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Original Research

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Background: The purpose of this study is the evaluation of fixation disparity curve (FDC) parameters with instruments including a central fusion lock (the modified near Mallett unit) and without it (the Wesson fixation disparity card) to determine which is useful for diagnosis of symptomatic from asymptomatic subjects.

Methods: In this analytical-descriptive study, 100 students were selected randomly and divided into symptomatic and asymptomatic groups. FDC parameters were determined with the Wesson card and the modified near Mallett unit for each subject and compared in symptomatic and asymptomatic groups. Data were analyzed by Wilcoxon, Mann-Whitney U, Spearman correlation coefficient and Chi-square tests.

Results: The mean slope, fixation disparity, and associated phoria (x-intercept) with the Mallett unit and the Wesson card were significantly different in symptomatic and asymptomatic group (p< 0.001). Significant correlation was obtained between symptomatic and asymptomatic subjects in fixation disparity, associated phoria and slope with the Mallett unit and the Wesson card (p< 0.001). Also there was a significant difference in the distributions of fixation disparity curve types in the two symptom groups with the Mallett unit (P=0.01) and the Wesson card (p= 0.002) by Chi-square test.

Conclusions: The associated phorias (x-intercepts) were on average displaced the base-in direction, y-intercepts (point at which the FDC crosses the y-axis) were shifted in the exo direction. The slopes were steeper with Wesson FD card compared with the modified near Mallett unit but this may be related to the design of two devices. Also, these differences were more significant in symptomatic groups.


This is an original research manuscript. It has not undergone peer review.