A Comparative Study of Fusional Vergence According to Ocular Dominance in Healthy Subjects

In orthoptics, fusional vergence (FV) measurement is one of the key tests used to assess fusion ability. These amplitudes are generally reduced in individuals with convergence insufficiency. Fusional vergence are quantified by placing a prism bar randomly in front of one eye-first for distance vision and then for near vision. Rehabilitation exercises using the prism bar are also performed in the same manner. Depending on orthoptic practice, the prism bar is placed in front of the left eye or the right eye, in a dogmatic manner, or in front of the right eye, in a conventional manner, without any study providing any justification. However, some practitioners report interocular differences when measuring fusion amplitudes depending on which eye is prismed, while others do not. The hypothesis put forward is that, in individuals with an interocular difference in fusional vergence, ocular dominance (sensory and motor) could be the cause of this measurement discrepancy. The objective is therefore to compare the difference in fusional vergence depending on whether the dominant or non-dominant eye is tested, using a randomized order of eyes and tests. If this hypothesis is confirmed (greater fusional vergence when tested on the dominant eye), this would: (1) enhance our understanding of fusion mechanisms, (2) provide more targeted insights for the assessment, quantification, and rehabilitation of fusional vergence to improve the effectiveness of orthoptic intervention.