There are several surgical strategies for the correction of hypertropia due to unilateral superior oblique muscle palsy, the most common type of cyclovertical muscle palsy.1 One of these procedures that has been proven to be effective is inferior oblique myectomy of the affected eye.2–5 Isolated inferior oblique myectomy is effective in treating hyperdeviation of greater than 15 prism diopters (PD) in primary position. For hyperdeviation of greater than 15 PD, operating on two cyclovertical muscles is advised.6,7 Some studies reported that isolated inferior oblique myectomy can be more effective in resolving primary position hypertropia (up to 34 PD)8 and operating on two muscles can result in overcorrection.8,9
There is not wide agreement on the effectiveness of isolated inferior oblique myectomy for hyperdeviation. One study has noted the risk of overcorrection in two-muscle surgery, especially in moderate amounts of hypertropia (14 to 25 PD).9 There is also little information about the effect of inferior oblique myectomy on improving abnormal head posture and incomitancy of hyperdeviation. We conducted this study to evaluate the amount of effectiveness of inferior oblique myectomy in primary position, side gazes, and head tilts and its effect on improving the most presenting symptom of unilateral superior oblique palsy (abnormal head posture). We tried to find any relationship between the magnitude of hypertropia before surgery and the range of its change postoperatively and the effect of this procedure on incomitancy of hyperdeviation.
Patients and Methods
This prospective interventional case series was conducted in the strabismus clinic of Farabi Eye Hospital. The study was approved by the institutional review board of Farabi Eye Research Center. Informed consent was obtained from the patient or parent before beginning the study. The study was compliant with the tenets of the Declaration of Helsinki. It included 59 patients with presumed unilateral superior oblique palsy who underwent a single inferior oblique myectomy procedure between 2011 and 2015. All of the included patients had inferior oblique overaction and none of them had significant superior oblique underaction. Prior history of eye muscle surgery, orbital lesion, additional eye muscle surgery at the time of inferior oblique muscle surgery, presence of dissociated vertical deviation, severe amblyopia, and other ocular disease or mental disorders were the criteria of exclusion. In the traumatic cases, the surgery was done at least 6 months after the trauma.
Age, sex, etiology, and affected eye were recorded for each patient. Visual acuity and refractive error were examined. The presence and type of abnormal head posture was also recorded for each patient. A full orthoptic assessment was performed before surgery and in every follow-up examination by a single experienced orthoptist, including deviation measurement in primary position (at 33 cm and 6 m), right and left gaze, and right and left head tilts, by alternate prism-cover testing.
The criteria used for diagnosing superior oblique palsy were: hypertropia increasing in adduction and head tilt toward the hypertropic side. Bielschowsky's head tilt test was always performed prior to surgical planning. Other clinical conditions mimicking unilateral superior oblique palsy (eg, skew deviation) were considered and excluded.
The patients underwent a fornix-based inferotemporal peritomy. After disinsertion and 8-mm inferior oblique myectomy, the distal stump of the muscle was repositioned in the Tenon capsule opening and conjunctiva was sutured. All of the operations were done by a single strabismus surgeon (MRA).
The patients were observed at least 4 months after the operation. Success rate was defined as residual hypertropia of 5 PD or less and no hypotropia in distance and near fixation at the last follow-up examination. In the cases with preoperative hypertropia of 5 PD or less, success was defined as improvement of hypertropia, resolution of abnormal head posture, and resolution of diplopia (if present).
Statistical analysis was undertaken using SPSS software (version 24; IBM Corporation, Armonk, NY). A P value less than .05 was considered significant. The non-parametric Wilcoxon signed-rank test was used to compare preoperative and postoperative results. Spearman correlation was used to assess the correlation between the amount of reduction in hypertropia and preoperative hypertropia. To compare the success rate between subgroups, Fisher's exact test was used.
Of 59 patients, 39 met the criteria to participate in this study. The remaining 20 patients who were excluded had concurrent surgery on another muscle or were lost to follow-up. Twenty-two of 39 (56.4%) patients were male and 25 (64.1%) patients had left-sided palsy. Five (12.8%) patients had a clear history of trauma severe enough to injure the fourth nerve. The mean age at the operation was 21.2 ± 15.0 years (range: 4 to 58 years) and mean follow-up was 7.5 ± 6.7 months (range: 4 to 34 months).
