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K. Totonelly, D. Troilo; Eye Shape and Off-Axis Refractive State Following Induced Axial Refractive Errors in Marmoset Monkeys. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3589.
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Eye shape and off-axis refractive state differ between emmetropes and myopes suggesting that they may be factors in the development of on-axis refractive state. We examined the development of eye shape and off-axis refractive state in our primate model of eye growth to determine whether changes occur after axial refractive errors are induced.
Marmosets wore soft contact lenses to impose hyperopic or myopic defocus and produce changes in axial length and refractive state. Treatment groups include monocular and binocular conditions (R/L): -/plano, +/plano, -/+. Untreated marmosets were used as controls. Following treatment, cycloplegic refractive state was measured on- and off-axis and MRI was performed to analyze eye shape and vitreous chamber (VC) depth on a central horizontal plane.
Using a simple measure (equatorial diameter/axial VC depth) we found that eye shape is significantly correlated with the powers of the lenses used (r=0.60), the degree of axial refractive error induced (r=0.47), and the depth of the VC on-axis (r=0.44). Negative lenses induced eyes to grow significantly longer and more myopic on-axis, and become less oblate (wider-than-long) or, in some cases, prolate (longer-than-wide). Eyes that wore positive lenses were significantly shorter, more hyperopic on-axis, and more oblate. Marmoset eyes exhibit a nasal-temporal asymmetry in shape: the VC depth 20-40 deg nasal to the optic axis is significantly shallower than along the central 20 deg or from 20-40 deg temporal to the optic axis, which were not significantly different from each other. The nasal VC depth in negative-lens-treated eyes was significantly shallower than in positive-lens-treated eyes and corresponds to differences in off-axis refractive state in the two groups. Relative to the on-axis refractions, off-axis refractive state measured along the nasal retina in the negative-lens-treated eyes was significantly more hyperopic than in positive-lens treated or control eyes. The relative off-axis refractive state measured along the temporal retina did not differ between the experimental groups and controls.
Lens-induced axial myopia is associated with a change in overall eye shape and an increase in nasal-temporal asymmetry that corresponds to an increase in off-axis hyperopia on the nasal retina. While these changes are consequences of the response to lens rearing, the altered shape and off-axis refractive state in myopic eyes may increase their risk for developing more axial myopia.
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