For the surgical management of esotropia, deciding whether to perform unilateral or bilateral MR recession varies depending on the amount of deviation, type of esotropia, and the surgeon’s preference.11, 15, 16 Because of the merit of less operating time, less potential complications17 and preventing overcorrection, there have been many reports describing the success of UMR recession in patients with esotropia. Procianoy and Justo17 mentioned that esotropia of 15–35 PD were treated with 6–8 mm UMR recession and achieved 96% of surgical success without overcorrection. Stack et al.11 have reported that a 6 mm UMR recession resulted in 13.2 PD total change in deviation, and one muscle surgery was adequate for esotropia < 25 PD. Grin and Nelson18 have also reported that a UMR recession of 6–6.5 mm is a safe method for treating moderate-angle esotropia of 30–35 PD, with a success rate of 80%. However, direct application of these positive outcomes of UMR recession to PAET should be considered carefully because the main cause of surgical failure of PAET is undercorrection or postoperative drift to esodeviation.8, 19, 20
It is clear that multiple muscle surgeries are performed for the treatment of large-angle of PAET.16 However, determining the number of surgical muscles required for the treatment of small- to moderate -angle PAET remains a concern. Therefore, we aimed to investigate the efficacy of UMR recession for treating PAET ≤ 25 PD and achieving complete weaning of prism glasses. As a result, the success rate was 53.6% at 4.4 ± 3.1 years postoperatively.
The surgical success rate of UMR recession in various types of esotropia, resulting in a favorable deviation of ≤ 10–12 PD, ranges from 80 to 100%.15, 17, 18, 21 The success rate of our study is lower than that which may be attributed to the relatively long postoperative follow-up period of 4.4 years and a strict success criteria within 5 PD of phoria. Furthermore, the success rate was only 10% among the 10 patients who underwent UMR recession of ≤ 5 mm in our study. The mean surgical amount of these 10 patients was 4.85 ± 0.34 mm (range, 4–5 mm). This indicates that a sufficient amount of recession of more than 5 mm is required to manage PAET effectively.
In our study, three patients were overcorrected with consecutive exotropia of > 5 PD, who had undergone 4.5–5 mm recession for treating 14–19 PD of esotropia. In all of these patients, immediate overcorrection of more than 10 PD was found at 1 month after operation, and at the last follow-up examination, their exodeviation were 8 PD, 10 PD, and 28 PD at distance fixation, respectively. No specific risk factor for overcorrection was identified, such as surgical age at surgery, angle of preoperative deviation, period of wearing prism glasses, degree of stereoacuity, presence of amblyopia, history of preterm, and degree of hyperopia, probably due to the small number of patients. In previous reports regarding surgical overcorrection of esotropia, a history of cerebral palsy and preterm were mentioned as risk factors of surgical overcorrection.22, 23 In spite of the reduced amount of MR recession, esotropia with cerebral palsy resulted in a greater surgical response, and the significantly higher incidence of late overcorrection than patients without cerebral palsy.22 In addition, the surgical dose-response was significantly greater in preterm patients than in full-term patients with infantile esotropia.23 Common mechanisms mentioned in these studies include abnormal cortical reflexes in patients with neurological deficits, defective substrate of binocularity and muscle tone abnormality. However, authors could not find risk factor corresponding to hypothesis of previous research because there are only three patients with overcorrection in our study and none of them had any neurological deficits, developmental delays. Future studies with a large number of enrolled patients will be meaningful.
Many studies have demonstrated that stereopsis improves after strabismus surgery for PAET.16, 24 Kim and Cho24 have reported that patients with partial stereopsis increased from 20–78% after surgery, and the prognosis for stereopsis of PAET was favorable, which is different from congenital esotropia that causes consistent low stereopsis throughout one’s life. Kurup et al.16 also demonstrated favorable postoperative sensory outcomes of 100 arcsec. Unlike previous studies,16, 24 good fusional response in W4D test and degree of stereopsis in the Randot test were not significantly changed after surgery in our study. Moreover, no significant differences in binocularity or stereopsis were observed between the success and failure groups. Regarding this result, the author believes that the binocularity and stereopsis of enrolled patients were already preserved through wearing of prism glasses preoperatively. We have previously reported the efficacy of prismatic correction in PAET.1, 2 In that study, 30.6% in PAET with ≤ 20 PD achieved binocular fusion and stable alignment of ≤ 10 PD after 3 years of prism-wear. In the present study, twenty-three patients (82.1%) in our study were treated with preoperative prismatic correction for improving binocularity, and the mean period of prism glasses wearing was 27.7 ± 29.0 months. Hence, 64.3% of the patients in this study demonstrated good fusional responses with the W4D test preoperatively, and this value was higher than that in previous studies of 14.3–35%.8, 15
The period of preoperative prism glasses wearing in the success group was significantly longer than that in the failure group, indicating that preoperative prismatic correction may improve the surgical success rate. Although the patients in this study were those who could not obtain stable ocular alignment < 10 PD even with prismatic correction and eventually underwent surgery2, it can be seen that sufficient prismatic correction before surgery may contribute to improve surgical outcomes.
The mean preoperative refractive errors of + 3.09 ± 1.73 D in the success group was not statistically significant different from + 2.70 ± 2.17 D in the failure group. This indicates that the degree of hyperopia did not play a role in surgical outcomes, which is similar to those of previous studies4, 15, 16 demonstrating that preoperative refractive errors were not different among the favorable, under-corrected, and over-corrected groups.
The limitations of this study include its retrospective nature and small sample size of 28 patients. However, although our study was conducted with a small number of patients, the result of this study is valuable in that no previous studies that included a long-term observation of one muscle surgery in PAET. Additionally, as a second limitation, patients with amblyopia were enrolled. Amblyopia was treated with glasses and occlusion therapy as much as possible; however, amblyopia persisted despite sufficient treatment in three patients (10.7%). Further evaluations with more patients and strict exclusion criteria would be meaningful.
In conclusion, successful results of UMR recession were obtained in 53.6% of patients with PAET of 25 PD or less. Considering that the period of wearing prism glasses was longer in the success group, sufficient preoperative prismatic correction for maintaining binocularity can positively influence surgical success. Furthermore, because the amount of hyperopia does not affect the surgical result, modifying the amount of recession according to hyperopia may not be necessary, and full surgical correction based on the maximum non-accommodative component of the deviation may improve surgical success. Finally, sufficient amount of UMR recession of more than 5 mm is required to manage PAET effectively.