Grating acuity and contrast tests for clinical trials of severe vision loss

Abstract

Purpose: To evaluate the reliability and validity of grating visual acuity (VA) and contrast sensitivity (CS) tests, which could be useful outcome measures to assess changes in severely reduced vision.

Methods: The Grating Acuity Test (GAT) and Grating Contrast Sensitivity (GCS) tests, which involve the detection of grating orientation in a four-Alternative Forced Choice paradigm on a liquid crystal display screen, were compared with the well-validated Early Treatment of Diabetic Retinopathy Study (ETDRS) and Pelli-Robson (PR) charts. Grating tests were repeated two or three times within-visit, across three or four sessions, in 20 legally blind subjects: 8 with retinitis pigmentosa (RP) (16 eyes) and 12 with other retinal diseases (OR) (16 eyes).

Results: VA determined by ETDRS and GAT was in good agreement and scaled very similarly, as shown by regression of the within-session difference between the two measures against their mean [RP group: slope (m) = 0.11; 95% confidence interval [CI]: -0.06, 0.29; p = 0.21; OR group: m = -0.07; 95% CI: 0.33, 0.20; p = 0.62]. On average, higher logCS levels were obtained using the GCS than the PR in both groups. The two CS measures scaled similarly in the RP group (m = 0.07; 95% CI: -0.09, 0.22; p = 0.39) but not in the OR group (m = 0.41; 95% CI: 0.12, 0.70; p = 0.005). The within- and between-visit 95% coefficient of repeatability (CR.95) were 0.11 to 0.17 log units for the ETDRS charts and GAT in both groups and 0.14 to 0.15 log units for the PR and GCS in the RP group, whereas the OR group demonstrated more variability in CS. Between-visit CR.95 did not significantly change with mean VA or CS for the ETDRS, PR, or GCS tests, but RP patients' CR.95 on the GAT increased significantly with decreasing VA. Floor effects occurred for some RP eyes with ETDRS and PR charts but not with the GAT and GCS.

Conclusions: Computer-driven grating tests appear to be reliable, capable of evaluating vision that may fall outside of the range of standard clinical tests and may be useful during clinical trials for advanced eye disease.

FIGURE 1

Photographs of the LCD monitor displaying a GAT stimulus in the diagonal right orientation (A) and the dot pattern that is presented between gratings (B).

FIGURE 2

Scatter plot demonstrating the relationship between VA with the ETDRS charts and the GAT for the RP (A) and OR (B) groups. Each subject’s ETDRS acuity (y-axis) was plotted against his/her within-session mean GAT acuity (x-axis) at each visit for each eye and each group. Snellen equivalents are shown on top. The lines represent the least-squares bivariate regressions of the two measures by subject group. The better eye is designated by open symbols, the worse eye with filled symbols, with a unique symbol for each subject.

FIGURE 3

Bland-Altman plots of the difference between the two VA tests (grating test minus letter chart) vs. the mean of both tests for the RP (A) and OR (B) groups.

FIGURE 4

(A, B) Scatter plot demonstrating the relationship between CS obtained with the PR charts and the GCS test. Each subject’s PR acuity was plotted against his/her within-session mean GCS at each visit for each eye and each group. The GCS values presented are a Weber conversion to match the contrast values of the PR. Measures are dichotomized into two subgroups according to the difference between the individual’s mean ETDRS VA and the PR letter size (1.54 logMAR or 20/700). Subjects demonstrating ≤0.3 (20/350) difference, i.e., poorer acuity limits, are presented with open symbols, whereas subjects demonstrating better acuity are presented with filled symbols. The lines represent the least-squares bivariate regressions of the two measures. No line was fit for the RP group in the poorer acuity range (designated by open squares) because of the small number of data points in this category (n = 2). Triangles represent measures obtained at floor for the both groups and were not included in the regression fit.

FIGURE 5

Box plots of the 95% coefficients of repeatability (CR_.95) between test sessions (A) and within test sessions (B) for each test in each group. The bottom and top of the box are the 25th and 75th percentile (the lower and upper quartiles, respectively), and the band near the middle of the box is the 50th percentile (the median). The ends of the whiskers represent the lowest datum within 1.5 times the interquartile range of the lower quartile, and the highest datum still within 1.5 times the interquartile range of the upper quartile. Any data not included between the whiskers are plotted as an outlier indicated by a plus sign. The OR group GCS data were subdivided according to their acuity limits as either ≤0.3 or >0.3 (equivalent to 20/350 per Fig. 4). Subjects with measures at floor were excluded from the analysis.

FIGURE 6

Scatter plots indicating the relationship between the 95% coefficient of repeatability (CR_.95) and mean VA measured with the ETDRS charts and GAT for each eye tested for each group (A), including a regression line for the GAT in the RP subjects indicating a statistically significant relationship. The scatter plots indicating the relationship between CR_.95 and mean CS measured with the PR charts and GCS test for each eye tested and each group are presented on the bottom (B). Triangles represent measures obtained for one method (e.g., GAT) but excluded from the plot in the alternate method (e.g., ETDRS) due to floor effects. The OR group GCS values (6B, right) are subdivided according to their acuity limit as either ≤0.3 (filled symbols) or >0.3 (open symbols).