Superior oblique tuck—effectiveness in reducing vertical deviations when performed as a primary versus secondary procedure in superior oblique paresis

To quantify the effectiveness of superior oblique tuck (SOT) surgery in patients with a hyperdeviation secondary to superior oblique paresis (SOP). Surgical outcomes were compared in patients undergoing SOT surgery as a primary procedure with those who had previously undergone ipsilateral inferior oblique weakening surgery. This retrospective study assessed surgical outcomes from all patients undergoing SOT surgery for SOP between 2012 and 2021 across 2 hospitals. The effectiveness of SOT surgery in reducing the hyperdeviation was assessed in the primary position (PP) and in contralateral elevation and depression. Results were compared between those undergoing primary SOT surgery with those who had previously undergone ipsilateral inferior oblique weakening surgery. A total of 60 SOT procedures were performed between 2012 and 2021. 7 were removed due to incomplete data. The remaining 53 cases experienced a mean reduction in hyperdeviation of 6.5 prism dioptres (PD), 6.7PD and 12.0PD in the PP, contralateral elevation and contralateral depression respectively. In eyes with previous IO weakening, the reduction of hyperdeviation was larger than in those eyes with no previous IO weakening surgery, with mean reductions of 8.0PD vs 5.2PD, 7.4PD vs 6.2PD and 12.4PD vs 11.6PD in the PP, contralateral elevation and contralateral depression respectively. SOT surgery is a safe and effective procedure with high patient satisfaction and resolution of symptoms in those patients with troublesome diplopia in downgaze secondary to SOP. This is true in both unoperated eyes and in those who have previously undergone inferior oblique weakening surgery.


INTRODUCTION
Superior oblique paresis (SOP) is the most common cause of vertical strabismus secondary to an isolated muscle paresis [1][2][3].In SOP the typical ocular motility pattern consists of an incomitant vertical deviation, worst on contralateral gaze and improved on contralateral head tilt [4].As time progresses, secondary changes can occur resulting in a spread of comitance.SOP can be congenital or acquired (most commonly secondary to trauma) and unilateral or bilateral [5].
Various surgical procedures have been described to treat hyperdeviations in SOP.These include weakening the overacting inferior oblique in the form of inferior oblique anterior transposition, recession, myectomy and myotomy [6,7].Alternatively, superior oblique tightening (tucking or advancing the superior oblique tendon) can be performed [8].Finally, recession of the contralateral inferior rectus or ipsilateral superior rectus muscle for comitant vertical deviations can be performed.These procedures can also be combined with SOT surgery for larger vertical deviations.The appropriateness of each approach depends on the position of maximal deviation, its incomitance/comitance, the presence or absence of excyclotorsion, previous surgery, and the patients' symptoms.When vertical deviations are incomitant and maximal on contralateral downgaze, or if significant excyclotorsion is present, a SOT procedure should be considered.Multiple authors have retrospectively reviewed operative data on superior oblique outcomes, however these have largely focussed on primary position outcomes or have been limited to when the SOT is being performed as the primary procedure [9,10].
In this paper we quantify the reduction in hyperdeviation that can be expected from SOT surgery in the primary position and in contralateral elevation and depression.In addition, we compare the surgical outcomes of patients undergoing SOT surgery as a primary procedure, with those who have already undergone ipsilateral inferior oblique weakening surgery.One theoretical concern with tightening the superior oblique tendon following inferior oblique weakening surgery, in particular after inferior oblique anterior transposition surgery, is that this will carry a risk of inducing a significant iatrogenic Brown's syndrome [11,12].We therefore report the rates and size of any post-operative limitation of elevation in adduction as well as post-operative resolution of diplopia and patient satisfaction.Finally, we compare the effectiveness of SOT surgery in congenital vs acquired aetiologies.

