Complete and partial sixth nerve palsies create debilitating symptoms for patients with varying degrees of diplopia depending on gaze. If the deviation is permanent, surgical correction aims to restore the largest binocular field of single vision, eliminate abnormal head posture, reduce or eliminate esotropia, and restore abduction while preserving ipsilateral adduction. Surgical options to improve the alignment include large recessions of the medial rectus ipsilaterally or contralaterally, botulinum toxin injections in the ipsilateral medial rectus, resections of the ipsilateral lateral rectus, and complete or partial transpositions of the superior and inferior rectus to the lateral rectus with and without augmentation sutures. Large recessions of the medial rectus limit full adduction of the eye, reducing the field of single vision. Although Hummelshein first described a partial tendon transposition in 1907 , many modifications on the procedure have ensued, including full or partial tendon transposition, augmentation with resection of vertical recti, posterior fixation sutures  addition of ipsilateral medial rectus recessions, and botox into the ipsilateral medial rectus. Recently, full tendon transposition of only the superior rectus to the lateral rectus with posterior fixation has also been proposed to treat sixth nerve palsies . This article discusses the variations in types of transpositions, reported results, and complications associated with transpositions.
The main purpose of a vertical transposition is to allow better rotation of the eye into the field of the palsied muscle by creating tone through the transposed muscles in primary position. Addition of ipsilateral medial rectus recession will further enhance the rotation of the globe to midline by relaxation of the antagonist. Transpositions increase the area of binocular vision and shift the binocular field toward the palsied gaze. If there is residual action in a partially palsied muscle, resection of that muscle will have greater effect with fewer complications than a transposition . Rotation of the globe beyond the midline toward the palsied muscle indicates residual muscle function or only partial palsy . Lack of rotation beyond the midline may occasional occur because of contraction of the ipsilateral antagonist in long-standing palsies, or restrictive processes that must be ruled out prior to establishing a surgical plan. Narrowing of the lid fissure with attempted abduction may be indicative of contracture of the medial rectus.
IMAGING DATA ON TRANSPOSITIONS
Transpositions function because of the muscle tone present in the transposed muscle, which allows a change in the vector forces on the eye in the primary position. The transposed muscle is stabilized by a pulley that resists the change in the muscle pathway . MRI evaluations confirm that the posterior portion of the transposed muscle is not as deviated toward the palsied muscle as would be expected from the transposition because of the resistance from the pulley. The transposed muscles do not have active innervation in the field of gaze of the palsy. Therefore, procedures that cause accompanying relaxation of the antagonist muscle are more effective in realigning the eyes in primary position and in gaze toward the palsied muscle. Posterior fixations increase the force created by the transposition [6,7,8▪]. On MRI examinations following transpositions with posterior fixation sutures, there is a larger shift in the position of the belly of the transposed muscle in the direction of the transposition compared with transpositions without posterior fixation sutures. In addition, MRI reveals that the transposed muscle pulley is shifted posteriorly, which also increases the transposition effect [5–7].
SURGICAL TECHNIQUES FOR TRANSPOSITIONS
The surgical technique for vertical rectus transpositions can vary. Most reports employ a conjunctival peritomy  or fornix-based conjunctival incision  in the superior and inferior quadrants to initiate the surgery. Most authors report isolating the muscle to be transposed of adjacent structures such as the capsulopalpebral fascia, superior oblique tendon, and upper eyelid attachments. Rosenbaum describes reattachment of the temporal border of the transposed muscle adjacent to the lateral rectus . The nasal border reattached following the spiral of Tillaux. The nasal border may also be reattached adjacent to the temporal pole of the old insertion of the transposed muscle . In addition, the transposed muscle may be recessed if there is restriction to adduction, elevation or depression, respectively, following transposition . Alternatively, the transposed muscle can be resected symmetrically prior to transposition [9,10,11▪]. As originally described by Foster, a posterior fixation suture is a single-armed permanent suture placed in the sclera 16 mm from the limbus at superior border of the lateral rectus. This suture is then passed 8 mm from the insertion of the superior rectus muscle incorporating approximately 25% of the transposed muscle . A similar suture is passed though the transposed inferior rectus. Forced ductions at the conclusion of the transposition with posterior fixation should be free. Paysee describes a partial tendon transposition of 75% of the inferior and superior rectus muscle with posterior fixation as described by Foster . Mehendale describes a loop myopexy to close the gap in the transposition procedure in place of the posterior fixation suture . A double-armed polyester suture is placed 8–12 mm from the insertions through the superior quarter of the lateral rectus and temporal quarter of the superior rectus, respectively. No scleral pass is performed. Other authors report injection of botulinum toxin into the ipsilateral medial rectus at the conclusion of vertical rectus transposition or in the immediate postoperative period [13,14]. One other additional surgical technique to help prevent induced torsion is marking the 12 o’clock and six o’clock positions on the cornea preoperatively . Assessing the markings following transposition may reveal a torsional shift, which may accompany an induced vertical deviation. In response to torsion, posterior fixation sutures can be loosened to relieve the torsion created and prevent vertical misalignment.
