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MJ Collins, A Shaw, E Menkens, B Davis, R Franklin; The Effect Of Pupil Size On Subjective Refraction With Irregular Corneas . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2058.
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Purpose: The subjective refraction of normal eyes changes little as a function of pupil size. But when the optical properties of the eye show large regional variations, we might expect that the refractive error of the eye would also change as the entrance pupil size varies. Methods: Nineteen young subjects were recruited for the study, 10 had normal corneal topography and normal levels of visual acuity. The other 9 subjects were selected for the presence of irregular corneal shape, related to early to moderate levels of keratoconus. Videokeratoscopy was performed on the selected eye of each subject and a best-fit sphero-cylinder and RMSE was calculated for corneal diameters of 3, 5 and 7 mm. The subjects’ pupil was dilated with phenylephrine and subjective refraction was performed through fixed artificial pupil diameters of 3, 5 and 7 mm. High and low contrast visual acuity was measured through the optimal sphero-cylindrical refraction for each of the artificial pupils. Results: The group mean change between the 3 and 7 mm pupils in subjective refraction sphere and cylinder components, were 0.86 ±0.18 D and 1.14 ±0.25 D respectively for the irregular cornea group. For the group with normal corneas, the corresponding changes were much smaller at 0.18 ±0.24 D (sphere) and 0.25 ±0.35 D (cylinder). There was a statistically significant difference between groups in the change in the subjective sphere with pupil size (ANOVA, p = 0.01), while the change in subjective cylinder with pupil size approached significance (ANOVA, p = 0.07). But the change in subjective and objective sphere and cylinder were not significantly correlated. There was a significant correlation between corneal RMSE and both high contrast visual acuity (r = 0.67) and low contrast visual acuity (r = 0.77). Conclusion: Keratoconus patients may experience significant changes in optimal sphero-cylinder correction as pupil size varies. If rigid contact lenses are used as a correction, this change is effectively masked by the post-lens tear layer. If the patient uses spectacles or soft contact lens correction, some will benefit from different corrections for different pupil sizes (eg, daytime versus night driving). However changes in corneal sphero-cylinder with pupil size do not reliably predict these individuals.
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