A- and V-pattern strabismus are unique forms of vertical incomitant strabismus characterized by a change in the horizontal deviation from the primary position to the upgaze and downgaze . The A pattern generally indicates a difference in exodeviation between the upgaze and downgaze of greater than 10 prism diopters (PD), and the V pattern indicates a minimum difference of 15 PD . Although the precise etiology and pathogenesis remain unclear, oblique muscle dysfunction is accepted as a predominant mechanism for the A and V patterns [3,4,5,6,7]. The oblique muscle dysfunction etiology indicates that A-pattern strabismus is frequently associated with superior oblique overaction (SOOA), and V-pattern strabismus is closely related to inferior oblique overaction (IOOA).
IOOA in V-pattern strabismus can be divided into two main categories: primary IOOA and secondary IOOA. In the clinic, inferior oblique (IO) weakening procedures are mainly used to treat primary or secondary IOOA and associated V pattern strabismus. For the surgical treatment of IOOA, several different techniques have been developed. Costenbader FD et al. analyzed three different IO weakening procedures and suggested that the effects of myotomy at its origin, recession at the scleral insertion and partial myectomy were similar . Ercan Ozsoy et al. studied the efficacy of IO weakening treatment in 179 patients with IOOA and showed that the cure rates of IO recession, myectomy and anterior transposition were 96%, 98.4% and 93.9%, respectively . Antielevation syndrome was found in patients with IOOA who underwent IO anterior transposition.
Several studies have focused on the effects of unilateral IO weakening surgeries on bilateral IOOA [10,11,12]. The authors found that if only the side of the IO with more severe overaction was performed, IOOA may be aggravated in the fellow eye in 36.1% of patients. Ahmed Awadein et al. studied the effects of bilateral symmetric IO myectomy on patients with asymmetric IOOA . They concluded that IOOA was significantly improved after symmetrical surgery without dysfunctional IO. However, currently reported data examining the effectiveness and safety of symmetric IO partial myectomy for V-pattern exotropia with bilateral IOOA are limited [9,10,11,12,13]. In addition, the efficacy of symmetric IO partial myectomy has not been assessed quantitatively using methodologically sound research. The purpose of this study was to evaluate the efficacy of symmetrical IO partial myectomy for the treatment of V-pattern exotropia with bilateral IOOA and to compare the effectiveness in the correction of symmetric and asymmetric IOOA.
Patients and methods
This retrospective study was conducted in the First Affiliated Hospital of Soochow University between August 2017 and August 2019. The study was reviewed and approved by the Ethics Committee of the First Affiliated Hospital of Soochow University and complied with the principles of the Declaration of Helsinki. Informed consent from a parent or legal guardian for study participation was obtained for all children. The medical records of V-pattern exotropia patients with bilateral primary or secondary IOOA, either symmetric or asymmetric IOOA, who underwent bilateral symmetric IO partial myectomy were collected. The exclusion criteria for this study are listed below: (1) patients with dissociated vertical deviation, amblyopia, anisometropia, insertion anomalies of IO muscles and ocular structural abnormalities; (2) patients with an extraocular muscle surgery history; and (3) patients with central nervous system abnormalities or other systemic disorders.
Complete ocular examinations, including visual acuity testing, intraocular pressure, cycloplegic refraction, corneal topography, slit-lamp and fundus examinations, were performed on all patients by the same ophthalmologist. The angle of deviation was measured by the alternate prism cover test under optimal corrected visual acuity at both 6 m and 33 cm in the primary position, 25 degrees upgaze and 35 degrees downgaze, respectively . To guarantee accurate data, patients were patched into either eye for more than 30 minutes prior to deviation angle examination, and the examination was performed at least 3 times. For patients with exotropia, the maximum deviation angle was recorded. The Titmus stereotest (Stereo Fly Test, Stereo Optical Company, Inc., Chicago, IL, USA) was performed at a distance of 40 cm. The Titmus stereotest detected stereoacuity varying from 40, 50, 60, 80, 100, 140, 200, 400 to 800 seconds of arc by the circle figures.
Ductions and versions were evaluated in all patients. The grades of IOOA were classified as follows: 1) If the abducting eye was fixating straight in abduction, a slight upshoot of the adducting eye was recorded as grade 1 overaction. 2) Grade 2 overaction was recorded if there was an obvious upshoot of the adducting eye when the abducting eye was in horizontal abduction. 3) A severe upshoot of the adducting eye was considered grade 3 overaction. 4) An extremely severe upshoot of the adducting eye was regarded as grade 4 overaction .
All of the patients underwent fundus photography provided by a stereoscopic fundus camera pre- and postoperatively. The patient was asked to keep the head straight and look at an internal fixation with either eye occluded. The fovea-disc angle (FDA) was examined between a horizontal line drawn through the fovea and a line connecting the fovea to the optic disc center from the good focus photograph using Auto CAD 2014 imaging software . The torsion angle was defined as “+” if the fovea was located below the optic disc center, which means extorsion. Conversely, the fovea was located upon the optic disc center, defined as “-” , which means intorsion  (Fig. 1).
Residual IOOA, collapse of the V pattern, and FDA were evaluated on postoperative day 1, day 7 and at 1, 3, 6, 12 and 24 months postoperatively. Data recorded at the final follow-up postoperatively were included in this study. The difference in correction between the FDA and IOOA was compared in each group. The successful outcome was regarded as no or a small deviation in primary position (≤ 10 pd), IO muscle function of grade 0 and the collapse of the V pattern.
All procedures were performed under general anesthesia to guarantee the safety of the operation and alleviate patients pain. In all procedures, initially, an 8 mm long standard inferotemporal fornix conjunctival incision parallel to the corneal limbus was made between the inferior rectus and lateral rectus. After Tenon’s capsule was dissected and lifted, operation of the horizontal muscles was performed. The IO was identified and isolated under direct vision using a muscle hook. Then, the surrounding fascia tissue was separated. The IO was stretched and exposed using two muscle hooks. A 3–5 mm segment of muscle from both sides was clamped with two small hemostats. Next, the IO was cut at the temporal hemostatic forceps, and the 3–5 mm segment of the IO muscle was excised from the nasal hemostatic forceps. The insertion site was burned to stop the bleeding. The muscle was released into the tenon’s capsule. Finally, the surgeon closed the conjunctival incisions with 8–0 absorbable sutures and anti-inflammatory ointments (Fig. 2).
All statistical analyses were performed using SPSS software version 25.0 (SPSS Inc. Chicago, IL, USA). Data are presented as the median, minimum (min), and maximum (max) or as percentages. Significance was tested using t tests, Mann–Whitney tests, chi-square tests, and the nonparametric Wilcoxon signed rank test as appropriate. A P value under 0.05 was considered significant.