A comparative study of a subjective heterophoria testing with a phoropter and trial frame among health science students at University of Limpopo, South Africa

Abstract

Background: There are several clinical techniques for the subjective measurement of heterophoria. In South Africa, von Graefe is one of the most commonly used techniques to quantify heterophoria using the phoropter. However, most rural community clinics have trial frames rather than phoropters to perform heterophoria measurements and other clinical tests. Heterophoria or phoria is the misalignment of an eye that occurs when binocular sensory fusion is blocked. The distance heterophoria is determined by the tonic vergence resting state and negative accommodative vergence. In distance vision, normal heterophoria is zero. The tonic vergence resting state is the vergence angle dictated by tonic vergence innervation alone. However, during a near heterophoria test, the vergence angle observed involves multiple innervational factors. Blocking binocular fusion eliminates disparity vergence innervation. Because of the dual interaction, the loss of disparity vergence innervation initiates simultaneous changes of accommodation innervation. Purpose: The purpose of the study was to investigate the agreement of von Graefe heterophoria measurement using the phoropter and a trial frame. Setting: The study was conducted at an Optometry Clinic, University of Limpopo, South Africa. Methods: Distance and near horizontal and vertical heterophoria measurements were performed on 88 visually-normal university students using the phoropter and a trial frame. The 95% limits of agreement were compared using the exact Bland-Altman statistical test. To measure the horizontal heterophoria, 12 prism base-in was placed before the right eye and 6 prism base-up before the left eye. The prism in front of the right eye was reduced until the participant reported that the two images were vertically aligned. The vertical heterophoria was measured by reducing the prism in front of the left eye until the participant reported that the two images were horizontally aligned. Zero deviation was recorded as ortho or orthophoria. Results: For distance horizontal heterophoria, the Von Graefe values were 0.39±2.0 and 0.38±1.8Δ with the phoropter and trial frame, respectively. The mean near v horizontal heterophoria were 3.69±3.3 and 4.13±3.27Δ with the phoropter and trial frame. There were no significant differences between the mean heterophorias measured using the phoropter and the trial frame, p ˃ 0.05. For the vertical heterophorias at distance and near vision, the means were close to orthophoria. The mean differences and limits of agreement showed good agreement of Von Graefe test using the phoropter and trial frame. Conclusion: Measurement of Von Graefe testing with the phoropter and trial frame showed a high level of agreement for both distance and near vision performed through the phoropter and a trial frame. For clinical and research purposes, the phoropter and trial frame can be used interchangeably for measuring heterophoria. Keywords: heterophoria, phoropter, trial frame, von Graefe, prism