The effect of target size and accommodation on the distant angle of deviation in intermittent exotropia

To evaluate the effect of size and accommodative characteristics of fixation targets on the distant angle of deviation in patients with intermittent exotropia in the context of interobserver and intraobserver variability of the prism cover test.

Cross-sectional observational study.

Institutional.

Sixty subjects with intermittent exotropia.

Prism cover tests were performed by 2 independent examiners. The distant angle of deviation was measured with 4 different targets at 6 m. The targets varied in size and accommodative characteristics: black-on-white optotypes with a visual angle of 50 minutes of arc (MOA; large figure) and 10 MOA (small figure) and green lights with a visual angle of 50 MOA (large light) and 10 MOA (small light).

Interobserver variability, intraobserver variability, test-retest reliability using different targets, and the influence of factors on the magnitude of absolute test-retest variability.

The 95% limits of agreement of the prism cover test between different targets (± 3.0 to 6.0 prism diopters [PD]) were similar to the intraobserver variability (± 3.9-4.8 PD) and slightly smaller than the interobserver variability (± 6.3-6.5 PD). The only factor strongly correlated to the magnitude of interobserver variability was the magnitude of target-related intraobserver variability.

The size and accommodative characteristics of a target do not cause significant variability beyond the natural variability of the prism cover test for distance in intermittent exotropia. Differences less than 6 PD from one prism cover test measurement are likely to represent test-retest variability.