Outcomes of Asymmetric Primary Inferior Oblique Muscle Overaction Managed by Bilateral Myectomy and Tucking of Proximal Muscle End: A Cohort Study.

BACKGROUND: We present the outcomes of bilateral myectomy and tucking of the proximal end of the muscle for the treatment of asymmetric primary inferior oblique (IO) overaction.
METHODS: This was a one-armed prospective cohort study. An ophthalmologist and orthoptist evaluated cases of primary IO muscle overaction presenting between January 2010 and December 2013. All eyes underwent bilateral myectomy and tucking of the proximal end of the IO muscle. Data were collected on ocular motility, the angle of deviation, postoperative complications, and status of hypertropia at 6 months postoperatively. The 95% confidence intervals (CI) were calculated. The statistical significance was indicated by P < 0.05.
RESULTS: The patient cohort was comprised of 51 patients with primary IO muscle overaction. Preoperatively, all eyes had +2 or greater overaction of the IO muscle except one patient with +1 and +3 overaction in the right and left eyes, respectively. At 6 months postoperatively, the reduction in the angle of deviation for distance and near was 32.6 prism diopters (PD) ([95% CI 30.3-34.9], P < 0.001) and 32.6 PD ([95% CI: 29.8-35.3], P < 0.001), respectively. There was no significant difference in the postoperative variation of the reduction in the angle of deviation based on gender, right or left eye, and type of horizontal strabismus. There were no cases of "A" or "V" patterns, clinically a significant IO underaction or "adherence syndrome" postoperatively.
CONCLUSION: Bilateral myectomy and tucking of the proximal end of the muscle is likely an effective method of treating asymmetric primary IO muscle overaction.

INTRODUCTION

Overaction of the inferior oblique (IO) muscle can be present in eyes with horizontal strabismus.1 Often, surgery aims to weaken the overacting IO muscle. Surgeons prefer myectomy or a graded recession of the IO muscle due to its speed, simplicity, and safety when compared to a recession.23 Unfortunately, recurrence of overaction following IO muscle surgery results in less-than-optimal outcomes. Parks4 reported lower recurrence rates of overaction following recession compared to myectomy. The proximal muscle end after the IO muscle is cut is not adequately addressed in such procedures. Tucking the proximal muscle end into the space where the IO muscle penetrates Tenon's capsule following myectomy was attempted as a modification in both superior and inferior oblique muscle surgeries.56 However, to the best of our knowledge, there is limited information in the literature on the steps and 6-month outcomes after the combined procedure.

In this study, we review the surgical procedure and success rate for bilateral myectomy and tucking of the proximal end of the muscle for asymmetric primary IO muscle overaction.

METHODS

The hospital Ethics Committee approved the study, and all patients had to sign a written informed consent to participate in this one-armed cohort study. Patients were included if they had bilateral asymmetric primary IO overaction (Grade +1 or +2 in one eye and IO overaction Grade +3 or +4 in the other eye). All patients presented to our eye hospital between January 2010 and December 2013. Patients were excluded if they had previous strabismus surgery, IO overaction secondary to superior oblique palsy, thyroid eye disease, myasthenia gravis, Duane syndrome, and craniofacial anomalies.

A strabismus specialist, two ophthalmologists, and an orthoptist were the field staff for this study. Prior to the surgery, patients with hyperopia <+2.00 diopter (D) were prescribed full cycloplegic refractive correction. Patients with amblyopia had to undergo part-time occlusion for 3 months or until visual acuity stopped improving, despite adequate compliance.

