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C.M. Dickinson, I. Cacho, R. Harper, H. Smith; Using Visual Acuity (VA) to Investigate Contour Interaction and Gaze Selection Defects in Age-Related Macular Degeneration (AMD) . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1816.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose To investigate the relationships between three measures of VA in AMD. The difference between Single Letter (SL) and Crowded Letter (CL) VA is attributed to contour interaction. As this is thought to increase in peripheral vision, the (SL - CL) difference should correlate with the scotoma size in AMD patients. Repeated Letter (RL) VA uses a two-dimensional array of identical targets across the central field to simultaneously present targets to both foveal and parafoveal retina, measuring the optimum VA without accurate fixation. The (RL - CL) VA difference should correlate to fixation ability, and RL VA should most accurately predict the final acuity which can be achieved by the patient using prism relocation. Methods Different arrangements of Landolt-C stimuli were used to measure the SL, CL and RL logMAR VAs of 194 subjects with bilateral AMD aged from 56 to 97 years who were recruited as part of a randomised controlled trial of prism relocation spectacles. All tests were performed on the optimally-refracted better eye. Acuity (ETDRS) ranged from 0.42 - 1.60 (mean 0.92 sd 0.31). The size of the scotoma was determined from a Bjerrum screen plot in terms of its area and the angular distance from the edge of the scotoma to the fovea. Fixation quality was scored for stability, consistency and pursuit ability using direct observation of a fixation target imaged on the fundus. The best "relocation prism" was determined subjectively by optimising recognition of a complex visual target, and used to measure logMAR VA (ETDRS). Results SL VA was found to be better than CL VA in almost all subjects (mean difference 0.11 95% CI 0.09-0.13 p<0.0001). There was however no relation of the difference between SL and CL VA to scotoma area (r = -0.14) or to the distance from the scotoma edge to the fovea (r= -0.09). RL VA was also better than CL VA (mean difference 0.13 95% CI 0.11-0.15 p<0.0001). Although the different measures of fixation quality were highly correlated with each other (r from 0.69 to 0.77), there was no correlation to the difference between RL and CL VAs. The optimum relocation prism varied from 2 to 15 Δ (mean 7.3 sd 2.7). Although the average RL VA was well matched to the prism VA (mean difference 0.02 and 37% of results differed by less than 0.1), 58% of subjects still had better prism VA than predicted. Conclusions The marked inter-individual variation in AMD suggests that simple measures of VA cannot be used to draw conclusions about complex fixation behaviour in patients with central scotoma.
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