Nilpa Shah

Vanishing Optotype acuity: repeatability and effect of the number of alternatives.

Citation information: Shah N, Dakin SC, Redmond T & Anderson RS. Vanishing Optotype acuity: repeatability and effect of the number of alternatives. Ophthalmic Physiol Opt 2011, 31, 17–22.

Effect of Scoring and Termination Rules on Test–Retest Variability of a Novel High-Pass Letter Acuity Chart

PURPOSE Test-retest variability (TRV) limits our ability to detect clinically significant changes in visual acuity (VA). We wanted to compare the effect of scoring and termination rules on TRV for logMAR charts, employing either conventional or pseudo high-pass (Vanishing Optotype) letters. METHODS VA measurements and TRV were compared in 50 uncorrected normal observers (17 male, mean age 42.8 ± 16.2 years) using both conventional logMAR-style charts and letter charts of the same layout but containing pseudo high-pass letters (Moorfields Acuity Chart [MAC]). Additional charts employing a different 10-letter alphabet to the Sloan set were also tested. Mean spherical refractive error was -0.93 diopters (D; range, -5.38 to +3.00 D). Acuity scores were calculated using three methods: letter-by-letter, with either line- or chart-based termination, and line-by-line scoring. Bland-Altman methods were used to calculate 95% ranges for TRV. RESULTS While acuity thresholds were higher for the MAC, they were less affected by termination criteria and displayed significantly lower 95% TRV values across all scoring techniques. Ordinary least squares regression analysis confirmed a proportional as well as systematic bias between conventional and MAC measurements (r(2) = 0.217, P = 0.001) such that the difference between the two was greater with better VA. CONCLUSIONS TRV was consistently lower for a logMAR chart employing high-pass rather than conventional letters in uncorrected refractive error and was less affected by termination and scoring methods. The MAC was also less affected by optical defocus. Further work is required to determine the usefulness of different charts to differentiate between optical and neural losses of vision.

Visual acuity loss in patients with age-related macular degeneration measured using a novel high-pass letter chart

Background/aims Conventional Logarithm of the Minimum Angle of Resolution (logMAR) acuity is the current gold standard for assessing visual function in age-related macular degeneration (AMD). However, visual acuity (VA) often remains ‘normal’ when measured with these charts, even with advanced retinal changes. We wished to investigate how VA measurements with the Moorfields Acuity Chart (MAC), which employs high-pass filtered letters, compares to conventional letter charts in subjects with AMD. Methods Monocular best-corrected VA measurements and test–retest variability (TRV) were compared for conventional and MAC charts in 38 normal observers (mean age 52.1 years) and 80 patients (mean age 80.6 years) with varying degrees of acuity loss owing to AMD. Methods of Bland–Altman and ordinary least-squares regression were employed for data analysis. Results A proportional bias was confirmed between conventional and MAC measurements (r2=0.133, p=0.001) such that MAC acuity was −0.45 logMAR ‘worse’ at the 0.00 logMAR acuity level, but only −0.26 logMAR ‘worse’ at the 1.00 logMAR level. The mean bias was much smaller in the normal subject group (−0.16 logMAR). Similar TRV (ranging from ±0.09 to ±0.12 logMAR) was found for both charts in both subject groups. Conclusions VA measurements with the MAC chart appear to be more sensitive to functional loss in AMD compared with conventional letter charts, with similar TRV. Simulations indicate this may be because the high-pass filtered letters are more vulnerable to undersampling as a result of retinal cell loss in the disease process.