An automated and objective cover test to measure heterophoria

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.隐斜视（Heterophoria）是指双眼在缺乏融合性集合（fusional vergence）时的相对偏斜。融合性集合可被人为剥夺，例如在一眼注视视标的情况下遮盖对侧眼。此时，被遮盖眼将偏离初始位置，其偏斜量与隐斜视程度相符。临床实践中对隐斜视的评估，对于诊断集合不足（convergence insufficiency）等非斜视性双眼视功能异常至关重要。传统临床检测方法，如遮盖试验（cover test）或改良Thorington试验（modified Thorington test），存在检查者主观性强、无法观测被遮盖眼，或需特殊观测条件等局限。采用眼动追踪系统（eye-tracker）可客观测量隐斜视，从而克服上述局限。本研究的核心目的为：对比使用眼动仪测量近距离隐斜视的自动化客观方法，与两种常规临床方法——遮盖-去遮盖试验（cover-uncover test）及改良Thorington试验的检测性能。该眼动追踪方法可通过两种路径测量隐斜视：其一以被遮盖眼的偏斜量作为指标（模拟遮盖试验的检测逻辑），其二同时测量被遮盖眼与注视眼的偏斜量（契合隐斜视的理论定义）。后者所得检测结果较前者更小，尽管平均差异在临床层面通常不具备显著意义。所提出的客观检测方法的重复性显著优于两种常规临床方法，且与改良Thorington试验的一致性优于遮盖-去遮盖试验；两种常规临床方法之间的一致性水平则较为相近。综上，使用眼动仪测量隐斜视可获得客观且重复性更佳的检测结果。随着眼动仪逐步成为临床环境中的常用工具，采用其测量隐斜视有望成为新的临床金标准。