MATERIALS AND METHODS
This retrospective study was exempt from ethics approval and was conducted in accordance with the tenets of the Declaration of Helsinki.
The medical records of all patients who underwent SOT surgery for SOP at Addenbrookes Hospital, Cambridge and West Suffolk Hospital, Bury St Edmonds between January 2012 and January 2021 were reviewed.Exclusion criteria included a history of any previous ipsilateral strabismus surgery other than inferior oblique weakening.In addition, patients were excluded if the patient was lost to follow up or if data from the 3 month post-operative review was incomplete.
The diagnosis of congenital or acquired SOP was made based on a combination of history, ocular motility examination using the Parks threestep test, vertical fusion ranges and examination of old photos [13].We included all patients who underwent SOT surgery with and without a history of previous inferior oblique weakening surgery.The decision to perform SOT surgery at the time was based on ocular motility findings of an incomitant deviation greatest in contralateral downgaze, and either a relatively small deviation in primary gaze (<15 dioptres), or if significant excyclortorsion (>10 degrees) was present using Bagolini lenses.In our cohort the majority of cases underwent SOT surgery to correct the symptomatic incomitant vertical deviation in downgaze, with a small PP deviation.Significant excyclortsion (>10 degrees) was rare in our cases.
All patients had a pre-operative orthoptic assessment 1 week prior to surgery.All measurements were recorded using the alternate prism cover test (APCT) with loose prisms held in front of the paretic eye, in 9 positions of gaze.Torsion was measured using 2 Bagolini lenses in a trial frame with the gratings orientated horizontally.The patient was then asked to rotate the lens in each side of the trial frame until a vertically orientated line was seen and the angle of rotation read off the trial frame.Measurements of torsion were recorded in the primary position and in 15 degrees of downgaze.
All SOT procedures were either performed by, or under direct supervision of a single consultant ophthalmologist (AJV) using the same technique.This consisted of a force duction test followed by insertion of a traction suture at 12 o'clock.A fornix conjunctival incision in the superotemporal quadrant was followed by hooking of superior rectus and isolation of the superior oblique tendon.A Green's hook was positioned and the tendon lifted.Two 6-0 half round vicyl sutures were passed through the tendon on each side at the muscle insertion, and again at a distance from the insertion to create a tuck of the required size, and temporarily secured on a loop.The tuck size was 10 mm in 96% of cases and an intraoperative force duction test was performed in all patients to confirm elevation above the midline was possible and an improved spring back test was present [14].Any adjustments were made as necessary before finalising the 6-0 vicryl suture knot.The conjunctival wound was closed with 7/0 vicryl rapide suture.
Post-operative reviews were performed 2 weeks and 3 months following surgery.Additional follow-up visits occurred as necessary.Examinations at each review included visual acuity, cover test and alternate prism cover test using loose prisms at 1/3 m and 6 m.At the 3 month review, measurements were recorded in all 9 positions of gaze.For the purpose of this study, pre and post-operative distance measurements were used.Inferior oblique overaction or underaction was graded from +4 to -4 units according to the standard scale [15].In addition, we collected outcomes regarding the presence or absence of post-operative diplopia as well as patient satisfaction and the need for any further surgery.
Statistical analysis was performed using SPSS.Distance deviation measurements were used for analysis.Probability values were calculated using t tests and χ2 test.A p value of <0.05 was considered statistically significant.

