In this retrospective study, three different IO weakening procedures were compared in the treatment of unilateral SO palsy in terms of effect of each procedure on HT in different positions of gaze, anomalous head tilt, oblique muscles’ dysfunction and postoperative complications. In general, and relative to IOM, we have found that both IORAT and IOAT procedures attained significant improvement of HT in primary position, contralateral gaze, ipsilateral head tilt and IOOA with more significance of IORAT in correction of HT in contralateral gaze and ipsilateral head tilt. In addition, we have also found that all procedures were equally effective in improving SO underaction and HT in both ipsilateral gaze and contralateral head tilt. However, significant higher rates of postoperative anti-elevation secondary to IORAT could complicate its use in the management of unilateral superior oblique palsy.
Weakening of overacting IO muscle has been widely considered as the first surgical option in treatment of symptomatic SO palsy, and this is because its effectiveness in alleviation of ocular deviation and abnormal head positions while avoiding adverse side effects associated with other forms of strabismus surgeries such as iatrogenic Brown syndrome and overcorrections [1, 11].In previous reports concerning IOM, the average correction of HT in primary position ranged from 12.5 to 14 PD and unit correction of IOOA ranged from 2 to 2.4 units with favorable outcomes regarding correction of AHP and symptomatic diplopia [12,13,14]. These results closely match ours in the IOM group where there is an average correction of HT in the primary position and IOOA by 13 PD and 2.0 units, respectively.
IOAT has been advocated as an effective weakening procedure for controlling IOOA associated with SO palsy [15, 16]. In previous reports, the range of average correction of HT was 9.5 to 19.2 PD in the primary position and 17 to 27.7 PD in contralateral gaze, while average correction of IOOA ranged between 2.5 to 4 units [15,16,17,18,19]. In the current study, IOAT achieved an average 20.7 PD correction of HT in the primary position and 30 PD correction in contralateral gaze, while IOOA was improved by an average of 3 units. The incidence of postoperative anti-elevation syndrome following IOAT was reported to range between 0 and 27% [6, 15,16,17,18,19] while in the current trial, the incidence of anti-elevation syndrome following IOAT was 13%.
Farvardin and colleagues have reported satisfactory results of combined resection and anterior transposition of IO muscle in cases of dissociated vertical deviation associated with IOOA. They postulated that segment resection of the anteriorly transposed IO muscle augments its anti-elevating effect . Later, they studied the effect of the combined procedure on 27 unilateral SO palsy patients with large (20-25 PD) HT in the primary position, and have found that the procedure improved HT in the primary position and in contralateral gaze by an average of 21.2 PD and 27.8 PD respectively with development of mild defective elevation in only one case  These results are comparable to ours whereas the IORAT procedure improved HT in the primary position and in contralateral gaze by an average of 23.7 PD and 36 PD respectively. However, the IORAT procedure in our study induced a significant number of postoperative anti-elevation syndrome (6 out of 19 cases, 31%).
We believe that the present study is the first to specifically compare three different IO weakening procedures in the treatment of SO palsy. Of note, there were some previous reports which compared three different IO weakening procedures, but those studies were mainly concerning IOOA secondary to horizontal strabismus [21, 22]. In contrast to the current study which evaluates the effect of each procedure on HT in all gaze positions as well as oblique muscle dysfunction, outcome measures of most of the previous reports were mainly concerning procedure effect on IOOA and HT in the primary position. Few studies concerned HT in contralateral gaze [14, 20] and SOUA , and none up to our knowledge, have evaluated the result of a procedure on HT in all relevant gaze positions, as well as IOOA and SOUA.