Postoperatively, the mean preoperative hypertropia in primary position decreased significantly (Table 1). There was no overcorrection, even in the small amounts of hypertropia (3 PD). Three cases with pre-operative hypertropia of 30 PD became orthotropic after the surgery. The mean amount of hypertropia in all gazes and tilts diminished significantly (Table 1).
Preoperative and Postoperative Data for Patients Undergoing Single Inferior Oblique Myectomy
We also calculated the amount of incomitance (contralateral-ipsilateral gaze hypertropia) preoperatively and postoperatively, and it showed a significant change (Table 1) from 15.0 ± 7.4 PD (range: 3 to 35 PD) preoperatively to 2.8 ± 4.1 PD (range: 0 to 16 PD) postoperatively (Table 1, P = .001).
We also assessed head tilt difference (ipsilateralcontralateral head tilt hypertropia), which was reduced from 13.8 ± 7.7 PD (range: 2 to 30 PD) preoperatively to 4.1 ± 5.4 PD (range: 0 to 21 PD) postoperatively, with a significant change (Table 1, P = .001).
There was a strong correlation between the amount of reduction in primary position hypertropia and the amount of primary position hypertropia before surgery (ρ = 0.909, P ≤ .001). Similarly, there was such a correlation between the degree of reduction in contralateral gaze and ipsilateral head tilt hypertropia and the preoperative amount (contralateral gaze: ρ = 0.729 and P < .001, ipsilateral head tilt: ρ = 0.628 and P ≤ .001).
Thirty-two (82%) patients had abnormal head posture, of which 10 (25.6%) patients had both head tilt and face turn to the contralateral side of the palsy, 7 (17.9%) patients had only face turn, and 15 (38.4%) had only head tilt to the contralateral side of the palsy. In 29 patients with abnormal head posture, the posture resolved postoperatively.
The success rate in all cases was 89.7% (35 of 39 patients). We divided the patients into two groups: 15 PD or less and greater than 15 PD of hypertropia in primary position. The success rate was 94.7% (18 of 19 patients) in the first group and 85% (17 of 20 patients) in the second group, and there was no statistically significant difference between groups (P = .605). We also divided the patients in another two groups: 20 PD or less and greater than 20 PD of hypertropia in primary position. The success rate was 93.3% (28 of 30) in the first group and 77.8% (7 of 9) in the second group. There was no statistically significant difference between groups (P = .223).
We also categorized our patients by traumatic and congenital superior oblique palsy. The success rate was 80% (4 of 5) in the traumatic group and 91.2% (31 of 34) in the congenital group, with no statistically significant difference (P = .436)
Twenty-two patients had concomitant exotropia of 9.3 ± 5.2 PD (range: 5 to 18 PD) in primary position, 4 patients had esotropia of 4.8 ± 2.5 PD (range: 2 to 8 PD), and 13 patients did not have any horizontal deviation. In the exotropia group, the mean value of exotropia reduced to 2.1 ± 4.4 PD (range: 0 to 16 PD) postoperatively (P = .001). In the esotropia group, 3 of 4 patients had no horizontal deviation postoperatively, and one of them became more esotropic (from 5 to 18 PD) after the operation. Of 13 patients who did not have any horizontal deviation, one of them had 7 PD of esotropia after the operation and the others did not show any change in the horizontal deviation.
Two patients required a second operation. The possibility of missed inferior oblique muscle fibers in both reoperations was considered. First, the site of previous inferior oblique myectomy was explored. Because we found no residual muscle fibers, we operated on the second muscle. The first patient with 10 PD of residual hypertropia and 18 PD of esotropia underwent 3.5-mm contralateral inferior rectus recession and 5.5-mm ipsilateral medial rectus recession. The second patient with 16 PD of residual hypertropia underwent 5-mm contralateral inferior rectus recession. They became orthotropic after the second operation.
This study indicated that isolated inferior oblique myectomy was an effective surgical method in resolving large hypertropia of unilateral superior oblique palsy. Hypertropia of 30 PD resolved in 3 cases with isolated inferior oblique myectomy. It is always important to consider the “least” amount required in our surgical decision process, especially when we can correct residual small angle deviations with prism in symptomatic patients. Raoof and Burke8 observed that isolated inferior oblique myectomy produced a maximum reduction of 34 PD in primary position hypertropia, which is comparable to this study.
