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Amelia J. Leotta, Hannah E. Bowrey, Sally A. McFadden; Characteristics of the Response to Myopic Defocus in a Mammalian Eye. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6302.
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Young growing eyes inhibit ocular growth in response to wearing positive lenses in chicks, mammals, fish and primates. In mammals and primates, it is unclear whether this is due to myopic defocus per se since young eyes are hyperopic, the range of response is limited, and the effects wane in older animals. Thus in general, the plus lens simply corrects the hyperopia rather then imposes myopic defocus. In this study, using positive lenses, we deliberately imposed myopic defocus on the mammalian eye and studied the response over time.
23 guinea pigs wore either a +5D, +10D or +15D lens monocularly from 10-23 days of age. Refractive error (with steak retinoscopy after cycloplegia), corneal curvature (using IR videokeratometry) and ocular length (using high frequency ultrasound in anaesthetised animals) were measured before and after the lens-wear period. Animals wearing +5D lenses were also measured repeatedly after 2, 5, 9, and 13 days of lens wear.
The degree of imposed myopic defocus at the start and end of the of lens wear period was -0.05 to -1.5D (+5D lens); -5.7 to -7.8D (+10D) and -11 to -14.5D (+15D). In every group, the lens-wearing eye responded by slowing its vitreous chamber elongation and expanding its choroid with the greatest changes in the +10D group (-96µm; +16µm); and a characteristic thickening of the sclera (+11 µm). These changes increased over time and peaked after 9 days of lens wear. The crystalline lens increased in thickness in the lens-wearing eye shortly after lenses were first worn, an effect which dissipated over time. The anterior chamber increased in the lens-wearing eyes, and significantly more in the +5D and +10D groups (+54µm; +60 µm) counteracting the shrinkage at the back of the eye resulting in no significant net refractive error difference between the eyes in the +5D (-0.8D) and +10D (-0.4D) groups and a small amount of myopia in the +15D group (-1.7D).
The guinea pig eye responds rapidly up to -10 D of imposed myopic defocus with characteristic changes in choroid and sclera and inhibition in vitreous chamber growth showing that the eye responds to myopic defocus. This does not translate to hyperopic refractive errors due to an increased anterior chamber, and thus the neurons involved are unlikely to be using refractive state to guide this compensatory feedback mechanism. Furthermore, we find that the crystalline lens may possibly change its tone initially to imposed myopic defocus.
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