The result of this study showed that patients exhibiting fast exodrift after the first surgery were not more likely to exhibit fast exodrift after the second surgery. Patients with exotropia usually experience postoperative exodrift over time. The rate of postoperative exodrift varies among patients, with some exhibiting faster exodrift than others [5,6,7]. Park and Kim  reported drift rates over 12 months of postoperative follow-up; they indicated that drift rate was fastest at postoperative weeks 1–3, and showed the strongest correlation with overall drift rate. Additionally, previous studies [8, 9] showed that more than one-half of the total amount of postoperative exodrift occurred during the first postoperative year. Therefore, fast postoperative exodrift would occur relatively early period after the surgical treatment of exotropia. Those results are consistent with the results of the present study, which showed that mean deviation observed at postoperative month 6 in group F was approximately one-half of the mean preoperative deviation.
If fast exodrift occurs after surgery, a second surgery to restore ocular alignment can be considered. Whether patients with recurrent exotropia who had experienced fast exodrift will exhibit fast exodrift after the second surgery is of concern; however, a search of the literature did not reveal any study that evaluated the surgical outcomes of recurrent exotropia after the second surgery according to the rate of exodrift after the first surgery. This study investigated the surgical outcome after the second surgery in patients with recurrent exotropia according to the rate of exodrift after the first surgery. The patients with recurrent exotropia were divided into two groups according to rate of exodrift after the first surgery. The fast exodrift after the first surgery was defined as more than 10 PD at 6 months after the first surgery. This value was selected because most studies evaluating surgical outcome of exotropia use 10 PD as the reference in their definition of successful alignment [2,3,4, 10]. Because patients with recurrent exotropia may exhibit exodrift over time after the second surgery [2, 3], this study included patients who were followed up for at least 24 months postoperatively to investigate long-term surgical outcomes after the second surgery and compare these outcomes between patients with fast and slow exodrift after the first surgery.
In this study, there was no significant difference in the surgical outcome after the second surgery according to the rate of exodrift after the first surgery. Even when fast exodrift occurred after the first surgery, similar surgical outcomes were not necessarily seen after the second surgery. The results of the present study indicate that a second surgery may be considered for recurrent exotropia even though fast exodrift occurred after the first surgery. We suspect that both motor and sensory improvements after surgery were the reason that no significant differences in surgical outcomes were seen after the second surgery. From a motor perspective, the mechanical force from the resected medial rectus muscle in both eyes is considered one of the reasons for result of this study . Kim and Kim  reported that the clinical course after a second surgery for recurrent exotropia was improved compared with the clinical course of both recurrent exotropia after the first of two surgeries and exotropia after a single surgery. From sensory perspective, even though fast exodrift occurred after the first surgery, it was assumed that there would be an improvement in the fusional ability compared with that before surgical treatment. Previous studies revealed an improvement in binocularity after surgical treatment in patients with exotropia, even constant exotropia [13,14,15,16,17]. These improvements in both motor and sensory aspects might lead to similar exodrift outcomes after the second surgery between the two groups.
Univariate analysis of the associated factors related to fast exodrift after the first surgery revealed its association with immediate postoperative deviation. This is consistent with a previous studies, which reported that the rate of exodrift correlated with the initial postoperative overcorrection [6, 7, 10]. However, the result of this study might be interpreted differently from previous studies, because previous studies evaluated exodrift in patients with exotropia who underwent a single surgery. This present study included only patients with recurrent exotropia who underwent a second surgery. The immediate deviation after the first surgery reached a statistical difference between the two groups, but these differences are very small from a clinical perspective. In group S, the mean deviation 6 months after first surgery progressed to 6.3 PD, only one-half of that in group F, but all included patients underwent a second surgery. Therefore, immediate postoperative deviation shows an association with fast exodrift after the first surgery, but it may not guarantee good surgical outcome over the long term.
The present retrospective study has some limitations. The mean interval between the first and second surgeries differed for each group, likely because fast exodrift after surgery can lead to earlier consideration of a second surgery. This study only included patients who underwent an R&R procedure as the first surgery. Another common surgery for the treatment of exotropia is the bilateral lateral rectus recession (BLR) procedure, and exodrift after the BLR procedure and its effect on surgical outcome after second surgery should also be determined. In addition, all surgeries were performed at the same institution and by the single surgeon. A future prospective study based on multiple institutions with a fixed interval between the first and second surgery and including both the R&R and the BLR procedure will provide more information on the clinical course of recurrent exotropia.