Pilot Study Comparing the Ocular Deviation Measures of Basic Intermittent Exotropia Using 2WIN-S and Prism Cover Test

Intermittent exotropia (IXT) is a common form of strabismus in children, but the fluctuating angle of deviation and control make measuring the ocular deviation complicated. The prism cover test (PCT) is the gold standard, yet examiner dependent. We assessed whether the 2WIN-S photoscreener (2WIN plus Kaleidos corneal-reflex wand) can quantify deviation in basic IXT and whether control level modifies agreement with PCT.

Nineteen children with basic-type IXT and monocular acuity 0.1 log MAR or better were enrolled, ocular pathology or significant refractive error excluded. Control was graded (0-2 good; 3-5 fair-to-poor). PCT measured deviation at 4m, 1m and 40 cm with 5 s occlusion; 2WIN-S measured at 1 m after a 30 min rest. Analyses compared 10-20 versus > 20 prism dioptres (pd) and used Bland-Altman plots.

Mean age was 10.26 ± 3.76 years. Median deviation was 25.0 pd (IQR 20-30) with PCT versus 8.5 pd (IQR 0-19.5) with 2WIN-S (z = -3.82, p < 0.001). Differences were significant in both 10-20 pd (median 11.25; z = -2.20, p = 0.028) and > 20 pd groups (median 18.0; z = -3.18, p = 0.001). In five cases, 2WIN-S reported orthophoria; four had ≥ 20 pd on PCT and good control. Good-control cases showed a 20.0 pd median difference; fair-to-poor control showed 3.5 pd within the 10 pd clinical margin. Bland-Altman mean difference was 14.13 pd with limits of agreement from -4.96 to 33.22.

Overall agreement between 2WIN-S and PCT was poor. Until methodological or optical refinements are made, 2WIN-S should not be used for quantifying basic-type IXT.