DVD is a dissociated strabismus characterized by upward drifting of one or both eyes when binocularity is interrupted. DVD is usually bilaterally. Asymmetric DVD is probably associated with unequal IOOA or superior rectus overaction [7]. The management of DVD is challenging for strabismus surgeons. Multiple approaches have been proposed for the treatment of DVD, but there is no agreement among surgeons regarding the best practice [7]. However, when DVD coexisted with IOOA, IOAT is preferred by most clinicians as it reduces the IOOA while simultaneously restricting the superior floating phenomenon of DVD [2,3,4,8,11,12]. Full IOAT includes the posterior fibers with J deformity that form a neurofibrovascular bundle. The neurovascular bundle provides the inferior oblique muscle with a new functional origin and convert the inferior oblique muscle from an elevator to a depressor. The depressor effect is likely due to a combination of active contraction of the distal inferior oblique muscle and mechanical restriction on the elevation of the eye [10,13].
A number of reports have showed satisfactory clinical results for the IOAT procedure in DVD with IOOA [2,3,4,11]. Symmetric DVD was always treated with symmetric surgery, whereas asymmetric DVD or IOOA was preferred for bilateral asymmetric surgery or unilateral surgery. Snir et al. [10] suggested the use of bilateral IOAT with monocular-graded inferior oblique resection for asymmetric DVD with IOOA as this procedure could improve DVD more than that can be achieved by equal bilateral IOAT. Pineles et al. [8] used bilateral asymmetric IOAT to treat incomitant asymmetric DVD and showed that this procedure led to improvements in incomitant DVD, a V-pattern and IOOA. Rajavi et al. [11] demonstrated both graded and ungraded asymmetric binocular IOAT could effectively reduce DVD and IOOA. Nevertheless, a unilateral surgical procedure was used only in patients with unilateral markedly asymmetric DVD. Burke [6] observed that unilateral IOAT achieved satisfactory results for DVD with IOOA when the preoperative DVD was less than 15 PD. However, Bothun and Summers [4] considered unilateral IOAT could be an effective treatment for unilateral or markedly asymmetric DVD in patients with a strong, contralateral fixation preference. In their study, 9 patients with DVD measuring 17 to 33 PD were also successfully corrected. In our study, the patient’s inclusion criteria were similar to those described in Bothun’s study, in which all cases in a case series of patients with primary DVD measuring 14 to 36 PD in the operated eye were successfully treated for motility disturbance.
In the literature, when performing inferior oblique anterior transposition, the optimal placement of the muscle is various and controversial. The standard placement is at the temporal border of the inferior rectus muscle insertion. Kratz [14] performed graded procedures at a position 1 mm posterior to and 1 mm anterior to the temporal position of the inferior rectus insertion site depending on the severity of DVD. Seawright and Gole [15] performed graded IOAT to positions located 2 mm posterior and 2 mm anterior to the temporal position of the inferior rectus insertion site depending on the presence or amount of preoperative IOOA, V pattern, hypertropia, and DVD. Placing the inferior oblique at a position 2 to 4 mm anterior to the lateral end of the inferior rectus muscle insertion site did not increase the effectiveness of IOAT, but it might increase the risk of postoperative anti-elevation syndrome [16,17]. Recently, Ford [3] and Farid [2] reported that anterior and nasal transposition of the inferior oblique might mechanically restrict the elevation of the eye and improve DVD more effectively than is achieved by IOAT; however, it may also induce hypotropia and consecutive horizontal strabismus. We performed the procedure at a placement position similar to that used by Kratz, but we graded for the position based on the degree of IOOA. In the monocular procedure, IOOA and DVD were significantly improved, and no cases developed obvious anti-elevation syndrome.
With the generalization IOAT, some side effects, such as hypotropia [6,9] and anti-elevation syndrome have been mentioned [16,17]. Although hypotropia and anti-elevation were also reported in unilateral IOAT surgery, its complications were often mild [4,6]. In our study, IOOA and DVD were significantly reduced in all patients, and there were no related complications. This result is probably related to our inclusion criteria, which required unilateral IOOA, significantly different primary positions of DVD in both eyes, and surgery on the non-fixing eye. Because the non-fixing eye always occupied a higher position, performing IOAT on the non-fixing eye could improve the floating phenomenon observed in DVD with fewer complications. Moreover, the patients in our study had worse vision or amblyopia on the operated eye. Patients with paralytic strabismus or alternate fixation were excluded. These inclusion criteria may contribute to less postoperative impaction on the contralateral eye. Meanwhile, individual difference in the strength of the muscle may impact the efficiency of IOOA.
There are some limitations to this study. It is a retrospective study performed without a control group. The sample size was relatively small due to the specific inclusion and exclusion criteria applied in this study. The follow-up duration was not long enough to observe some delayed complications. Moreover, we did not evaluate the development of the V pattern or limitations on elevation during abduction. Concurrent horizontal muscle surgery was preferred in most of previous studies. However, we performed secondary horizontal muscle surgery at 3 months after IOOA, considering the impact of DVD and IOOA on horizontal strabismus. These factors make our results less comparable to those achieved in previous studies.