Efficacy Analysis of Inferior Oblique Muscle Belly Transposition for Small-Angle Asymmetric Superior Oblique Palsy
Asymmetric superior oblique palsy (SOP) is a complex and non-comitant form of strabismus, often presenting with small-angle hypertropia in the affected eye accompanied by inferior oblique muscle overaction (IOOA). Patients with congenital SOP typically exhibit compensatory head posture and asymmetric facial development, while those with acquired SOP commonly experience diplopia. While prism glasses can alleviate diplopia and vertical deviation in the primary position, they are ineffective in correcting non-comitant paralytic strabismus. Moreover, vertical strabismus and compensatory head posture recur once prism glasses are removed, and their prolonged use can significantly impair vision in children. Consequently, surgical intervention, such as an inferior oblique (IO) muscle weakening procedure, is often necessary. However, conventional IO weakening techniques, including myectomy and recession, carry risks of overcorrection and secondary A pattern, particularly in patients with mild to moderate IOOA and small-angle asymmetric SOP. These complications underscore the clinical challenge of selecting the most appropriate surgical approach for this condition.
Inferior oblique muscle belly transposition (IOMBT) is a novel IO weakening procedure that has demonstrated efficacy in treating small-angle asymmetric SOP while minimizing the risk of overcorrection and secondary A pattern. This study evaluates the efficacy and safety of IOMBT in pediatric patients with small-angle asymmetric SOP, providing valuable insights into its potential as a preferred surgical option.
The study included 30 pediatric patients (42 affected eyes) with small-angle asymmetric SOP who underwent the IOMBT procedure at Anhui Province Children’s Hospital between June 2018 and August 2020. Inclusion criteria comprised: (1) a diagnosis of asymmetric SOP; (2) concurrent mild to moderate unilateral or bilateral IOOA, graded “+” to “++” based on Kenneth’s scale; (3) vertical deviation in the primary position of ≤5 prism diopters (PD); and (4) complete follow-up information. Ethical approval was obtained from the Clinical Ethics Committee of Anhui Province Children’s Hospital, and informed consent was secured from all patients and their guardians.
Pre-operative comprehensive eye examinations assessed visual acuity, anterior segment, fundus, refraction, and eye movements. Angles of deviation, including horizontal and vertical deviations, were measured using alternate prism cover testing at distances of 5 meters and 33 centimeters in the primary gaze position. Fundus photographs were captured using a retinal camera, and the fovea-disc angle (FDA) was calculated using CorelDRAW X4 software. The FDA, defined as the angle between the disc-fovea line and the horizontal line through the fovea, was automatically determined by the software. Post-operative follow-up examinations were conducted at 1 and 6 months to evaluate the outcomes.
The IOMBT procedure was performed as follows: for patients under 12 years of age, general anesthesia was administered, and the surgical field was disinfected. For patients aged 12 and above, topical and subconjunctival anesthesia was used. A fornix incision was made in the inferotemporal quadrant, and the IO muscle was isolated and hooked 10 to 11 mm from its temporal insertion. The muscle was secured with a double-armed 6-0 absorbable suture using a loop suture technique, fixed 5 mm posterior to the temporal insertion of the inferior rectus muscle. The fornix incision was then sutured to complete the operation. Patients with concurrent horizontal strabismus underwent simultaneous surgical correction.
Post-operative assessments focused on gaze positions, eye movements, improvements in diplopia, and compensatory head posture. Success criteria at the 6-month follow-up included: disappearance of vertical deviation in the primary position; absence of IOOA; resolution of compensatory head posture in congenital SOP cases; elimination of diplopia in acquired SOP cases; a difference of <10 PD in angles of deviation between upgaze (25°) and downgaze (25°) positions in patients with V pattern; and no complications such as overcorrection, secondary A pattern, or IO insufficiency. Statistical analysis was performed using SPSS software, with categorical variables analyzed using the chi-square test and continuous variables assessed using paired t-tests. A P value of <0.050 was considered statistically significant.
The study included 42 affected eyes of 30 pediatric patients, comprising 12 bilateral and 18 unilateral cases. Baseline characteristics revealed pre-operative vertical deviations in the primary position averaging 4.5 ± 0.6 PD. Post-operatively, mean vertical deviations significantly decreased to 0.2 ± 0.1 PD at 1 month and 0.3 ± 0.1 PD at 6 months, representing a mean reduction of 4.2 ± 0.4 PD. The FDA also showed significant improvement, decreasing from 10.5° ± 4.1° pre-operatively to 4.0° ± 2.5° at 1 month and 4.2° ± 2.2° at 6 months, with a mean reduction of 6.5° ± 2.3°. Patients with concurrent V pattern exhibited a mean decrease in V pattern from 18.0 ± 3.5 PD pre-operatively to 1.6 ± 0.9 PD post-operatively, representing a mean reduction of 16.4 ± 2.5 PD.
Pre-operatively, 6 patients had IOOA graded as “+” and 24 as “++.” Post-operatively, IOOA disappeared in 26 patients, and 4 patients improved to IOOA “+,” yielding a success rate of 86.7%. Compensatory head posture, present in 13 patients pre-operatively, disappeared in 11 patients and improved in 1 patient, resulting in a success rate of 84.62%. Diplopia resolved in 5 of 6 patients, achieving a success rate of 83.30%. Importantly, none of the 30 patients experienced complications such as overcorrection, secondary A pattern, or IO insufficiency.
The IOMBT procedure differs from conventional IO weakening techniques by suturing the IO muscle belly 10 to 11 mm from its insertion and fixing it 5 mm posterior to the temporal insertion of the inferior rectus muscle. This approach reduces the operating length of the IO muscle, diminishing its action strength and correcting vertical deviation in the primary position without the risks associated with other procedures. The study’s findings align with those of Dr. Yang, who first implemented IOMBT for small-angle hypertropia with IOOA, although the small sample size of that study limited its generalizability.
The success rates observed in this study suggest that IOMBT is a viable alternative to conventional IO weakening procedures for treating small-angle asymmetric SOP. Its efficacy in correcting V pattern strabismus with mild to moderate IOOA further highlights its potential as a preferred surgical option. The absence of complications such as overcorrection and secondary A pattern underscores the safety of the IOMBT procedure.
In conclusion, this study demonstrates the efficacy and safety of IOMBT in treating small-angle asymmetric SOP, particularly in cases with concurrent V pattern strabismus and mild to moderate IOOA. The procedure’s success in reducing vertical deviation, correcting compensatory head posture, and resolving diplopia, coupled with its low risk of complications, positions it as a promising surgical option. However, the observational design of this study necessitates further research, including randomized controlled trials, to validate these findings and establish IOMBT as a standard treatment for small-angle asymmetric SOP.
doi.org/10.1097/CM9.0000000000001591
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