This study showed that the procedure was effective in the cases with hypertropia of 15 PD or greater in primary position. This was in contrast with the previous literature, which defined the boundary for effectiveness of isolated inferior oblique myectomy to be approximately 15 PD. As Plager1 reported for surgery for unilateral superior oblique palsy, one-muscle surgery will probably be sufficient if the patient has less than 15 PD of hypertropia in primary position and it is advised to operate on two muscles for deviations greater than 15 PD. Similarly, Hatz et al.7 suggested that isolated inferior oblique weakening is an effective procedure for superior oblique palsy up to 15 PD of vertical deviation in primary position and two-muscle surgery should be reserved for patients with larger vertical deviations. Likewise, Nejad et al.10 observed that undercorrections are frequent following surgery for unilateral superior oblique palsy, with preoperative deviations of greater than 20 PD in primary position, especially after single-muscle surgery (inferior oblique recess or superior oblique tuck). Their success rate was 14% in patients with single-muscle surgery compared to 58% in patients with two-muscle surgery.10 However, similar to our study, Nash et al.9 reported no benefit for two-muscle surgery for moderate angle hyperdeviations (15 to 24 PD).
In this study, risk of undercorrection was low (10.3%) in contrast to the study by Raoof and Burke8 (29% undercorrection), which defined the success as residual hypertropia of 5 PD or greater. This may be because of the discrepancy between the amounts of preoperative hypertropia in these two studies. Their study assessed hypertropia of at least 20 PD, but we divided our patients into two groups of 20 PD or less and greater than 20 PD and observed a 22.2% failure rate in the second group, which is comparable to Raoof and Burke's study.
We also found a negligible risk for overcorrection, even in small amounts of primary position hypertropia (3 PD). Raoof and Burke's study had similar results. We also observed that the greater the hyperdeviation, the more efficient the inferior oblique myectomy, which indicates the self-titrating effect of the procedure. Shipman and Burke6 found that unilateral inferior oblique myectomy is somewhat self-grading and greater effects are produced when inferior oblique overaction is greater.
Abnormal head posture improved in all of our patients, even in those who had residual hypertropia. This is comparable to previous studies. Elliott and Nankin11 reported an abnormal head posture correction rate of 88% after inferior oblique anterior transposition surgery. Yumusak et al.12 reported 80% improvement of abnormal head posture after inferior oblique myectomy.
There was no difference in the success rate between the two etiologies for superior oblique palsy, congenital and traumatic. We concluded that inferior oblique myectomy can be effective in both cases if there is significant overaction of inferior oblique muscle and minimal underaction of superior oblique muscle.
Inferior oblique myectomy may have a significant effect on the concomitant horizontal deviation. In the current study, the exotropic cases showed significant decrease in the amount of exotropia. In Nejad et al.'s10 study, most patients had exotropias that resolved by surgery for superior oblique palsy. The logical explanation is that the inferior oblique muscle is an abductor muscle and its weakening can reduce its abducting effect. But in the current study, 3 of 4 patients with esotropia also had improvement of their horizontal deviation postoperatively. The explanation for this finding is that with improving vertical deviation, fusional convergence or divergence begins to work and correct small horizontal deviations. Thus, it is advised to take a higher threshold for horizontal muscle surgery because it may resolve without handling the horizontal muscles when operating on the inferior oblique muscle.
This study had some limitations. The traumatic superior oblique palsy group was small and there was no quantification for abnormal head posture. In addition, objective torsion was not measured in this study.
Isolated inferior oblique myectomy is a simple and effective procedure in improving hypertropia due to superior oblique palsy. It can resolve large amounts of hypertropia with low risk of undercorrection and overcorrection. It is effective in resolving abnormal head posture and concurrent small amounts of horizontal deviation. In addition, inferior oblique myectomy is a self-adjusting measure that also decreases incomitancy. It is recommended that isolated inferior oblique myectomy be the first procedure for every patient with hypertropia of 30 PD or less due to unilateral superior oblique palsy with negligible superior oblique underaction.
- Plager DA. Superior oblique palsy and superior oblique myokymia. In: Santiago AP, Rosenbaum AL, eds. Clinical Strabismus Management: Principles and Surgical Techniques. Philadelphia: W.B. Saunders; 1999:219–229.
- Helveston EM, Mora JS, Lipsky SN, et al. Surgical treatment of superior oblique palsy. Trans Am Ophthalmol Soc. 1996;94:315–328.
