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Shrinivas Pundlik, Matteo Tomasi, Kevin Houston, Gang Luo; Use of a smartphone app for heterophoria measurement. Invest. Ophthalmol. Vis. Sci. 2016;57(12):1509.
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© ARVO (1962-2015); The Authors (2016-present)
In a prior study, a smartphone app that implements the photographic Hirschberg method was shown to accurately measure eye deviations. We aimed to develop and evaluate a method to measure heterophoria with this smartphone app.
Normally sighted subjects without known strabismus (n = 12) fixated at targets placed at 3m (far) and 40cm (near) distances. The smartphone was placed 30cm from the eyes such that it occluded the view of the target for one eye. Three measurements each at far and near fixations were made with the smartphone app by capturing pictures using the flash. Deviations between the two eyes at far and near fixation distance were computed by the app, and the mean magnitude and direction of the deviation were compared to the Modified Thorington Test (MTT).
The range of phoria on MTT was from -16 Δ (exophoria) to 17 Δ (esophoria) for near fixation (median: -4 Δ, IQR: 11.75 Δ), and -20 Δ to 18 Δ at far fixations (median: -1 Δ, IQR: 2.5 Δ). The app measurements for phoria at far distance closely matched MTT (r2=0.96, p<0.001), and the median of the difference between the two methods was 0.8Δ (IQR: 1.1 Δ). However, the two methods had disagreement at a clinically relevant level (> 3Δ) for the near fixating distance in 6 of the 12 subjects (median for 6 subjects: 7 Δ, IQR: 6 Δ). For the remaining subjects, the median difference between the two methods at near fixation was 1.7 Δ (IQR: 0.5 Δ) The direction of the measured phorias matched in both the methods for 10 subjects. In the case of two subjects where the directions did not match, the phorias measured by MTT were ≤ 3 Δ.
The agreement in phoria measurements at far distance between the two methods is consistent with our prior simulated strabismus measurement studies, suggesting that the smartphone app for measuring eye alignment can be adopted to measure phoria at far fixations. For near fixations, there could be a sizeable inconsistency between the two methods for some people. The causes for this discrepancy need to be further investigated. One possible reason might be accommodation changes during the test.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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