Transposition of vertical rectus muscles has been considered as mainstay of treatment of complete sixth nerve palsy. Despite their various forms, all transposition procedures aim for improvement of the compromised abduction with correction of esotropia, face turn and expansion of the field of binocular single vision . Traditional VRT procedures involve the simultaneous lateral transposition of part or the whole of both vertical rectus muscles at the same time. Single vertical rectus muscle transposition in the form of superior rectus transposition was proposed aiming for reduction of the incidence of postoperative anterior segment ischemia, especially when medial rectus muscle contracture mandates simultaneous recession of the medial rectus muscle [3, 4]. Since then, SRT has been studied in many articles concerning cases with defective ocular abduction secondary to chronic sixth nerve palsy and esotropic Duane retraction syndrome and its efficacy has been well established [4, 8, 9] [8,9,10]. Recently, a large retrospective analysis of induced vertical deviations secondary to SRT in sixth nerve palsy and esotropic Duane retraction syndrome was performed. The study comprised 69 patients who underwent SRT, 32 patients with sixth nerve palsy and 37 patients with esotropic Duane retraction. The study revealed that 14 patients developed hypertropic shift while 23 patients developed hypotropic shift. Out of these cases, 5 cases developed persistent vertical diplopia which required intervention .
The idea of IRT was first proposed by Velez and colleagues in 2017 to treat complete sixth nerve palsy in a series of 5 patients . Because of their experience with induced hypertropia after SRT in sixth nerve palsy , they started transposing the inferior rectus muscle in patients with preoperative hypertropia. Their case series comprised 5 consecutive patients with sixth nerve palsy associated with preoperative hypertropia or more defective abduction in the down gaze. In terms of results, IRT scored significant improvement of esotropia, face turn and limited abduction with no reported cases of symptomatic induced vertical deviation. In the current study, we have evaluated our results with IRT on a series of 11 patients with chronic sixth nerve palsy. In general, we have found that IRT produces significant correction of esotropia, face turn, limited abduction with no reported cases of symptomatic induced vertical deviation, torsional diplopia or anterior segment ischemia.
In Velez study , average esotropia improvement was 27 PD, face turn correction was 26.4° with an average improvement of limited abduction by 1.0 unit. Compared with Velez study, our results are quite superior with an average improvement of esotropia by 35.9 PD, face turn correction by 25.9° and an average improvement of abduction limitation by 2.2 unit. We believe that our technique of dual augmentation of transposition compared with single muscle to muscle augmentation technique used by Velez and colleagues could explain such discrepancy. In addition, smaller number of patients in Velez study (5 patients) compared with the current one (11 patients) could also plays a role in such difference. One of the similar aspects between the current study and that of Velez is that IRT in both studies lead to collapse of the preoperative hypertropia by an average of 2.5 PD in the former and 1.2 PD in the latter. In Velez study, one patient developed 2PD induced hypertropia while another patient developed 3PD induced hypotropia. In the current study, two patients developed 2 PD induced hypotropia. In both studies, the induced vertical deviations were innocuous and of small magnitude which warranted no further interventions.
Recently, Sener and colleagues in 2019 evaluated IRT in management of 7 esotropic Duane retraction syndrome patients who had either more defective abduction in the inferior gaze or had V-pattern esotropia . In their study, average esotropic correction was 19.6 PD, V-pattern collapse was 19.9 PD, face turn improvement was 16.4° with an average improvement of limitation of abduction by 1.3 unit. Induced hypertropia was reported in two patients which mandated secondary surgical intervention in one patient. Comparatively, the results of the current study in correction of esotropia, face turn and limited abduction are slightly better than those of Sener study. However, we believe that comparisons are quite irrelevant because of different characters of patients in both studies (esotropic Dune retraction syndrome vs. sixth nerve palsy). In addition, different augmentation techniques were used in Sener study which vary between resection of inferior rectus muscle in some cases and posterior scleral fixation suture in other cases. Nevertheless, we believe that Sener study holds a beneficial role in the evaluation of the efficacy and safety pattern of IRT procedure.
Limitations of the current study include its retrospective nature, small number of included patients, in addition to paucity of data regarding the effect of IRT on fundus torsion. However, and taking into account the relative recentness of IRT procedure, this study provides valuable insights into outcomes of a recently practiced transposition procedure. We have found that IRT is effective in management of esotropia, face turn, limited abduction associated with chronic sixth nerve palsy with collapse of preexisting V-pattern. In addition, the impact of IRT on collapse of preexisting hypertropia with low incidence of significant induced hypotropia, which was observed in Velez study as well as in the current study, could open new insights into potential implications of IRT in cases of Sixth nerve palsy with preoperative hypertropia. Base on its efficacy and low rate of significant postoperative complications such as induced vertical deviations and torsional diplopia, we believe that IRT could be considered in management options of chronic sixth nerve palsy especially in cases with V-Pattern esotropia and more abduction limitation in the downgaze. However, and as data on outcomes of IRT on cases of sixth nerve palsy which do not exhibit the aforementioned criteria are currently lacking, future studies with larger sample size are recommended.