The efficacy of superior oblique posterior tenotomy in the treatment of A-pattern strabismus without ocular intorsion: A retrospective study

Background: Superior oblique weakening is a common method to treat A-pattern strabismus. This study aim to evaluate the surgical results of superior oblique posterior tenotomy procedures to treat A-pattern strabismus patients who had bilateral superior oblique overaction without ocular objective intorsion. Methods: The records of 18 consecutive patients who underwent surgery of superior oblique posterior tenotomy close to its insertion with superior oblique overaction (SOOA)-associated A-pattern strabismus between September 1, 2015 and August 31, 2018 were retrospectively reviewed. Ocular alignment, objective torsion, A-pattern and ocular motility were assessed. Ocular alignment was measured in the primary position, 25° upgaze, and 25° downgaze using the prism bar cover test, and torsion was measured using fundus photographs. Results: A total of 18 patients (mean age: 15 years; 6 female, 12 male) underwent bilateral superior oblique posterior tenotomy and simultaneous horizontal rectus muscle surgery were included. The mean preoperative A-pattern deviation was 15 PD and the mean postoperative A-pattern deviation was 2.25 PD with a mean reduction of 12.75 PD. The mean preoperative superior oblique overaction was 2.28 and the mean postoperative superior oblique overaction was 0.43 with a mean reduction of 1.85. There was no significant correlation between the ocular torsional, vertical and horizontal alignment change and the superior oblique posterior tenotomy procedure. Conclusions: Superior oblique posterior tenotomy surgery selectively improved the A-pattern and superior oblique overaction but not affect the primary position vertical and horizontal deviation, as well as the ocular torsion. So it is an effective procedure to treat the mild to moderate superior oblique overaction (SOOA)-associated A pattern strabismus without ocular intorsion.

mean preoperative A-pattern deviation was 15 PD and the mean postoperative A-pattern deviation was 2.25 PD with a mean reduction of 12.75 PD. The mean preoperative superior oblique overaction was 2.28 and the mean postoperative superior oblique overaction was 0.43 with a mean reduction of 1.85. There was no significant correlation between the ocular torsional, vertical and horizontal alignment change and the superior oblique posterior tenotomy procedure.
Conclusions: Superior oblique posterior tenotomy surgery selectively improved the A-pattern and superior oblique overaction but not affect the primary position vertical and horizontal deviation, as well as the ocular torsion. So it is an effective procedure to treat the mild to moderate superior oblique overaction (SOOA)-associated A pattern strabismus without ocular intorsion.

Background
Superior oblique overaction is usually seen in A-pattern strabismus, and superior oblique weakening on one or both sides is helpful to solve this situation. Superior oblique posterior tenotomy (SOPT) was proposed first in 1976 to correct A-pattern deviations [1]. The operation aims to weaken the abduction and depression effect of the superior oblique muscle through partial cutting of the posterior fibers at the insertion site, and induce clinically insignificant changes in the amount of torsion by leaving the anterior fibers intact. However, it has been reported that the magnitude of correction obtained by SOPT is less than that obtained by other procedures like superior oblique tenotomy or split lengthening [2,3], and few studies have evaluated concretely the A pattern, superior oblique function and torsional changes after selectively weakening of the superior oblique [4,5]. The purpose of this study was to evaluate the effect of SOPT on A-pattern strabismus and its association with the preoperative primary position vertical and torsional deviation to provide further information for the preoperative surgical design of these patients with A-pattern and overaction of superior oblique, but not bothered with intorsion.