- Toosi SH, von Noorden GK. Effect of isolated inferior oblique muscle myectomy in the management of superior oblique muscle palsy. Am J Ophthalmol. 1979;88:602–608. doi:10.1016/0002-9394(79)90522-1 [CrossRef]
- Bahl RS, Marcotty A, Rychwalski PJ, Traboulsi EI. Comparison of inferior oblique myectomy to recession for the treatment of superior oblique palsy. Br J Ophthalmol. 2013;97:184–188. doi:10.1136/bjophthalmol-2012-301485 [CrossRef]
- Ghazawy S, Reddy AR, Kipioti A, McShane P, Arora S, Bradbury JA. Myectomy versus anterior transposition for inferior oblique overaction. J AAPOS. 2007;11:601–605. doi:10.1016/j.jaapos.2007.06.011 [CrossRef]
- Shipman T, Burke J. Unilateral inferior oblique muscle myectomy and recession in the treatment of inferior oblique muscle overaction: a longitudinal study. Eye (Lond). 2003;17:1013–1018. doi:10.1038/sj.eye.6700488 [CrossRef]
- Hatz KB, Brodsky MC, Killer HE. When is isolated inferior muscle surgery an appropriate treatment for superior oblique palsy?Eur J Ophthalmol. 2006;16:10–16. doi:10.1177/112067210601600103 [CrossRef]
- Raoof N, Burke JP. Isolated inferior oblique myectomy for vertical deviation of at least 20 prism diopters in the primary position. J AAPOS. 2016;20:112–116. doi:10.1016/j.jaapos.2015.11.009 [CrossRef]
- Nash DL, Hatt SR, Leske DA, et al. One-versus two-muscle surgery for presumed unilateral fourth nerve palsy associated with moderate angle hyperdeviations. Am J Ophthalmol. 2017;182:1–7. doi:10.1016/j.ajo.2017.06.030 [CrossRef]
- Nejad M, Thacker N, Velez FG, Rosenbaum AL, Pineles SL. Surgical results of patients with unilateral superior oblique palsy presenting with large hypertropias. J Pediatr Ophthalmol Strabismus. 2013;50:44–52. doi:10.3928/01913913-20121113-01 [CrossRef]
- Elliott RI, Nankin SJ. Anterior transposition of the inferior oblique. J Pediatr Ophthalmol Strabismus. 1981;18:35–38.
- Yumusak E, Yolcu Ü, Küçükevcilioglu M, Diner O, Mutlu FM. Outcomes of unilateral inferior oblique myectomy surgery in inferior oblique overaction due to superior oblique palsy. Turk J Ophthalmol. 2016;46:21–24. doi:10.4274/tjo.02170 [CrossRef]
Preoperative and Postoperative Data for Patients Undergoing Single Inferior Oblique Myectomya
|Parameter||Preoperative (PD)||Postoperative (PD)||Change (PD)||P|
|Primary position hypertropia (distance)||15.7 ± 7.7 (3 to 30)||1.5 ± 3.3 (0 to 16)||14.2 ± 7.8 (3 to 30)||< .001|
|Primary position hypertropia (near)||13.9 ± 7.2 (2 to 30)||1.5 ± 3.4 (0 to 16)||12.4 ± 7.2 (0 to 30)||< .001|
|Ipsilateral gaze hypertropia||6.7 ± 5.0 (0 to 20)||0.9 ± 2.4 (0 to 12)||5.8 ± 4.5 (0 to 18)||< .001|
|Contralateral gaze hypertropia||21.7 ± 9.0 (5 to 45)||3.6 ± 5.1 (0 to 20)||18.1 ± 9.3 (4 to 45)||< .001|
|Ipsilateral tilt hypertropia||21.9 ± 8.4 (8 to 40)||5.0 ± 5.9 (0 to 24)||16.9 ± 9.0 (2 to 40)||< .001|
|Contralateral tilt hypertropia||8.1 ± 6.2 (0 to 28)||0.9 ± 1.9 (0 to 10)||7.1 ± 5.5 (0 to 18)||< .001|
|Horizontal incomitance||15.0 ± 7.4 (3 to 35)||2.8 ± 4.1 (0 to 16)||12.3 ± 8.2 (1 to 33)||.001|
|Head tilt difference||13.8 ± 7.7 (2 to 30)||4.1 ± 5.4 (0 to 21)||9.7 ± 7.7 (0 to 30)||.001|