Our results are in agreement with previously published reports that demonstrate relative hyperopia in the horizontal retinal periphery of uncorrected myopic eyes.13,18–21 However, the relative hyperopia observed in our study population was higher than that reported from a group of (mostly white) myopic children (+0.80 ± 1.29 D, 30° nasal field) with a similar age range (6 to 14 years) and myopic refractive error (−2.84 ± 2.09 D).21 It is well known that the prevalence of myopia in children is higher in Chinese population22 in comparison with a white population,23 and the annualized progression of myopia is also greater in the Chinese population.24 Thus, the increased relative peripheral hyperopia observed in our population may provide some evidence of ethnic differences in eye shape or distortion in myopic Chinese children and consequently provide us with an alternate explanation for the differences seen in progression of myopia in children from different ethnicity.
It has been hypothesized that peripheral hyperopia provides a stimulus for axial elongation and progression of myopia as measured at the fovea.25 Also, studies in infant monkeys showed that from deprivation in the retinal periphery despite clear images at the fovea can produce axial myopia at the fovea.11 Our data show that when uncorrected, the myopic eye experiences absolute myopic defocus along the horizontal visual field. However, when corrected with SVLs, the myopic eye experiences absolute hyperopic defocus at the periphery, which may be associated with the progression of myopia. More importantly, when the magnitude of hyperopic defocus was compared between corrected vs. uncorrected states, it was seen the correcting SVLs increased the amount of hyperopic defocus in eyes with moderate myopia. Calver et al.26 using trial lenses to correct adults with low myopia did not find differences in peripheral mean spherical equivalent error between corrected and uncorrected state of myopia. Although our results concur with Calver et al. for the low myopic group, we found a significant increase in hyperopic defocus with SVLs for the moderately myopic group. This increase was present at all the peripheral angles that were assessed and was as expected due to higher powered negative lenses being prescribed for moderate myopic eyes. Given these results, the question then asked is to whether correction with SVLs is likely to accelerate myopia progression. Smith et al.11 suggested that prescribing lenses that impose hyperopic defocus in the periphery could effectively promote axial elongation and suggested it may be possible to slow the progression of myopia in children by prescribing lenses that correct central refractive errors and at the same time increase the curvature of field of the image plane, thus either correcting for any peripheral hyperopia or actually imposing myopic defocus in the periphery. Because of the cross-sectional nature and the methodology adopted in this study, we are unable to draw any inferences on the effect of hyperopic defocus with SVLs on progression of myopia. Studies considering the peripheral refractive error state of stable myopic eyes vs. progressive myopic eyes with SVLs or a longitudinal study monitoring the progression of myopia with or without interventions will shed more light on this issue. However, despite the lack of the evidence for efficacy in terms of control of progression of myopia, it could be argued that a correcting lens that reduces or eliminates hyperopic defocus may have inherent advantages over the present form of spectacles in optimizing aspects of peripheral visual performance such as detection acuity. Wang et al.27 demonstrated that grating detection acuity depends strongly on optical blur in the periphery, and Whatham et al.28 demonstrated that contrast sensitivity at peripheral retinal locations improved with correction of peripheral refractive errors.
In agreement with previous data, asymmetry was observed in the peripheral refraction with the temporal retina being more hyperopic in comparison with nasal retina.29 Also, astigmatism increased with an increase in eccentricity in the uncorrected myopic eyes.19 These findings have been suggested to be the result of the combined effects of the cornea, crystalline lens, and retina.30 Correction with SVLs had a small impact on the magnitude of J180 in both the low and moderate myopic eyes; however, J45 increased with eccentricity, especially in moderate myopic eyes. One possible explanation for this increase is the pantoscopic tilt induced by spectacle lenses.31 We rule out misalignment of the spectacle lenses as a possible explanation for this increase as the methodology adopted in the study involved the use of appropriate head turn to ensure that the spectacle plane was always at right angles to the visual axis. Furthermore, for the farthest peripheral angle measured in the study, i.e., 40° the rays passed approximately 15 to 20 mm from the lens center and therefore was not subject to increased astigmatism and other aberrations.
In conclusion, our results provide evidence that correction of SVLs induced absolute hyperopic defocus on the retinal periphery of low and moderate myopic eyes. Although the effect of this induced hyperopia on the progression of myopia at the fovea remains to be assessed, it is reasonable to suppose that improving on the designs of the existing optical interventions may confer additional benefits.
We thank Mr. Les Donovan for his help with the preparation of this article.
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