Methods
The clinical records of consecutive patients with A-pattern strabismus associated with bilateral superior oblique overaction from September 1, 2015 and August 31, 2018 were retrospectively reviewed, to identify patients who had undergone bilateral superior oblique posterior 3/4 tenectomy for A-pattern strabismus without ocular objective intorsion during the study period, and had at least 3 months of postoperative follow-up.
The data collected included age, sex, preoperative and postoperative prism cover test at 33cm and 6 m, A pattern, ocular movement, objective torsion and history of previous and subsequent squint surgery. An A pattern was defined as >10PD difference between up-and downgaze at 6 m by use of the alternate prism and cover test. A dilated fundus evaluation and fundus color photography were performed to evaluate objective torsion both before and after surgery. SO overaction was measured on a 9-cardinal system, from -4 to +4. Exaggerated traction test was performed before and after SO posterior tenectomy. Patients who had previous vertical or oblique muscle surgery or who previously underwent corrective surgery for A-or V-pattern strabismus were excluded.
Statistical analysis was carried out using SPSS software version 12.0 (SPSS Inc, Chicago, Illinois, USA).
A paired t-test was used to determine the statistical significance between pre-and 3 months postoperative results. P values less than 0.05 were considered statistically significant.

Results
Eighteen patients (thirty-six eyes) were included in the study. There were 6 female and 12 male patients in age ranging from 3 to 41 years, with a mean age of 15.22 years. All patients showed Apattern exotropia and bilateral superior oblique overaction without intorsion, and underwent bilateral superior oblique posterior tenectomy with horizontal strabismus surgery.

A Pattern
Surgery was successful in collapsing the A pattern in all the patients. The mean A patterns were 15±3.87PD preoperatively (range, 10PD-22PD) and 2.25±2.73PD postoperatively (range, 0PD-8PD) ( Figure 1). There was 12.75PD (85%) reduction in the size of the A pattern postoperatively.

Objective Torsional Deviation
Based on the fundus examination, the mean preoperative extorsion was 5.97° ± 2.45° and postoperative extorsion was 5.8° ± 1.35° (Figure 3). No patient who underwent SO posterior tenectomy showed significant fundus torsion change or complained of torsional diplopia after surgery.

Primary Position Deviation
All the 18 patients showed primary position exotropia without vertical deviation at presentation, and had simultaneous surgery on their horizontal rectus muscles for exotropia. The mean exodeviations were 33.11 ±13.21PD preoperatively (range, 8-50PD) and 4.11±2.03PD postoperatively (range, 1-8PD). None of the patients showed postoperative primary position vertical deviation.

Complications
In our series, no complications such as consecutive superior oblique palsy, postoperative V pattern, or damage to the superior rectus muscle, were encountered.

Discussion
Our study recruited 18 patients with an overall A-pattern of 15 PD for whom a correction of 12.75PD (85%) was achieved. The mean SO overaction grade decreased from 2.28 before surgery to 0.43 after surgery. These were statistically significant change. No induced vertical deviations were seen in those patients and no patient showed significant fundus torsion change or complained of torsional diplopia postoperatively. This study suggests that SO posterior tenectomy effectively collapses mild to moderate A-pattern deviation and SO overaction, but not affect the primary position vertical and horizontal deviation, as well as the ocular torsion. Several techniques have been described to weaken the superior oblique muscle, including superior oblique tenotomy or tenectomy, recession, posterior tenectomy and tenotomy with a prosthetic spacer. The main complications associated with complete tenectomy and tenotomy are that it may lead to iatrogenic superior oblique palsy, conversion of an A -pattern to a V -pattern and induced excyclotorsion [6,7]. Superior oblique posterior tenotomy leaving the anterior fibers intact will result in selective weakening of the abduction in downgaze while preserving its intorsion, and the risks of inducing superior oblique palsy or V-pattern were rare. Therefore this should be the good choice for A pattern patients with superior oblique overaction but no intorsion.

Conclusions
In our study, bilateral superior oblique posterior tenectomy proved to be an effective treatment for mild to moderate A-pattern and superior oblique overaction, without causing torsional deviation

Consent for publication
Not applicable.

Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding Preoperative and postoperative objective torsional deviation in patients who underwent bilateral SO posterior tenectomy (mean and SD).