Exotropia disturbs stereopsis development, especially in children, by causing diplopia or its inhibition in the binocular vision system;thus, treatment must be administered early. However, intermittent exotropia often relapses after surgery. Although the angle between the visual lines of both eyes is correlated with relapse, no significant difference is observed in this angle between patients with and without relapse. Since the visual lines are not parallel but form an angle even in non-patients, this angle is not directly related to diplopia. Herein, we quantitatively measured the binocular parallax degree in the visual field, and verified its significance to distinguish between patients with and without relapse. In the normal binocular vision system, the 3D point gazed at is projected onto the fovea of each eye by ocular convergence. The other 3D points around the gazed point are projected onto different positions on both retinas to generate retinal images with a binocular parallax. The 3D points with sufficiently small binocular parallaxes constitute Panum’s fusional area, wherein retinal images fusion causes depth perception by stereopsis, whereas retinal images of other 3D points with larger binocular parallax are observed doubly without binocular fusion. Ocular convergence is insufficient in binocular vision systems with exotropia;thus, binocular parallax should be larger for retinal images of 3D points corresponding to Panum’s fusional area of the normal binocular vision system with sufficient ocular convergence. Thus, herein, the binocular parallax degree for both eyes with exotropia is estimated for points included in Panum’s fusional area of the normal binocular vision system gazing at different points. The estimated binocular parallax degree for upward gazing exhibits significant differences between patients with and without exotropia.
