Long-term astigmatism changes following horizontal muscle recession: a prospective cohort study

Strabismus surgery may cause refractive changes, particularly in astigmatism. The aim of this study is to evaluate those changes in astigmatism two and 12 months following unilateral recession of horizontal rectus muscles in children. The authors prospectively evaluated 66 children with esotropia or exotropia, that would undergo a unilateral recession strabismus surgery. Comparisons were made between the 66 eyes that would undergo strabismus surgery and the fellow unoperated 66 eyes of the same children. The 66 eyes that would undergo strabismus surgery were divided into medial (38 eyes) and lateral (28 eyes) rectus muscle subgroups, and further, into subgroups based on the astigmatism axis preoperatively (with-the-rule astigmatism: 35 eyes, no astigmatism: 20 eyes, oblique astigmatism: 10 eyes, against-the-rule astigmatism: 1 eye). All patients were examined one day preoperatively, and then, two and 12 months postoperatively. Paired tests were conducted, and the significant level was set to 0.05 or was adjusted for subgroups. Mean age of children included was 6.73 years (SD = 3.19). Mean astigmatism values preoperatively, 2 and 12 months postoperatively were 0.92D (SD = 0.95), 1.45D (SD = 1.04) and 1.50D (SD = 1.10), respectively, for the eyes that underwent strabismus surgery. A statistically significant mean increase of 0.58D in astigmatism values in the eyes that underwent strabismus surgery was observed 12 months postoperatively (p < .005). Astigmatism values in the eyes that did not undergo strabismus surgery did not statistically significantly change during the observation period. The increase of the absolute values of astigmatism in medial and lateral rectus muscle subgroups was similar, 0.59D (SD = 0.10) and 0.57D (SD = 0.11), respectively. For the eyes that had with-the-rule astigmatism and no astigmatism preoperatively, a statistically significant increase was shown 12 months postoperatively (0.64D and 0.66D respectively) (p < .005). Changes in astigmatism were observed in the eyes which underwent recession of horizontal rectus muscles compared to the fellow eyes, which did not undergo any intervention. An increase in cylindrical power was noted in the eyes that had with-the-rule and no astigmatism prior to surgery. This increase may be interpreted by the decreased tension of the recessed rectus muscle following strabismus surgery. Decreased forces, caused by the recessed horizontal rectus muscle, acting on the sclera on 180-degree meridian may lead to corneal flattening on this particular meridian and consequently, a corneal steepening on the 90-degree meridian. These changes seem to be stable during the first 12 postoperative months.