Purpose: To determine the most accurate method of strabismus measurement in patients with constant manifest esotropia.
Methods: Patients underwent routine cover test assessment with the cover-uncover test and the alternating cover test with any increase in the angle of deviation being noted between the two methods. The simultaneous and alternating prism cover test was then performed to see if a qualitative increase in angle deviation was represented by a quantitative measurement. Patients also were assessed for the presence of binocular vision in free space using Bagolini striated lenses and motor fusion responses to base out prisms.
Results: There was a significant difference between the alternating and simultaneous prism cover test measurements for all patients (mean difference for near fixation: 4.72 prism diopters [delta]; 95% confidence interval [CI], 1.68-7.75 delta; df=51; P<.001). For patients who showed a qualitative change in angle based on observational judgement using the cover tests, the mean difference in angle measured by simultaneous and alternating prism cover test for near fixation was 7.42 delta (95% CI, 5.75-10.30 delta; df=25; P<.001).
Conclusion: To obtain an accurate interpretation of the size of manifest esotropia, the angle of deviation needs to be measured by the simultaneous and alternating prism cover test when an observable difference is noted on the cover test. The presence of a latent component to a manifest deviation should prompt the investigation of binocular reflexes even if the manifest angle is too large to be commensurate with binocular vision.