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C. F. Wildsoet, Y. Liu; Effect of Defocusing Lenses on Peripheral Refractions and Eye Shape in Young Chicks. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3714. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Experimental studies of refractive development have traditionally used single focus lenses which are worn continuously. While such studies provide convincing evidence that ocular growth is sensitive to defocus, they only poorly simulate the dynamic state of natural viewing conditions. Here we made use of a multifocal "simultaneous vision" contact lens design to create a dynamic defocus situation.
Single-focus (+10, -10 D) and multifocal (+10/-10 D) lenses were fitted monocularly to young chicks; the multifocal lens presented the 2 powers in alternate rings of approximately 0.5 mm width across the 12 mm total lens diameter. The lens designs offered unobstructed visual fileds of 70-90 degrees, i.e. panoramic vision. Refractions were measured after a week of lens wear on-axis and in the nasal and temporal fields, at 30 degrees eccentricity, and effects on eye shape assessed by measuring external ocular dimensions of enucleated eyes. High frequency A-scan ultrasonography was used to track axial ocular changes in vivo.
The mean (±SD) on-axis refractive errors of treated eyes were consistent with compensation for the +10 and -10 D lenses (+9.94±1.85 & -4.81±3.83 D), and the response to the multifocal lens was most similar to that of the +10 D lens (+7.83±2.08 D). Peripheral refractive errors shifted in the same direction as the on-axis refractive errors in all cases, but the changes were smaller in magnitude (e.g. by 2.12±1.84, 2.13±1.84, 1.31±3.20 D, for +10, -10 & +10/-10 D lenses, temporal field). External ocular dimensions were minimally affected although significant changes in choroidal thickness, vitreous chamber depth and optical axial length were recorded for treated eyes, consistent with their refractive error changes.
Young chicks show relatively little decrease in sensitivity to defocus over the central 60 degrees of their visual field. The mismatch between changes in refractions, internal and external ocular dimensions is likely to reflect the well documented slower response to the imposed defocus of the sclera compared to that of the choroid.
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