The use of distance stereoacuity assessment in determining the effectiveness of minus lenses in intermittent exotropia
Minus lens treatment has been advocated for intermittent exotropia to prevent progression and has been found to be effective in improving the control of the deviation in the distance. However, the effectiveness of this treatment is difficult to ascertain because of the lack of standardized outcome measures. Distance stereoacuity has been reported to be a reliable assessment of control of intermittent exotropia. In this study, its role as an outcome measure in the efficacy of minus lens treatment was examined by the use of the Frisby-Davis Distance stereotest.Patients (n = 24) with intermittent exotropia participated. Their distance stereoacuity was tested with the Frisby-Davis Distance stereotest at baseline and with varying minus lens powers (of -1, -2 and -3 D) that were tested randomly, as were the distance binocular visual acuity and angle of deviation.Varying minus lens powers had a significant effect on the angle of deviation and binocular visual acuity but not distance stereoacuity. However, both measures of binocularity, distance stereoacuity, and binocular visual acuity tended to diminish with the stronger minus lens power of -3 D. Participants actually demonstrated difficulty in accommodating through the stronger lenses.Distance stereoacuity (and binocular visual acuity) cannot reliably be used to determine the optimum minus lens strength that could be used in these patients to reduce the angle of deviation and regain binocularity in the distance. An additional observation was made that stronger lenses should perhaps be avoided to prevent binocular and visual discomfort.