The threshold for the detection of strabismus

To identify the threshold at which horizontal or vertical strabismus becomes reliably detectable by observers and to determine the effects of interpupillary distance, age, gender, and observer experience. Six models of different gender, age, and interpupillary distance were digitally photographed in several predetermined gaze positions off-axis in the horizontal and vertical planes. Standardized distance, zoom factor, and lighting were used. The images were digitally altered to exactly superimpose one eye deviated and one eye aligned with the axis of the camera. This simulated horizontal and vertical strabismus ranged from 2.5 to 20 prism diopters (PD). The images were arranged in random order and presented to groups of lay and professional observers, and their responses were recorded. The statistically significant threshold for detecting esotropia, exotropia, and hypertropia was 12.5 PD. Hypotropia had a higher threshold of 20 PD. Observer experience and model age each had a significant effect on the ability to detect strabismus. Our study demonstrates a unique method for assessing the significance of different types and degrees of strabismus. Our findings may be used to help patients with strabismus as they consider others' perception of their ocular misalignment. Physicians can also use this information in making decisions regarding surgery.