New tests of distance stereoacuity and their role in evaluating intermittent exotropia

Abstract

Purpose: Poor control of intermittent exotropia has been considered an indication for surgical intervention, and poor distance stereoacuity may be an indicator of poor control. Two new measures of distance stereoacuity, the Frisby-Davis Distance test (FD2) and Distance Randot test (DR), both of which have been validated in normal and strabismic subjects, were evaluated, and we compared stereoacuity with scores on a recently described control scale.

Design: Prospective case series.

Participants: Twenty-five consecutive patients with intermittent exotropia.

Methods: Office-based control was graded at distance and near on a 0 to 5 scale, and distance control ranged from 1 (recovery in 1-5 seconds after monocular occlusion) to 4 (>50% spontaneously tropic). Stereoacuity was measured using the FD2 and DR at distance and the Preschool Randot and Frisby tests at near.

Main outcome measure: Distance stereoacuity measured using the FD2 and DR.

Results: Measurable distance stereoacuity thresholds in intermittent exotropia were poor with the DR and excellent with the FD2 (medians, nil and 40''; P<0.0001). Near stereoacuity was excellent with both the Preschool Randot and Frisby (medians, 60'' and 60''; P = 0.99). There was poor correlation between distance control score and either FD2 (r(s) = 0.1, P = 0.6) or DR (r(s) = 0.3, P = 0.2). Control scores correlated with magnitude of deviation at distance (r(s) = 0.5, P = 0.02) and near (r(s) = 0.5, P = 0.01).

Conclusions: The real-world contour-based targets of the new distance FD2 appear to stimulate fusion in intermittent exotropia, even when distance control is poor. In contrast, the new Polaroid vectograph-based DR is very sensitive to disturbances of binocularity. Two new distance stereoacuity tests appear sensitive to opposite ends of the intermittent exotropia spectrum; FD2 performance deteriorates when the patient is constantly tropic, whereas DR performance deteriorates at the earliest stages of intermittency.

Figure 1

Box and whiskers plots of measurable distance stereoacuity thresholds (A) measured using the Frisby Davis Distance (FD2) test and Distance Randot® (DR) test in 25 patients with intermittent exotropia and near stereoacuity (B) measured using the near Frisby test and Preschool Randot® test. Center bold lines represent the median, top and bottom of the boxes represent quartiles and the whiskers represent the extreme values.

Figure 2

Relationship between distance and near control. Typical of patients with IXT, all but one patient had distance control that was worse than, or the same as, near control.

Figure 3

Relationship between control and angle of exodeviation by alternate prism and cover test at distance fixation (A) and near fixation (B). Although there was a positive correlation (distance r_s = 0.5, p=0.02, near r_s = 0.5, p=0.01), there were notable examples of patients who had the same angle of deviation by alternate cover test and either excellent or poor control.

Figure 4

Relationship between distance control score and measurable distance stereoacuity thresholds either measured using the Frisby Davis 2 (FD2) test (A) or Distance Randot® (DR) test (B). The performance on the FD2 test was generally good, but the correlation with control score was poor (r_s=0.1, p=0.6). Performance on the DR test was generally poor and the correlation with the control score was similarly poor (r_s=0.3 p=0.2).

Figure 5

Relationship between near control score and measurable near stereoacuity thresholds either measured using the near Frisby test (A) or the Preschool Randot® test (B). The performance on both tests was generally excellent, but the correlation with control score was poor with the near Frisby (r_s= 0.02, p=0.9) and modest with the Preschool Randot® test (r_s=0.4, p=0.04).