312092
- Identifier
- uon:22342
- Rights
- Copyright 2016 Amelia Leotta
- Language
- eng
- Full Text
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