Comparison of visual acuity in macular degeneration patients measured with snellen and early treatment diabetic retinopathy study charts

Abstract

Purpose: To compare the measurements of visual acuity (VA) results measured with Snellen and Early Treatment Diabetic Retinopathy Study (ETDRS) charts in eyes with and without age-related macular degeneration (AMD).

Design: Cross-sectional study.

Participants: One hundred four participants (190 eyes) selected from a university retina practice; 80 participants (142 eyes) had some degree of AMD.

Methods: Visual acuity was measured in each patient using standard procedure with both Snellen and ETDRS charts in random order. Statistical analysis of the results was performed.

Main outcome measures: Difference in VA measured by both charts in logarithm of minimal angle of resolution (logMAR) notations.

Results: Overall, the mean Snellen VA was 0.78 logMAR (= 20/120), and the mean ETDRS VA in the same eye was 0.54 logMAR (= 20/70; P<0.001). In the low vision group (<20/200), represented by patients with AMD, the average difference in number of lines was considerably larger than in the good vision range (>20/30). On average, 20/200 on Snellen was 20/95 on ETDRS (>3 lines difference), and 20/30 on Snellen was 20/25 on ETDRS (<1 line difference).

Conclusion: Our results show poor agreement between the Snellen and ETDRS charts, and it was more pronounced in the group with poor vision. The ETDRS measurements yielded better VA, particularly in participants with vision <20/200 (representing more advanced AMD patients). We suggest taking these findings into consideration when comparing outcomes in clinical practices (which typically measure VA using standard Snellen charts) with outcomes from clinical trials (which typically measure VA using ETDRS charts).

Figure 1

Visual acuity (VA) measurement differences between Snellen and Early Treatment Diabetic Retinopathy Study (ETDRS) charts. This figure is a Bland-Altman chart, which graphically displays VA discrepancy between the 2 charts. The x-axis displays mean VA value between Snellen and ETDRS - (Snellen + ETDRS)/2; the y-axis displays the difference between measurements by both charts (ETDRS - Snellen). All numerical values on x-and y-axes are logarithm of the minimum angle of resolution (logMAR) units. All points below the solid line reflect Snellen score worse than ETDRS score in the same person and vice versa.