The angle of vertical deviation was measured by the prism, and alternate cover test for distance and near.7 For distance, we measured deviation in primary gaze, looking 25° superiorly and looking 35° inferiorly. If the angle of deviation could not be reliably measured with the cover tests, the modified Krimsky method was used. Patients were asked to focus on a target of 6 m away and then a prism with apex toward the side of deviation was placed over the patient's dominant eye. A light source was held 33 cm from the patient, was focused between the eyebrows, and then the prism strength was gradually increased until the cornea light reflection was centered in the cornea. This was considered the angle of deviation. We defined this method as the “distance Krimsky test.” The conventional Krimsky test is performed with the fixation target and the light source at 33 cm from the patient.8

Ductions and the versions were measured, with a light source beam held 33 cm from the patient.7 Duction and version testing were performed preoperatively and postoperatively. IO overaction was graded on levoversion (for the left IO) and on dextroversion (for the right IO) on a scale from “0” to “+4.” Grade 1+, 2+, 3+, and 4+ roughly translates to 1–15, 16–30, 31–45, and 46–60 prism diopters (PD) of hypertropia in adduction and on far gaze. Absence of hypertropia was defined as “0” Grade for IO oblique overaction.9101112

All patients underwent a fornix-based incision with bilateral symmetric IO myectomy, with tucking of the proximal ends into Tenon's. The IO muscle was isolated through an inferotemporal cul-de-sac approach. The muscle was dissected from the surrounding tissues, clamped at 12 mm from the original insertion, and then removed from the sclera. The portion of the IO muscle distal to the clamp (approximately 10–11 mm) was transected and cauterized. The short cauterized stump was tucked into its Tenon's sleeve. An IO exaggerated traction test was subsequently performed to ensure that no residual fibers remained.9 Surgery was then performed to correct horizontal misalignment. All patients had bilateral symmetrizing surgery on the horizontal muscles.

Success was defined as the elimination of IO muscle overaction and the elimination of hypertropia secondary to persistent ipsilateral IO muscle overaction in primary gaze.

We plotted the difference in the angle of deviation before and 6 months after surgery for distance and near to study the normality of distribution. The difference was normally distributed. The parametric method of statistical analysis was used with statistical package for social studies (SPSS 22.1; IBM Corp., New York, NY, USA). Data are presented as mean and standard deviation for the angle of deviation. The paired t-test was used to review the outcomes (reduction of angle of deviation) in relation to dependent factors such as gender, age, laterality of IO overaction, type of horizontal deviation, and fixating eye. The difference of the mean, 95% confidence intervals, and two-sided P value were calculated. A P < 0.05 was considered statistically significant.

RESULTS

The study cohort was comprised of 51 patients (25 males and 26 females). The mean age of the cohort was 13.1 ± 8.4 years (range, 2–48 years; median 10 years).

Preoperatively, there were 42 cases with an exodeviation, seven cases with an esodeviation, and two cases with no horizontal deviation. There were 23 right eyes and 14 left eyes with strabismus at distance and near. There were 12 eyes with alternating strabismus. In 43 eyes, IO overaction was bilateral but of different amplitude. Six right eyes and two left eyes had unilateral IO overaction.

Distance visual acuity was noted in 44 participants. The presenting vision in the better eye was 6/6 in 17 patients, 6/9 in 21 patients, 6/12 in two patients, 6/24 in one participant, 6/60 in two participants, and <6/60 in one participant.

The mean preoperative angle of deviation was 38.0 ± 7.5 PD at distance and 35.7 ± 8.7 PD at near in primary gaze. In upward gaze, it was 43 ± 8.4 PD and in downward gaze it was 18.4 ± 9.4 PD. Six months postoperatively, the mean angle of deviation for distance of all 51 patients was 6.5 ± 2.1 PD, and 4.9 ± 1.7 PD for near fixation point in straight gaze [Figure 1]. In upward gaze, it was 1.8 ± 0.4 (standard error [SE]) PD and in downward gaze it was 1.5 ± 0.4 (SE) PD. Postoperatively, there was a statistically significant reduction in the angle of deviation at distance in straight gaze (mean difference 31.5 ± 7.5 PD, P < 0.001) and at near (mean difference 30.8 ± 8.7 PD, P < 0.001).

Figure 1

Comparison of the angle of deviation for distance and near fixation in primary gaze before and after bilateral inferior oblique myectomy and tucking of proximal muscle stump into Tenon's capsule to treat primary inferior oblique overaction

The reduction in the angle of deviation, for distance, in eyes with IO muscle overaction, based on gender, age, laterality, and horizontal deviation is presented in Table 1. The variation in the reduction in angle of deviation following surgery in the subgroups was not statistically significant (P > 0.05, all cases).

