Knapp proposed the division of SOP into seven grades, with each grade requiring appropriate surgery. This has been considered as the basis for classification and treatment of the condition to date [13]. Surgical methods that weaken the IO muscle have been reported for treatment of SOP. Various IO muscle weakening surgeries have been reported since the first report of IO muscle tenotomy by Duane in 1906 [14]. In 1942, White first reported recession of the IO muscle [7]. Inferior oblique muscle weakening surgeries for IOOA include myotomy, disinsertion, denervation − extirpation, myectomy, anterior transposition, and various forms of recession [15, 16]. The choice of surgical method is determined in accordance with the experience and preference of the surgeon as well as the degree of IOOA. In general, although the above-mentioned surgeries are commonly used, in patients with an IOOA grade of +3, anterior transposition should be considered.
Elliott and Nankin reported successful anterior transposition of the IO muscle in patients with vertical deviation angles < 13 PD [8]. Engman et al. reported that anterior transposition of the IO muscle by suturing at 1, 2, and 3 mm anterior to the temporal border of the IR muscle can be used for the treatment of eyes with vertical deviation angles < 15 PD [17].
Patients with low IOOA are treated by recession rather than anterior transposition. However, recession is more challenging than myectomy and requires a longer operation time. Surgical myectomy is usually a short and simple procedure; however, the approach back to the ends of the broken muscle adhesions often results in recurrence of IOOA [18]. On the other hand, anterior transposition involves reinsertion of the IO muscle slightly forward along the temporal aspect of the IR muscle. It has been shown to be more effective than myectomy, particularly in patients with dissociated vertical deviation [19].
Guemes and Wright reported treatment of dissociative vertical deviation by concomitant recession of primary IOOA, unilateral SOP, and IOOA. This graded recession varied according to the severity of IOOA. Recession of the IO muscle was performed at 4 mm posterior and 2 mm temporal, 4 mm posterior, 3 mm posterior, 2 mm posterior, 1 mm posterior, or parallel to the temporal border of the IR muscle. Average corrections of 20, 18, and 15 PD were obtained, respectively, in patients who received 1, 2, and 3 mm posterior recessions [11].
In the present study, a single surgeon performed graded reattachment of the IO muscle to the posterior IR muscle according to the method of Guemes and Wright [11], but with some modifications. First, the extent of surgery performed in the present study was lower compared to that performed in the previous study. Moon and Lee, using the method of Guemes and Wright [11], reported IO muscle overaction in two eyes in which hypertropia developed in the opposite eye. They also reported the development of anti-elevation syndrome in patients who received 0 and 1-mm reattachments [20]. In the present study, none of the patients exhibited remaining head tilt or diplopia, although 4 patients in groups 2, 4, and 5 exhibited slight remaining hypertropia in the primary position, which was still categorized as a good outcome. At the 1-year follow-up, none of the patients exhibited upgaze limitation, except for one patient in group 6, or opposite vertical strabismus. Therefore, we deduced that the more conservative surgical approach used in the present study prevented upgaze limitation as well as hypertropia on the opposite site.
Secondly, Guemes and Wright used the extent of IOOA, which was determined subjectively, as a parameter to determine the extent of surgery [11]. In contrast, the present study employed the angle of vertical deviation in the primary position at far distance, which was determined objectively. This allowed the surgeon to determine the extent of surgery according to the angle of vertical deviation objectively, which allowed grouping.
