Mesuring near phoria with prism dissociation methods. Are the results interchangeables?J.C. Ondategui-Parra1, R. Borràs1, A. Sánchez-Magan2, M. Sánchez-Soler2, S. Gómez-Lopez2, J. Pujol2

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

Introduction: Phoria or heterophoria is a latent deviation of de visual axes of the eyes manifested in the absence of all stimuli of fusion. It is important to measure phoria in an optometric examination to diagnose any binocular dysfunction. There are several methods for measuring phoria and several factors which influence in the result such as the dissociation system, the distance, the prism change speed, the stimuli and the instructions to the subject. Our aim was to study the inter-examiner and intra-observer repeatability of the measurement of near lateral phoria by the prism dissociation method, modifying optotype design and dissociation external system. In addition, this study was also used to determine the best method to measure these dysfunctions by means of the new prototype based on Head Mounted Display (HMD) fully autonomous system (figure 3).

Results:

Conclusions: 1.  The agreement of the measurement results was good between the three methods for the repeatability. 2.  The agreement of the measurement results was low for reproducibility of the M1. 3.  The worse inter-examiner and intra-observer repeatability was for the phoropter and a vertical line of letters method. 4.  If the vertical line of letters is replaced by a single letter, the reproducibility of the test is improved considerably.

Figure 1: Example of phoropter

measurements.

Figure 4: Error bars of the measurements between the examiners

Methods: A group of 54 subjects underwent measurements of phoria at near vision. All subjects met the criteria for inclusion in the study, they wore adequately compensated refractive error and signed informed consent. Precise instructions were given to the subjects before the performance of the measures. The lighting was controlled and adequate for each test. There was good control of accommodation and tests were made at 40 cm. We have divided the study in 3 different methods in the same group of subjects. In each method have changed the optotype design and dissociation external system. Three measurements of phoria were obtained for all methods.

Phoropter and Vertical line of letters (M1)

Figure 2: Phoria measurement with prism

bar and single letter

•  External Risley prism was used in the phoropter (figure 1), with 15 PD BI in RE and 8 PD BU in LE.

•  Horizontal prismatic power was changed at a rate of 2 DP/s in RE until the subject indicated the first alignment.

•  The optotype used was a vertical line of letters of 0.2 logMAR.

Results: •  The analysis of variance shows for the phoropter and line of letters method (M1) a mean of -0.06 PD ± 3.53 (p= 0.898). The

Concordance Coefficient (CC) is 7.69. •  For the phoropter and a letter methods (M2) the mean is -0.24 PD ± 3.17 (p = 0.580). The CC is 6.40. •  For the prism bar method and a letter (M3) the mean is -0.46PD ± 2.69 (p = 0.218). The CC is 6.11.

REPEATABILITY REPRODUCIBILITY

Phoropter and Single letter (M2)

•  External Risley prism was used in the phoropter (figure 1), with 15 PD BI in RE and 8 PD BU in LE.

•  Horizontal prismatic power was changed at a rate of 2 DP/s in RE until the subject indicated the first alignment.

•  The optotype used was a single letter of 0.2 logMAR.

Prism bar and Single letter (M3)

•  A prism bar with 15 PD BI stood in RE and a 8 PD BU stood in the LE (figure 2).

•  Horizontal prismatic power was decreased every 2 seconds until the subject indicated the first alignment.

•  The optotype was a single letter of 0.2 logMAR held by a RAF rule.

References: 1.  Von Noorden, G.K. (any). Binocular Vision and ocular motility. Theory and management of strabismus. (6a Ed.). Florida: Mosby. 2.  Casillas, E., Rosenfiedl, M. (2006). Comparison of Subjective Heterophoria Testing With Phoropter and Trial Frame. Optometry and Vision Science. 83 (4) 237-241. 3.  Rainey, B. B., Goss, D. A. (1998). Inter-Examiner Repeatability of Heterophoria Test. Optometry and Vision Science. 75 (10) 719-726.

Design: To study the intra-observer repeatability were performed in 2 sessions of measurements by the same examiner, separated from 5 to 10 days. The method of measurement, instruments and measurement site was controlled.

Design: To inter-examiner repeatability in the same session 2 examiners performed the same tests in random order. In this study other variables were controlled: The method of measurement, instruments, conditions of use and lighting. Results: •  The inter-examiner variability for M1 is -2,67 PD ± 3.92 (p <0.05). The CC is 6.91. •  For the phoropter and a letter (M2) the inter-examiner variability is -0.34 PD ± 3.26 (p = 0.448). The CC is 6.22. •  The inter-examiner variability for M3 is -0.29 PD ±3.12 (p=0.497). The CC is 5.28.

Figure 5: Bland & Altman graph of the M1 Figure 6: Bland & Altman graph of the M2 Figure 7: Error bars of the measurements

between sessions Figure 8: Bland & Altman graph of the M1 Figure 9: Bland & Altman graph of the M3

Figure 3: Pictures of the prototype.

This study was performed in order to obtain the best optotype model and the best phoria measurement method to be included in the prototype of EVA (Eye Visual Analyzer, Davalor).

Acknowledgements: This study funded by DAVALOR SALUD.