Differences in the stability and amount of postoperative exodrift with age after unilateral lateral rectus muscle recession and medial rectus muscle resection of intermittent exotropia

We investigated variances in the stability and amount of postoperative exodrift among age groups of intermittent exotropia (XPT) patients who underwent unilateral lateral rectus muscle recession and medial rectus muscle resection. We analyzed the cases of 110 consecutive patients who underwent the surgery in 2004-2011, dividing the patients into groups by their age at surgery: < 10, 10-19, and ≥20 years. We performed a regression analysis (dependent variable: postoperative exodrift (°); independent variable: number of days post-surgery) using the formula of curve lines. When the tangent line slope was = 0.01 (°/days) for each group, we defined the numbers of days until alignment became stable as the 'stable days.' We evaluated the between-group differences in the amount of exodrift calculated for the stable days. The coefficients and coefficients of determination for the fitting curves were: < 10 year group: f(x)=12.2 (1-e^-0.0183x) (r²=0.588, p < 0.05); 10-19 year group: f(x)=10.0 (1-e^-0.0178x) (r²=0.453, p < 0.05); =20 year group: f(x)=3.40 (1-e^-0.0382x) (r²=0.217, p < 0.05). There were 389, 388, and 153 stable days, and the estimated postoperative exodrift with long-term follow-up was 11.5±3.7°, 9.3±4.4°, and 4.1±3.6° for the < 10 year, 10-19 year, and ≥ 20 year groups, respectively (≥20 year vs. other 2 groups, p < 0.05). Longer periods and more postoperative exodrift were associated with younger age at surgery. The postoperative evaluation was approx. ≥1 year post-surgery in patients aged < 20. These findings may contribute to evaluating XPT's success rate and prognoses.