Abstract
Purpose: :
Myopic human eyes are reported to have relatively less myopia, or even hyperopia, in the periphery relative to their axial refraction, and hyperopic eyes tend to have relatively less hyperopia peripherally. How the pattern of peripheral refractive state is related to the development of axial refractive error remains unclear. The purpose of this study is to measure peripheral refractive state in primate eyes with experimentally induced refractive errors.
Methods: :
Eleven juvenile marmosets were examined in this study; five served as controls and six were raised with soft contact lenses to induce anisometropia. Each experimental animal wore a positive lens (+3 or +5 D) over one eye and a negative lens (–3 or –5 D) over the other in order to induce as large a degree of anisometropia as possible. Following 7–13 weeks of lens wear, refractive errors were measured on–axis and along the horizontal meridian out to 40° nasal and temporal retinal eccentricities using an IR videorefractor (MCS PowerRefractor). Subjects were conscious and under cycloplegia.
Results: :
In the control eyes, the peripheral spherical equivalent refractions relative to the on–axis refraction (mean±se, 1.4±0.4 D) were increasingly myopic along both temporal and nasal retinal eccentricities (ANOVA, p<0.01), and astigmatism increased significantly in both nasal and temporal periphery (ANOVA, p<0.01). In the lens–treated marmosets there was, on average, 5.2 D of anisometropia induced (positive lens vs. negative lens: 2.2±0.6 D vs. –3.0±0.6 D, paired t–test, p<0.01). Only the negative–lens–treated eyes were significantly different from controls both on–axis and in the periphery, where they were relatively hyperopic along the nasal retina. The degree of relative hyperopia in the nasal periphery was significantly correlated and inversely proportional to the degree of myopia induced on–axis (R=0.78, p<0.01). There were no significant differences between lens–treated and control eyes in the temporal retina. Both positive–lens–treated and negative–lens–treated eyes had more astigmatism than controls on–axis (lenses vs. controls: 2.4±0.3 D vs. 1.4±0.2 D, unpaired t–test, p<0.05), but there was no difference in the magnitude of astigmatism between the lens–treated and control eyes in the periphery.
Conclusions: :
Induced axial myopia in marmosets is associated with relative peripheral hyperopia, similar to human myopes, but with a nasal–temporal asymmetry. Compared to untreated eyes, contact lens wear produced significantly greater astigmatism on–axis but not in the periphery suggesting that corneal shape changes are occurring in the lens–treated eyes.
Keywords: refractive error development • myopia • astigmatism