As in previous studies, a significant myopia shift in spherical equivalent could be observed after strabismus surgery[3,5,10], A major disagreement among previous publications regarded the duration and therefore clinical significance of the refractive change. While some investigators concluded that the refractive error change was transient [3, 5], others reported a long lasting clinically significant refractive change[11]. At present study agreed that the SE showed a myopia shift, and, it was not just a transient change,we could observe a long term and irreversible change in unilateral medial rectus recession. But, the results of this study suggested that the duration of SE changes was different among horizontal rectus surgery, which were inconsistent with S. W. Hong[12]. And Nardi and colleagues[13] found that the increase of postoperative astigmatism of patients with medial rectus muscle recession was greater than that of patients with lateral rectus muscle recession. Agree with Nardi, this study supports this conjecture that the effect of medial rectus is more lasting and the change of it has a greater impact on the eyes. Rajavi[3] concluded that due to the medial rectus was closer to the corneal limbus, and which had more power and effect on the adjacent sclera and cornea. As we know, The medial rectus muscle is nearest to the corneal limbus, so the strength is relatively strong. Besides that, the blood vessels of extraocular muscles are different, only 1 muscle rectus arterial supplies lateral rectus and medial rectus has two. So we infer the medial rectus muscle had much more influence on eyes. There were also proposed some other theories of refractive changes that involved the healing of scleral wound[14], orbital and eye lid edema[15], and the change of ciliary body circulation[4].
Similar to previous studies[2,3,12], we found a significant change in astigmatism towards to with-the-rule direction just in unilateral medial / lateral rectus recession, and it lasted for at least three months after surgery. The current study[5] found the corneal power of horizontal meridian became flat, and the vertical meridian became steeper one week after surgery, followed by the corneal power of vertical meridian one month after surgery. In this present study, we found flattening in corneal power of horizontal meridian continued until at least 3 months after operation only in the unilateral medial rectus recession, and induced steepening in vertical meridian until 1 week after surgery in all horizontal muscle surgery ( group I / II / III ). An important contributing mechanism was the influence of extraocular muscle tension on corneal topography[4]. The sclera was more malleable than the cornea, which maked it more prone to distortion due to pressure exerted by the movement of extraocular muscles[16], and the strength of reattached muscles was transmitted to the cornea through the sclera[2], the reduction in tension of the recessed extraocular muscle transmitted via the sclera to the cornea caused a decrease in corneal curvature[17]. And on the contrary, we found it induced steepening in vertical meridian. But, We think further research is needed to explain the reason of steepening in vertical meridian of this research.
Some investigators[15] believed that resection of rectus muscles had much lower influence than does recession on refraction. But in this study, there was short change in monocular lateral rectus recession and medial rectus resection after surgery, and it might be due to the effects of compensation between the surgical lateral and medial rectus muscle. And we found an obvious change in the best corrected vision acuity 1 week after surgery only in group III, we speculated that might be because of the greater number of muscles involved in the operation at the same time, whica could lead to more pronounced conjunctival or eyelid edema, more severe postoperative pain, and ultimately to a transient reduced vision. There was no statistical change in refraction and anterior segment parameters in disinsertion of the inferior oblique, so We considered that the anatomical position of the inferior oblique muscle was in the deep, so it showed little influence.
However, this study was limited by its retrospective design, the short follow-up period, and without self control groups. These factors might have influenced some results, and future studies are warranted to confirm the results of this work by addressing these limitations.