RESULTS
A total of 60 unilateral SOT procedures were performed between 2012 and 2021.Of the 60, 7 were excluded due to incomplete data or the presence of previous strabismus surgery not involving the inferior oblique.Of the remaining 53 patients, a summary of the baseline characteristics extracted from the records is shown in Table 1 and baseline orthoptic measurements in Table 2.
Baseline measurements revealed an overall pre-operative mean hyperdeviation of 7.7 PD, 8.2PD and 17.4PD in the primary position, contralateral elevation and contralateral depression respectively.Patients who had previous inferior oblique weakening surgery had a larger mean pre-operative PP hyperdeviation compared to those patients with no previous surgery (9.3PD vs 6.4PD).This was statistically significant (p = 0.03).The baseline hyperdeviation in contralateral elevation and depression were also larger in the previous inferior oblique weakening surgery group, but did not reach statistical significance (Table 2).
At the 3 month post-operative review, SOT surgery resulted in a significant reduction in hyperdeviation in all 3 positions of gaze.The mean post-operative reduction in vertical deviation was 6.5PD, 6.7PD and 12.0PD in the PP, contralateral elevation and  depression respectively.When SOT surgery was performed as a primary procedure, the mean reduction in vertical deviation was 5.2PD, 6.4PD and 11.6PD in the primary position, contralateral up gaze and contralateral downgaze respectively.In those patients who had previously undergone inferior oblique weakening surgery, SOT surgery resulted in a larger change in hyperdeviation, with a mean reduction in hyperdeviation of 8.0PD, 7.4PD and 12.4PD in the PP, contralateral elevation and contralateral depression respectively.Only the PP change in hyperdeviation reached statistical significance (Table 3).In both groups the maximum reduction in hyperdeviation was seen in contralateral depression.Torsional data was incomplete but of those patients where torsional data was present, a mean reduction in excyclortorsion of 7 degrees (range 5-15) was seen.No patient developed a new torsional deviation following SOT surgery.When congenital SOP cases were compared against acquired cases there was no significant difference in the post-operative change in hyperdeviation.For congenital vs acquired SOP cases respectively, post-operative changes in hyperdeviation were 7.4 vs 6.4 (p = 0.41) in the PP, 6.9 vs 5.2 (p = 0.26) in contralateral elevation and 12.9 vs 12.9 (p = 0.96) contralateral depression.
When the post-operative change in hyperdeviation was plotted against the pre-operative measurement, a strong positive correlation was seen.This correlation occurred despite 96% of cases having the same sized (10mm) tuck performed and indicates SOT is a self titrating procedure (Fig. 1).
SOT surgery, by its nature, can induce a limitation of elevation in adduction.This was seen in 21Table 5. Post-operative patient outcomes.
(40%) patients and was graded as −0.5 in 5 (9%), −1 limitation in 13 (25%) and −2 in 3 (6%).When present, this iatrogenic Brown Syndrome was not symptomatic or intrusive for the patient with 51 (96%) patients satisfied with the surgical outcome based on direct post-operative questioning.39 (74%) patients had a complete resolution of their pre-operative diplopia, whilst in 14 (26%), diplopia persisted in an area of eccentric gaze (Table 4).Over the 9 year period 5 patients underwent further surgery, which comprised of 3 ipsilateral inferior oblique weakening procedures, 1 contralateral inferior rectus recession and 1 reduction in the size of the tuck.There was no significant difference in the subsequent surgery rates between those having SOT surgery as a primary or secondary procedure.No increased risk of iatrogenic Brown Syndrome was found in those patients undergoing SOT who had previously had ipsilateral IO weakening surgery (most commonly IO anterior transposition).Likewise, when congenital and acquired aetiologies were compared there  was no significant difference seen in the magnitude of postoperative limitation of elevation in adduction (−0.45 vs −0.55 p = 0.45) for congenital vs acquired respectively.

