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Nancy J. Coletta, Meritza Frericks; Visual acuity at low luminance in myopia. Journal of Vision 2006;6(13):44. doi: 10.1167/6.13.44.
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Retinal stretching occurs in myopic eyes, but photopic visual acuity is only slightly reduced in myopia because the retinal image is magnified by the eye's increased axial length.1–4 Some studies report that myopic eyes have lower optical quality than emmetropic eyes,5,6 although optical factors do not affect photopic acuity in moderate myopia.7 Optical factors of the corrected myopic eye could have detrimental effects on acuity at low luminance, when the pupil is large and acuity is less likely to be sampling limited. The aim of this study was to determine whether acuity in dim light is affected by moderate amounts of myopia. Visual acuity was measured at photopic and mesopic luminance levels, using back-illuminated LogMAR letter charts, viewed through neutral density filters. The luminance levels were 7.3 and 0.12 cd/m2, providing either 142 or 2.3 td, respectively, for a 5mm pupil. Acuity was also assessed at the mesopic light level with laser interferometry, to determine whether any acuity deficits in myopia were of neural origin. Refractions ranged from plano to −6.50 D, and subjects viewed the charts with best spectacle correction. Acuity in cyc/deg was plotted against the refraction in Diopters and linear regressions were fit to the data. Myopic subjects showed a significant reduction of letter acuity at the mesopic light level (p=0.036) but not at the photopic level (p>0.05). Interferometric acuity was not significantly reduced with increasing myopia (p>0.05). The results imply that optical factors of the spectacle-corrected myopic eye are detrimental to acuity at dim light levels.
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