Measuring near phoria with prism dissociation methods. Are the results interchangeables?

Compartir 20/02/2015 Publicaciones Científicas

J.C. Ondategui-Parra1, R. Borràs1, A. Sánchez-Magan2, M. Sánchez-Soler2, S. Gómez-Lopez2, J. Pujol2

1 University Vision Center (CUV) -Universitat Politécnica de Catalunya (Barcelona Tech), Terrassa, Barcelona, Spain.
2 Centre for Sensors, Instruments and Systems Development (CD6) - Universitat Politécnica de Catalunya (Barcelona Tech), Terrassa, Barcelona, Spain.


To compare the values of near phoria measured with the clinical Alternating Cover Test (CACT) commonly used in clinics, objective method for the patient and subjective method for the examiner, with the objective method using the Alternating Cover Test Technique (OACT) implemented in a prototype of a new fully autonomous and automated vision analyzer (Eye and Vision Analyzer, EVA, DAVALOR, Spain). In this latter one eye movements are recorded while the patient watches a true3D short video game, with a precise stimulation of accommodation and vergence.


55 healthy subjects with no previous history of strabismus or amblyopia, no ocular pathology, and no history of eye surgery were enrolled in the study. All eyes achieved a visual acuity equal o higher to 0.00 logMAR. The CACT method was the commonly used in clinics consisting of alternating occlusion of each eye every 2 seconds and measuring the deviation angle with a prism bar in prismatic diopters (PD). The phoria value was defined as the mean value obtained with the maximum and minimal limit technique of the compensation ocular movement. The OACT method was implemented in the prototype of EVA device showing the video game only in one eye during 2 seconds in an alternant manner. The procedure was repeated 5 times. Phoria values were obtained as the deviation angle before the occlusion of the other eye recorded by the eye tracker. All measures were done at near vision (40 cm) and the visual acuity for the test stimuli was 0.2 logMAR. Three measurements were performed for each method.


The mean age (mean ± standard deviation) of the sample was 21.5±1.5 years (range: 19 to 24). The mean phoria values obtained were 1.0± 3.8 PD for CACT and 2.3± 3.4 PD for OACT. The mean value of difference between methods was 1.3±2.2 PD and was statistically significant (p<0.01). The Bland and Altman plot shows a confidence interval at 95% between 5.57 and 3.03 PD. The Pearson Correlation Coefficient between both methods was 0.82 and the Intraclass Correlation Coefficient (ICC) was 0.90, therefore, the strength of agreement is very good.


The EVA prototype is a useful device to use the Alternating Cover Test procedure to measure phoria. The results obtained with EVA were similar to the results with the method used commonly in clinics. Despite it is statistically significant; it is not clinically significant, because the difference is lower than 2PD.

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Juan Carlos Ondategui

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