[Application of modified fusion convergence index in intermittent exotropia]

Objective: To investigate a modified fusional convergence parameter-total convergence amplitude/distance angle and its relationship with exotropia control, stereoacuity, and other visual functions in intermittent exotropia. Methods: The cross-sectional study included children diagnosed with intermittent exotropia at the First Affiliated Hospital of Nanjing Medical University from August 2020 to June 2021. A modification was made by combining total convergence amplitude using synoptophore and distance angle at distance using prism bars to calculate total convergence amplitude/distance angle. Exotropia control at distance and near measured by Office-based Scale for Assessing Control was classified as good control (scale 0-1) and poor control (scale 2-5). Statistical analysis was performed using Spearman correlation analysis, Mann-Whitney U test, Fisher's exact test, χ² test, logistic regression analysis, and mediation effect analysis. Results: The study included 212 patients, of which 105 (49.5%) were male and 107 (50.5%) were female. The median (interquartile range) age was 9.0 (8.0, 10.0) years. Of 211 cases, 201 (94.8%) had binocular fusional function, while 11 cases (5.2%) did not have binocular fusional function. Among patients with binocular fusional function, inverse correlation was observed between total convergence amplitude and exotropia control scores for both distance (r=-0.427, P<0.001) and near (r=-0.194, P=0.006). Total convergence amplitude/distance angle was an independent predictive factor for exotropia control at distance (OR=0.195; 95%CI, 0.060-0.630; P=0.006) and near (OR=0.252; 95%CI, 0.085-0.746; P=0.013). Stereoacuity at distance (OR=3.110; 95%CI, 1.311-7.379; P=0.010) and near (OR=2.780; 95%CI, 1.401-5.517; P=0.003) were also factors associated with distance exotropia control. Mediation analysis revealed that stereoacuity was not a mediating factor between the ratio and distance control (distance: P=0.066; near: P=0.181). In patients with ratio≥1.5 °/PD, all the 15 patients demonstrated good control. On the contrary, patients with ratio<1.5 °/PD showed worse exotropia control (distance: P=0.001; near: P=0.040) and larger angles of deviation (distance: P<0.001; near: P<0.001). Conclusion: The modified fusional ratio, total convergence amplitude/distance angle, combining synoptophore and prism bars, could be used to evaluate the severity of intermittent exotropia. A higher ratio may be associated with poorer exotropia control. Though it may also be associated with distance exotropia control, stereoacuity is not the mediating factor between the modified ratio and distance exotropia control.