Results
Quantitative measurements of skew (with the VOG device) were feasible in all participants. The average examination time was 15 minutes including the calibration procedure. Figure 2 shows an example of positional data recorded with VOG at various conditions. We observed a corrective saccade either up- or downwards with a short latency after the change of eye viewing regardless of the prism size. However, 3 participants did not show any saccadic eye corrections with FP size of 1 PD (0.57°) and 3 had no eye alignment change with FP sizes of 2 PD (1.14°).
The mean value of the measurements was not significantly different between the APCT and the VOG except for 2 conditions (1 PD (0.57°) and 4 PD (2.28°)). In the 1 PD (0.57°) condition the mean-value in the APCT measurements was overestimated at 3.90 +/- 2.88 PD (2.22° +/- 1.64°) and for the VOG was 1.30 +/- 1.83 PD (0.74° +/- 1.04°)(p=0.09). In the 4 PD (2.28°) condition the mean-value in the APCT measurement was 3.50 +/- 1.43 PD (1.99° +/- 0.81°) and for the VOG was 2.14 +/- 1.69 PD (1.21° +/- 0.96°) (p=0.28). The detailed results of the mean-values for the APCT and the VOG are summarized in Table S1 (supplementary material).
We found a significant correlation between the reference APCT and the Skew -VOG (Figure 3A, Pearson’s R2 of 0.606, p <0.05). There was a good agreement between the two tests (Intraclass correlation coefficient 0.852, 95 CI 0.728-0.917, p<0.001). The overall accuracy of the VOG was estimated at 80.53% with an error rate of 19.46%. We performed a Bland-Altman plot (Figure 4), which demonstrated consistent variability across the graph with no proportional bias with a mean value of 1.8 PD (1.02°) and a half width of the 95% limit of agreement of +/- 5.2 PD (2.96°).
Figure S1 (supplementary material) shows the variance of both measurement methods (APCT and VOG) as a marker for test precision. There was a high variance across conditions ranging from 0.7 to 6.9 PD (0.40° to 3.93°). For conditions with small foil values (1 PD (0.57°)) the variance of APCT was markedly higher (s2=7.7PD (4.39°)) than Skew-VOG (s2=3.5 PD2(1.99°)).