Comparison of fixation disparity measured by saladin card and disparometer

The purpose of this study was to compare the fixation disparity (FD) measurements taken with the Saladin Near Point Balance Card (Saladin card) to those made with the Sheedy Disparometer, and to determine if the same clinical norms used with the Disparometer can be applied to the newer Saladin card.

Subjects were 44 optometry students (aged 23 to 34 years) without strabismus, amblyopia, or asthenopia during near work. They were randomized to begin at one of three examiners' stations: dissociated near phoria using Modified Thorington card; FD with Saladin card; and FD with Disparometer. Subjects proceeded to each station in turn. FD was measured for each subject through forced vergence demands of 0, 3, 6, and 9 base-in (BI), and 3, 6, 9, 16, and 20 base-out (BO), alternating BI and BO demands. Examiners were masked to subjects' results from the other stations.

FD curve (FDC) types were the same with the two instruments in most cases. However, statistically significant differences were found for FDC slopes (p = 0.048), y intercepts (p < 0.0001), and FD values through BI prisms (p < 0.0001), with the Disparometer finding the FD to be more eso/less exo than did the Saladin card. FD values through BO prisms showed no statistically significant differences but great variability.

The two instruments generally produce similar types of FDCs. However, the Disparometer tends to yield more eso/less exo FD measurements compared with the Saladin card. Although the newer Saladin card frequently produces FDC slopes and y intercepts within the expected range (as published for the Disparometer) for asymptomatic subjects, slopes and y intercepts obtained by the Saladin card are not sufficiently similar to those obtained by the Disparometer to warrant use of the same norms. Further study is needed to establish appropriate norms for the Saladin card.