Table 1

Angle of deviation (Δ D) for distance fixation at six months postoperatively compared to the preoperative deviation

The decrease in the angle of deviation at near, in eyes with IO muscle overaction, based on gender, age, laterality, and horizontal deviation is presented in Table 2. In these subgroups, the variation in decrease in the angle of deviation postoperatively was not statistically significant (P > 0.05, all cases).

Table 2

Angle of deviation (Δ D) for near fixation at six months postoperatively compared to the preoperative deviation

One patient who required a second surgery continued to demonstrate overaction of the IO muscle in one eye and developed underaction in the contralateral eye after bilateral IO muscle myectomy. A second patient had no IO muscle overaction following myectomy, and her hypertropia was comitant on side gaze, suggesting that the hypertropia was not secondary to persistent IO muscle overaction. The success rate of eliminating hypertropia secondary to persistent ipsilateral IO muscle overaction was 92%.

In 18 patients, the intraoperative behavior of the 26 IO muscles after myectomy was noted. The retraction were graded as: 6 (21%) patients were Grade 0 (minimal or no movement); 8 (29%) patients were Grade 1 (slow and incomplete retraction); 7 (25%) patients were Grade 2 (slow but complete retraction to Tenon's capsule); and 5 (18%) patients were Grade 3 (brisk retraction into Tenon's sheath). Postoperatively, no patient developed clinically significant IO underaction, “A” or “V” pattern after surgery or “adherence syndrome.”

DISCUSSION

This is a large cohort of eyes with asymmetric IO overaction managed by bilateral myectomy and tucking of proximal end of the muscle. The 6-month follow-up results suggest that this surgery had a 92% success rate in correcting overaction and deviations. The postoperative complication rate was low. The mean reduction in the angle of deviation was 31.5 PD at distance and 30.8 PD at near. The difference in the reduction in the angle of deviation in males and females, unilateral and bilateral IO overaction, and in patients with eso- or exodeviations was not statistically significant (P > 0.05, all cases).

Our literature search failed to find a study that combined bilateral myectomy and tucking of the proximal end of the muscle to address asymmetric primary IO overaction. Compared to our 92% success rate, Singh et al.12 had noted 77% success rate in their series of 15 patients of IO overaction. However, Singh et al.12 managed their cases with IO recession in addition to horizontal muscle surgeries (if required). A longitudinal study in the UK of unilateral IO myectomy and recession in 24 patients reported that outcomes were promising in all cases after 12 months postoperatively.13 However, there were cases of recurrence of hyperdeviation in those undergoing recession only in this study. Lee et al.14 had used Z myotomy to manage primary IO overaction in 13 patients. All cases had shown improvement postoperatively.14 All the eyes in Lee et al.'s14 study had Grade + 2 IO overaction, only while our series had eyes with Grade 3 and 4 deviations. Evaluating 100 eyes, Ghazawy et al. noted that both myectomy and anterior transposition of the IO gave similar results in regards to residual overaction postoperatively for correction of IO overaction.15 However, this large series included patients with both IO overaction and superior oblique underaction.

The magnitude of preoperative angle of deviation had a significant impact on the postoperative grades of vertical deviation after IO muscle surgeries.16 In our study, nearly 70% of eyes had Grade 3 and 4 deviations preoperatively. Therefore, the difference in outcomes could be due to differences in preoperative grades of deviations.

Although in cases of unilateral IO overaction, the outcomes of surgery were better following bilateral procedures, element of chance could not be ruled out. Unilateral management of IO was found to be more effective in reducing the angle of deviation by Singh et al.12

Due to small samples in the subgroups, we could not detect significant predictors of good outcomes in our study. Larger subgroup samples and a correct sample size calculation are recommended to address this issue.

Primary asymmetric overaction of IO is a less common problem in ophthalmic practice. We recommend management by IO myectomy and tucking of proximal end using a standard technique. Success rates may improve after adoption of this technique. Even in general ophthalmic practice, when patients with IO overaction seek a procedure with better predictability, IO myectomy and tucking of proximal muscle seem to be promising.

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Nil.

Conflicts of interest

There are no conflicts of interest.