Abstract
Purpose: :
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.
Methods: :
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.
Results: :
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.
Conclusions: :
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.
Keywords: myopia • refractive error development