An automated and objective cover test to measure heterophoria

Abstract

Heterophoria is the relative deviation of the eyes in absence of fusional vergence. Fusional vergence can be deprived by, for example, occluding one eye while the other fixates a visual target. Then, the occluded eye will presumably deviate from its initial position by an amount that corresponds to the heterophoria. Its assessment in clinical practice is crucial for the diagnosis of non-strabismic binocular dysfunctions such as convergence insufficiency. Traditional clinical methods, like the cover test or the modified Thorington test, suffer from practitioner’s subjectivity, impossibility to observe the occluding eye or unusual viewing conditions. These limitations could be overcome by using eye tracking systems to measure objectively the heterophoria. The main purpose of this study was to compare the performance of an automated and objective method to measure near heterophoria using an eye-tracker with two conventional methods: the cover-uncover test and the modified Thorington test. The eye tracking method gave us the possibility to measure the heterophoria as the deviation of the occluded eye (mimicking the cover test) or as the deviations of the occluded and fixating eyes (adhering to the theoretical definition of heterophoria). The latter method provided smaller results than the former, although on average the differences might not be clinically relevant. The proposed objective method exhibited considerably better repeatability than the two conventional clinical methods. It showed better agreement with the modified Thorington test than with the cover-uncover test, and a similar level of agreement was obtained between the two clinical methods. To conclude, the use of eye-trackers to measure heterophoria provides objective and more repeatable measures. As eye-trackers become common tools in clinical settings, their use to measure heterophoria should be the new gold standard.

Synchronization of a Removable Optical Element with an Eye Tracker: Test Case for Heterophoria Measurement

“…These values are larger than the intrasession repeatability reported with the EyeLink 1000 Plus 250 Hz ET, which was approximately 1 PD. 26 This discrepancy may be due to the fact that the standard deviation reported using the EyeLink was the mean of six measurements, whereas the value in this study is the mean of three measurements. It could also be due to the fact that subjects in the EyeLink experiment were restrained with a chin rest whereas subjects in the present study were unrestrained.…”

“…Future experiments can examine the agreement between ET recordings under restrained versus unrestrained conditions. Less agreement was reported for higher magnitudes of measurement by Mestre et al 26 One possible reason could be the number of participants presenting with larger magnitudes of heterophoria. Larger exophoria at near and certainly esophoric values at near viewing are less common in the population in general 35 and also in the sample included in this study, which results in larger variability.…”

“…Peli and McCormack 21 reported dissimilar heterophoria values with the covered eye only when there was a clearly dominant eye. Mestre et al 26 reported significant differences between heterophoria values measured with the right versus left eye covered, although the differences were smaller than 1 PD, which is not clinically significant. They also did not find a significant correlation between heterophoria value and the dominant eye.…”

“…Heterophoria is often defined as a relative deviation between the eyes. 26 Adopting this definition, the difference between the left and right eye position was calculated and traced. The heterophoria was calculated as the difference between the binocular eye position during the occlusion time as opposed to the binocular eye position during binocular viewing, that is, fusion, as detailed in the following equation: Where Δ Cover is the mean binocular eye position during the occlusion time that is averaged over three seconds and Δ Fusion is the mean binocular eye position averaged over three seconds of binocular viewing (fusion).…”

“…Some also did not examine the differences in heterophoria values measured when different eyes are covered, or the effect of cover duration. In a recent publication, 26 the EyeLink 1000 Plus (SR Research Ltd., Ontario, Canada) was used in combination with IR-filter and cross-polarized occluder driven by a stepper motor. The setup required a chinrest that can both introduce a burden for participants and is not as easily implemented in clinical practice, but provides increased accuracy relative to the unrestrained conditions.…”