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N. J. Coletta, M. Frericks, R. Groves; Mesopic Visual Acuity in Myopia. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1009.
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© ARVO (1962-2015); The Authors (2016-present)
Myopic eyes are elongated and have enlarged retinal images, but retinal stretching may increase the spacing between the photoreceptors. These two opposing factors apparently balance each other because photopic visual acuity is only slightly reduced in myopia. Myopic subjects may exhibit more severe losses of acuity in dim light, however. Myopic eyes should have dimmer retinal images than emmetropic eyes, due to the inverse square law of illumination, and myopes are reported to have lower optical quality than emmetropes. Acuity is less likely to be sampling limited at low luminance and should depend upon retinal illumination and image quality. We studied how acuity in dim light is related to refractive error, biometry and optical quality.
Spherical equivalent refractions of 18 subjects ranged from plano to -7.25 D; the best spectacle correction was determined from a subjective refraction prior to testing. Subjects dark adapted and monocularly viewed a back-illuminated high-contrast letter chart through neutral density filters. Acuity was measured at a mesopic luminance of 0.12 cd/m2 and a low photopic luminance of 7.3 cd/m.2 Corneal topography, biometry and ocular wavefront aberrations were measured and the retinal magnification factor (mm/deg) was calculated for each eye.
Mesopic visual acuity significantly decreased with increasing myopia (p=0.0004), even when the acuity data were adjusted for spectacle minification (p=0.003). Acuity at the low photopic light level decreased with increasing myopia but not significantly (p=0.055 for raw acuity data and p=0.29 with adjustment for minification). At the mesopic light level, retinal acuity in cyc/mm decreased at a rate of 0.08 log unit per mm increase in axial length (p=0.0006) while the low photopic retinal acuity decreased by 0.04 log unit per mm increase in axial length (p=0.056). Horizontal-vertical astigmatism (C22) was correlated with mesopic (p=0.033) but not low photopic retinal acuity (p=0.372), while lateral coma (C31) was correlated with both mesopic (p=0.026) and low photopic retinal acuity (p=0.021). Neither of these aberrations was correlated with the high photopic acuity recorded during the subjective refraction.
Spectacle-corrected myopes exhibit a significant reduction in mesopic acuity compared to emmetropes. This effect can be partially explained by the decreased retinal illumination that results from axial elongation of the eye. The acuity reduction in dim light is associated with an increase in certain ocular aberrations, but it may also involve the optics of spectacle correction as well as non-optical factors of the myopic eye.
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