DISCUSSION
Wheeler was the first to describe SOT surgery in 1935 [16].Since Wheeler, multiple authors have described SOT outcomes with a variety of success and complication/re-operation rates.Helveston reported on 59 cases of SOT in which high rates (17%) of tuck revision were required, however none of these cases occurred in patients having a SOT alone [11].Whilst more recently Dwivedi and Marsh [10] published a large 25 year retrospective review of SOT for SOP, which reported 15% post-operative iatrogenic Brown's syndrome, however only 2 patients of 162 (1%) required tuck revision.In the same paper a high (86%) post-operative patient satisfaction was reported.Over the 25 year follow up, 33% required further surgery at some point.Our paper quantifies not only the changes in vertical deviation that can be expected from SOT surgery in the PP, but also in contralateral elevation and depression, providing the surgeon with a more comprehensive understanding of the change in field of BSV that can be expected from SOT surgery.This will aid better patient counselling and consenting with regards to expected symptom resolution following SOT surgery, and possible need for further surgery.We show the mean change in PP hypertropia from SOT surgery is 6.5PD (range 0-20PD), and 12.0PD (range 3-37) in contralateral depression.
The change in PP deviation was strongly correlated with the pre-operative deviation (R 2 0.78) which is important for two reasons.Firstly, it confirms SOT is a self-titrating procedure, with the larger the pre-operative deviation the larger the expected outcome for a given size of tuck.Secondly, in our patient cohort, when there was no manifest deviation in PP and the patient was undergoing SOT for diplopia only in downgaze, our data shows that post-operative hypotropia in the PP did not occur.This is important in giving the surgeon confidence that disruption of the PP BSV, in an attempt to enlarge the inferior field of BSV, is very unlikely, even when there is no or very little pre-operative hyperdeviation in PP.However, intra-operative FDT is a crucial component in minimising the risk of over-correction.In the 1 patient that did have an overcorrection and therefore hypotropia in PP, this was related to overtucking in which all gaze positions were overcorrected and not an anomaly in which only the PP was over-corrected.This case had a positive outcome from further surgery to reduce the size of the tuck.
In addition to improving our understanding of the effects of SOT surgery in multiple positions of gaze, we also specifically assess and quantify the effects of SOT surgery in patients who have previously undergone ipsilateral inferior oblique weakening surgery.This is an important addition to the literature, as inferior oblique surgery is the most common primary procedure performed for patients with hypertropia secondary to SOP.In our cohort, 45% of patients undergoing SOT surgery had a history of previous inferior oblique weakening surgery.Patients who remain dissatisfied following initial inferior oblique weakening surgery, commonly have residual symptomatic diplopia in downgaze.If this residual hypertropia in downgaze is incomitant, SOT surgery is often the next procedure of choice.In our study we show the expected outcomes from SOT surgery in this cohort of patients tended to be larger than in those who have not had inferior oblique weakening.Only the primary position change in hyperdeviation reached statical significance between the 2 groups.However, the mean pre-operative PP difference in hyperdeviation was also significantly larger in the previous inferior oblique weakening group when compared to those with no previous surgery.As we have seen the expected change in hyperdeviation is strongly correlated to the preoperative deviation, therefore the larger post-operative change in PP may simply be reflecting the pre-operative differences.This is a limitation of our study being retrospective in nature and where we are not able to fully match baseline parameters.
One concern when performing SOT surgery in patients who have had previous inferior oblique weakening surgery, in particular inferior oblique anterior transposition, is that a significant iatrogenic restriction of elevation in adduction will be produced.We show that although some mild limitation of elevation in adduction was common following SOT surgery this limitation was not intrusive with 51 (96%) of patients satisfied with the surgical outcome.When patients with previous inferior oblique weakening were compared with primary SOT surgery, there was no difference in incidence or magnitude of iatrogenic Brown Syndrome.This is also the case when congenital and acquired SOP patients were compared.
We have shown that SOT surgery for patients with SOP with incomitant vertical deviations in downgaze, is a highly effective and safe procedure with high patient satisfaction.This is the case irrespective of whether performed as a primary procedure or following prior inferior oblique weakening and does not carry an increased risk of iatrogenic Brown Syndrome in the latter.

What was known before
• Superior oblique tuck is an accepted surgical option for incomitant vertical strabismus following superior oblique

Fig. 1
Fig. 1 Change in PP vertical deviation.Pre-operative PP vertical deviation plotted against change in vertical deviation following SOT surgery.Each point representing a different patient.

Table 1 .
Baseline characteristics of SOT patients.