Introduction: Acute acquired concomitant esotropia (AACE) has been increased dramatically in myopia. To clarify whether fusional vergence dysfunctions exist in AACE with myopia, and to explore the relationship between fusional vergence and myopia in AACEMethods: A prospective clinical study. Adult AACE patients as well as controls (aged >= 18 years) both with myopia were prospectively recruited from October 2016 to August 2021. Refractive error and fusional divergence and convergence amplitude at distance were measured, as determined with a comprehensive refractor. The main outcome measures were the real value of the fusional divergence amplitude (calculated by subtracting the break point value from the angle of deviation) and the real value of the fusional convergence amplitude (calculated by subtracting the angle of deviation from the break point value). Two-sided p value of less than 0.05 was considered to indicate statistical significance.Results: Fusional vergence consisting of fusional divergence and fusional convergence in patients with AACE of adulthood were different from the controls (p<0.001), with fusional convergence decreased and fusional divergence increased. In the linear regression analysis, spherical equivalents were significantly negatively correlated with fusional convergence (right eyes, p<0.001; left eyes, p<0.001) in patients with AACE of adulthood but not in controls. There was a significant difference in fusional convergence amplitude among different degrees of spherical equivalents (p<0.001).Conclusions: Fusional vergence dysfunction exist in AACE of adulthood, which may be attributed to self-adaptation based on concomitant esotropia to maintain fusion in AACE. Fusional convergence was associated with degrees of myopia, implying the adaption would be weakened as the degree of myopia becomes serious.