Agreement of Fixation Disparity Curve between Two Different Instruments

Abstract

SIGNIFICANCE: Fixation disparity tests with various characteristics of fusional stimulus are very important for assessment of decompensated heterophoria. The results suggest that there was no reasonable agreement between the fixation disparity curve's parameters of the modified near Mallett unit and the Sheedy disparometer. PURPOSE:The purpose of this study was to determine the agreement of the fixation disparity curve parameters between the modified near Mallett unit and the Sheedy disparometer in patients with decompensated near heterophoria.METHODS: A total of 147 young adults (mean age, 22.7 ± 4.8 years) participated in this cross-sectional study. After applying the exclusion criteria, the statistical analysis was done on the data of 134 individuals. All participants underwent preliminary optometric examinations including the measurement of uncorrected and best-corrected visual acuity, objective and subjective refraction, and unilateral and alternating cover tests. The fixation disparity was evaluated using the modified near Mallett unit and the Sheedy disparometer at 40 cm, and forced-vergence fixation disparity curves were generated.RESULTS: There were statistically significant differences in the fixation disparity curve parameters (except the center of symmetry) between the two devices. The median fixation disparity measured by the Sheedy disparometer was more positive compared with the modified near Mallett unit (toward more esodisparity or less exodisparity). The median associated phoria measured by the Sheedy disparometer was more positive compared with the Mallett unit. Also, the slope of the curve obtained by the Mallett unit was steeper. The wide limits of agreement indicated the poor agreement of all fixation disparity curve parameters between the two instruments.

Topical Review: Methodological Variables in Clinical and Laboratory Measurements of Fixation Disparity

“…54 Different devices often result in different fixation disparity measurements. [69][70][71][72][73][74][75][76][77][78][79][80][81] These differences may result because of variations in the location of the fusional stimulus, the method by which dichoptic separation is achieved, the strength of the accommodative stimulus provided by the fusion lock, and the likelihood of vergence adaptation during the measurements. It seems best to make measurements over time on a particular patient with only one fixation disparity measurement device.…”

“…Results from these devices have been compared with one another in several studies, [69][70][71][72][73][74][75][76][77][78][79][80][81] and in many cases, a lack of agreement between devices has been demonstrated. Studies in which results from the Sheedy disparometer and the Mallett unit have been compared have all reported a lack of agreement.…”

“…Studies in which results from the Sheedy disparometer and the Mallett unit have been compared have all reported a lack of agreement. [70][71][72]81 Results from comparisons of the Sheedy disparometer and the Wesson card have, in some cases, matched, 72,74 but in other cases, results from the two devices did not agree. [75][76][77] A comparison between the results from the Saladin card and the Sheedy disparometer failed to show agreement, 78 nor was agreement demonstrated in a study comparing the results from the Wesson and Saladin cards.…”

“…All the devices were deemed repeatable except the Sheedy disparometer. 90 In a recent article, Hashemi et al 81 compared forced vergence fixation disparity curves generated with the modified Mallett unit with those of the Sheedy disparometer in a group of subjects with decompensated heterophoria. A decompensated heterophoria is one that results in symptoms or, in some cases, foveal suppression.…”

“…These results were not predicted from those of Wildsoet and Cameron 70 and Debysingh et al 89 but do not necessarily disagree with other studies in which fixation disparity measurements from different devices were compared. 81 Hashemi and colleagues 81 suggested that the steeper slope obtained from the Mallett unit may have resulted from the fact that adjustment of the Nonius lines could occur more quickly with the modified Mallett unit compared with the Sheedy disparometer. If the Nonius line settings require more time as with the Sheedy disparometer, then this would allow more time for slow disparity vergence to build up.…”