ABSTRACT.Purpose: The aim of this study was to determine the variability of visual acuity in a large eye clinic. Methods: A cross-sectional study using 50, consecutively presenting adult patients with visual acuity of at least 6/60 and aged between 18 and 75 years was performed. Measurements of visual acuity obtained under normal clinical conditions were compared to measurements obtained using standard clinical research protocols. The variability of visual acuity was assessed by determining the 95% limits of agreement between test and retest measures. Results: There were no significant differences between test-retest measurements of visual acuity, either aided or unaided. Pearson r correlation coefficients between test and retest measurements were high for both aided and unaided visual acuity. The 95% limits of agreement revealed repeatability of about ∫1.5 log-MAR or 1.5 lines on a standard logMAR chart. Conclusion: In large eye clinics, in order to be confident that a real change in visual acuity has occurred between measurements, a difference of at least 0.15 logMAR (8 letters on a standard logMAR visual acuity chart) is required.
Previous investigators reported the impairment of foveal visual acuity by nearby flanking targets (contour interaction) is reduced or eliminated when acuity is measured using low contrast targets. Unlike earlier studies, we compared contour interaction for high and low contrast acuity targets using flankers at fixed angular separations, rather than at specific multiples of the acuity target's stroke width. Percent correct letter identification was determined in 4 adult observers for computer generated, high and low contrast dark Sloan letters surrounded by 4 equal contrast flanking bars. Two low contrast targets were selected to reduce each observer's visual acuity by 0.2 and 0.4 logMAR. The contour interaction functions measured for high and low contrast letters are very similar when percent correct letter identification is plotted against the flanker separation in min arc. These results indicate that contour interaction of foveal acuity targets occurs within a fixed angular zone of a few min arc, regardless of the size or contrast of the acuity target.
Tests of stereopsis have many uses in ophthalmic practice, especially in the examination of children. Stereotests are recommended as a way to improve paediatric vision screening programs, to provide an overall indication of binocularity and to monitor binocularity in amblyopia therapy or contact lens monovision. There are currently many stereotests available to practitioners and careful consideration must be given to the applicability of a particular test. The correct stereotest for a particular task is critically dependent on the perceptual age of the patient and the type of information the clinician wants to gather from the test.
For subjects with little or no accommodation, the NPC does not depend on the target used and is the same measured with the RAF rule, a pencil tip or finger tip. In non-presbyopic subjects there appears to be a small accommodative influence on the NPC, which is target dependent. However, the difference is probably not clinically important.
Our results show that maturation of line acuity is still taking place between the ages of 4 and 9 years. Measured acuity is affected by the amount of contour interaction induced by the type of optotype (letter or picture) and by the interoptotype separation. Another factor, probably a maturation of gaze control or selective attention is responsible for the reduction in crowding with age.
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