In addition to variations in the surgical technique, the degree of palsy of the lateral rectus muscle varies between patients. Therefore, comparing surgical results of the various techniques of transposition is difficult. Furthermore, the postoperative results of transposition procedures are reported differently in the varying articles. The postoperative results may include deviation change in the primary position or residual esotropia, change in degrees of binocular visual field, change in amount of head turn, and increase in abduction with use of scales or degrees. Comparative analysis consistently reveals improvements in degrees of binocular field with transpositions . Transposition of both vertical rectus muscles without antagonist recession or posterior fixation corrected on average 32 PD (prism diopters) of esotropia . Transposition with medial rectus recession or botulinum toxin injection increased the surgical effect to 30–50 PD of esotropic correction [13–14].
In Rosenbaum's analysis, if the initial esotropia in primary position was greater than 20 PD, augmentation with posterior fixation had a greater chance to relieve the esotropia in primary position. Transpositions with augmentation suture had an average of five PD residual esotropia compared with 12 PD residual esotropia without augmentation suture . Vertical transposition with Foster suture corrected an average of 40–55 PD of esotropia in primary [2,15,17▪▪,18,19].
A partial vertical rectus transposition of both superior and inferior rectus muscles with posterior fixation sutures corrected 50 PD of esotropia . Interestingly, partial vertical rectus transposition with resection of the transposed muscle achieved similar effect to full vertical rectus transposition of about 30 PD, from 30.6 ± 12.9 PD of esotropia preoperatively to 10.6 ± 8.8 PD esotropia postoperatively [11▪]. Addition of medial rectus recession to partial transposition achieved a larger effect of 39–52 PD [10,20].
Superior rectus transposition with myopexy suture as described by Mehendale corrected approximately 30–35 PD of esotropia [3,21▪,22▪]. One of the patients in Mehendale's series did have subsequent transposition of the inferior rectus to the lateral rectus with further improvement in alignment from 25 PD esotropia to 10 PD esotropia .
In summary, transposition of both vertical rectus muscles corrects a greater amount of esotropia than transposition of the superior rectus alone. Addition of medial rectus recession or botulinum toxin injection in the medial rectus causes a greater change in esotropic deviation, as does a posterior fixation suture. Finally, resection of the transposed muscles mildly increases the effect of the transposition (Table 1).
The binocular field increases with transposition procedures compared with recession of the medial rectus with resection of the lateral rectus for abducens palsy, from 25° to 41–51° . Addition of a posterior fixation suture to the transposition further increases the degrees of binocular field to 71° [2,17▪▪]. The residual face turn was similarly smaller with augmentation sutures . There is improvement in abduction with transposition and posterior fixation, but quantification remains difficult (Table 1).
COMPLICATIONS OF TRANSPOSITIONS
The potential complications of vertical rectus transpositions in addition to those of any extraocular muscle surgery are new vertical deviation in primary, new torsion, and anterior segment ischemia. With vertical rectus transposition with medial rectus recession, vertical deviations can occur in approximately 0–12% of patients [11▪,13]. The induced vertical deviation is generally small with ranges less than 4 PD [11▪]. Nevertheless, some cases require additional vertical rectus recession to correct the deviation. The vertical transposition with posterior fixation increases the incidence of vertical deviation with a range of 1–19% [2,15,19]. With transposition of the superior rectus with myopexy suture and recession of the medial rectus, a new onset vertical deviation occurred in 1–28% of patients [3,21▪,22▪]. The reported vertical deviations remain small for all types of transpositions. Small degrees of both incyclotorsion or excyclotorsion have been reported following both vertical rectus transpositions with posterior fixation  and superior rectus transposition with myopexy [21▪,22▪].
Another complication of transposition especially with accompanying medial rectus recession is anterior segment ischemia. Anterior segment ischemia may occur more frequently in patients with compromised circulation . There are a handful of case reports of anterior segment ischemia after transposition with posterior fixation without medial rectus recession [24,25]. The augmentation suture may have impeded the posterior ciliary artery circulation . In addition, the reported cases of anterior segment ischemia also had peritomy to open the conjunctiva, which might have contributed to the disruption of the anterior circulation.
Transposition procedures allow for some restoration of tone in the direction of action to a paralyzed or nonfunctional muscle. Meticulous attention to the techniques of surgery such as incision type, careful dissection to free the transposed muscle of attachments to the superior oblique, and eyelid attachments may reduce complications such as induced vertical deviations and anterior segment ischemia. Techniques such as marking the 12 and 6 o’clock positions may reduce the incidence of torsion. Forced ductions to ensure free adduction and elevation and depression posttransposition may also help guide the need to recess transposed muscles or loosen fixation sutures. Unfortunately, loosening fixation sutures and recessing the transposed muscles reduces the effect on the alignment in primary position. Assessing each patient for residual function of the palsied muscle, contracture of the antagonist muscle, and the amount of the deviation in primary should influence the choice of the transposition procedure.
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REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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