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Nilpa Shah, Roger Anderson, Adnan Tufail, Catherine Egan, Steven Dakin; Visual Acuity Loss In Patients With AMD, Measured Using A Vanishing Optotype Letter Chart. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5021.
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Vanishing Optotype (VO) letters have a pseudo high-pass design so that the mean luminance of the target is the same as the background and the letters thus ‘vanish’ once the resolution threshold is reached in the fovea. We wished to investigate how visual acuity measurements using charts constructed from these letters compared to charts of conventional letter design, in both normal subjects and patients with differing levels of AMD.
60 participants were recruited; 30 with no ocular abnormalities and 30 with AMD and no other significant ocular pathology. Following refractive error correction, each subject underwent monocular (single letter scored) visual acuity measurements using ETDRS charts 1 and 2, and charts of the same layout but constructed using VO letters (VO1 and VO2). All tests were performed in a random sequence. The methods of Bland and Altman were employed with test-retest variability (TRV) expressed as 95% confidence intervals for agreement.
Visual acuity measurements with the VO chart were, on average, approximately 3 logMAR lines ‘worse’ than those with conventional letter design in subjects with AMD compared to a difference of only 1.5 logMAR lines in normal subjects. This difference between the two groups was statistically significant (p<0.05, unpaired t-test). Similar TRV’s of approximately +/- 0.10 logMAR were found for both the ETDRS and VO charts in both groups.
AMD patients display a measurably larger visual acuity deficit using VO letter charts compared to conventional ETDRS charts, with no significant difference in test-retest variability.
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