Prospective evaluation of visual acuity assessment: a comparison of snellen versus ETDRS charts in clinical practice (An AOS Thesis)

Abstract

The purpose of this study was twofold: first, to prospectively compare visual acuity (VA) scores obtained with Snellen charts versus Early Treatment Diabetic Retinopathy Study (ETDRS) charts in a "real world" retinal practice, and second, to see if there was a difference in visual acuity measurements obtained with ETDRS charts starting at 4 or 2 meters.

Methods: Prospective, consecutive evaluation of patients who underwent best-corrected visual acuity testing of their right eye performed at a single seating by the same experienced, certified vision examiner in the same room with standardized low light conditions using a projected Snellen chart at 20 feet, and two different back-illuminated ETDRS charts placed 4 and 2 meters from the patient.

Results: One hundred sixty-three eyes were included in the study. The mean Snellen VA was 0.67 logMAR (20/94), ETDRS VA at 4 meters was 0.54 logMAR (~20/69), and ETDRS VA at 2 meters was 0.51 logMAR (~20/65). The mean difference was 6.5 letters better on the ETDRS chart (P=.000000001). As the VA worsened, there was increased variability between the charts and the mean discrepancy between charts also increased. Subgroup analysis revealed the greatest difference between charts was in the poor vision subgroup (<20/200) with a difference of 0.2 logMAR (10 letters; P=.0000002). Patients with exudative age-related macular degeneration (AMD) had the greatest disparity on vision testing, but patients with dry AMD and diabetic retinopathy also exhibited significant differences.

Conclusions: Visual acuity scores were significantly better on ETDRS charts compared to Snellen charts. The difference was greatest with poor visual acuity (<20/200) and in patients with exudative AMD. Thus, caution should be exercised when comparing data using the different charts.

FIGURE 1

Snellen projection charts used by the Nikon Chart Projector NP-3S.

FIGURE 2

Early Treatment Diabetic Retinopathy Study (ETDRS) chart 1 (left) and chart 2 (right) from Precision Vision.

FIGURE 3

Frequency distribution of ages for all patients in the clinical study.

FIGURE 4

Frequency distribution of logarithm of the minimum angle of resolution (logMAR) vision scores for all patients in the clinical study.

FIGURE 5

Correlation between logarithm of the minimum angle of resolution (logMAR) visual acuity scores on Early Treatment Diabetic Retinopathy Study (ETDRS) charts starting testing at either 4 meters or 2 meters from the patient. The x-axis and y-axis are the logMAR visual acuity scores. The yellow line represents the equivalence line, and the blue line represents the regression line.

FIGURE 6

Bland-Altman plot of the differences in logarithm of the minimum angle of resolution (logMAR) visual acuity scores between Snellen and Early Treatment Diabetic Retinopathy Study (ETDRS) charts. The y-axis displays the difference between measurements on both charts, and the x-axis displays the mean vision score value between the charts. All values represent the logMAR visual acuity scores. The solid line is the mean difference in logMAR acuity (−0.13), and the area within the dotted lines equals 95% limits of agreement between logMAR vision between the two charts. All points below the solid line reflect Snellen scores worse than ETDRS scores in the same eye.

FIGURE 7

Correlation between visual acuity scores measured on the Early Treatment Diabetic Retinopathy Study (ETDRS) and Snellen charts. The x-axis and y-axis are the logarithm of the minimum angle of resolution (logMAR) visual acuity scores. The yellow line represents the equivalence line, and the blue line represents the regression line. The dotted lines represent the 95% confidence intervals of the regression line.

FIGURE 8

Logarithm of the minimum angle of resolution (logMAR) visual acuity scores by age for Snellen and Early Treatment Diabetic Retinopathy Study (ETDRS) charts. The lines represent the regression line for Snellen and ETDRS charts.