Transposition Surgeries in Strabismus
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The history of transposition surgeries in strabismus dates back to 1908, when Hummelsheim first described the transposition procedure for the correction of esodeviation in sixth nerve palsy. A large number of modifications further took place resulting in a large number of variations in the procedure. These procedures however must be avoided in patients with vascular insufficiency, or when there is stiffness of the antagonist extraocular muscle resulting in restriction.
- Indications for transposition surgeries include
- Paralytic strabismus: The most common indication for transposition surgeries in strabismus is paralytic strabismus. Transposition surgeries must be planned only when the forced duction test is free and active force generation test reveals absence of any active force generation.(Figure1)
- Duane’s Retraction Syndrome
- Lost or slipped muscle
Mechanism of Action
The main aim of transposition surgery is to realign the eyes in primary position. Although the results of a transposition procedure are usually good, patients must understand that the ductions in the direction of action of a paralyzed muscle might not improve much. The mechanism of action of transposition procedures is still controversial. Some authors believe that improvement of alignment in the primary position can be due to a transfer of function of the transposed muscle in the direction of action of the paralyzed muscle, through a change in the direction of vector forces of the transposed muscle. Other hypothesis attributes it to the passive restraint created by the transposed muscle which assists in holding the eye in primary position. In augmented procedures with posterior fixation sutures, it has been suggested that rectus muscle pulleys are diverted posteriorly in the direction of transposition while translating the centre of the globe. Probably more than one mechanism acts together to achieve alignment in the primary position.
Transposition procedures for Sixth nerve palsy
Full tendon transposition
A full tendon transposition involves the dis-insertion of the full-width tendons of the vertical rectus muscles and transposition of these towards the lateral rectus.  To prevent the risk of anterior segment ischemia the vertical recti must be carefully dissected to conserve the anterior ciliary artery branches. Extensive dissection should be avoided. (Figure 2a)
The transposed vertical rectus muscles are passed beneath the lateral rectus muscle and attached at the respective opposite corners of the lateral rectus muscle insertion. This technique increases the transposed muscle path length while avoiding resection of the transposed muscles. In the original description for abducens nerve palsy, it reduced the need for simultaneous ipsilateral medial rectus weakening more than twofold.(Figure 2b)
After making a limbal based incision superiorly and inferiorly, the superior and inferior muscles are split half-tendon width up to approximately 14 mm from the insertion with the help of a muscle hook. The temporal halves of the superior and inferior rectus muscles are then secured, disinserted and further transposed towards the lateral rectus. The transposed halves of both vertical recti are then sutured to the sclera adjacent to the insertion of the lateral rectus. While splitting and dis-inserting the vertical recti, care must be taken to protect the anterior ciliary circulation. (Figure 3)
To increase the effectiveness of Hummelscheim procedure, resection of the transposed tendon can be added along with recession of medial rectus. (Figure 4)
Effectiveness of Hummelsheim procedure
Modified Hummelsheim procedure has been reported to correct a preoperative deviation of 43 Δ ± 5 Δ to postoperative 6 Δ ± 7 Δ .
Jensen’s procedure was first described by Jensen et al in 1964. In this procedure the muscle bellies of adjacent muscles are tied together without being dis-inserted, therefore decreasing the risk of anterior segment ischemia. Even with this procedure, however, some vascular compromise occurs, and cases of anterior ischemia have been reported.
When performing Jensen’s procedure for lateral rectus palsy, limbal base incisions are made to expose the bellies of the lateral, superior and inferior rectus. The intermuscular septa of all the three muscles is then dissected to expose 12-15 mm of the muscle belly behind its insertion. The muscle bellies of each of the exposed recti are then split half tendon width to at least 12-14 mm from the insertion with the help of a muscle hook. During this step, utmost care must be taken to preserve at least one anterior ciliary artery undisturbed. A 5-0 ethibond or Mersilene suture is then used to form a loop around each of the adjacent muscle belly halves (superior-lateral rectus and inferior-lateral rectus). The sutures are then tied to approximate the muscle bellies approximately 12 mm behind the insertion. (Figure 5)
Effectiveness of Jensen Procedure
Cline et al in their study on 26 patients who underwent Jansen’s procedure with medial rectus recession found that at 6 months follow up 18 eyes had less than 20% of normal abduction saccadic velocity; 9 eyes had 20% to 40%, and 2 patients could not be assessed with saccadic velocities.
Modifications of Jensen's Procedure
Vessel sparing modification: In this modification instead of looping the suture around the entire rectus muscle segments, the suture can be passed beneath the anterior ciliary vessels present on the orbital surface of each muscle.
The technique was described by Nishida et al in 2003. In this technique after making a limbal based conjunctival peritomy the vertical recti are hooked. Intermuscular septum and fascia along the lateral margin of each of the vertical recti are carefully dissected and the vertical muscle belly is then longitudinally split half tendon width to approximately 15 mm from the insertion with a short muscle hook. Following this, 6-0 nylon sutures are used to secure the temporal half of each muscle 8 to 10mm posterior to the insertion. The same sutures are then passed through the sclera 8mm posterior to the insertion of lateral rectus beside the superior or inferior margin. The temporal margin of each vertical rectus is then transposed and anchored onto the sclera. One scleral suture can be added on the inside edge of each transposed muscle to the sclera.(Figure 6)
Effectiveness of Nishida
According to Nishida et al, the surgery resulted in an average correction of 42.4PD.
Modification Of Nishida's Procedure
Nishida et al further modified the procedure and omitted the step of muscle splitting to prevent further damage during the surgical manipulation. 
A limbal based incision is made which helps for better exposure of the vertical recti. 6-0 polypropylene or ethibond sutures are passed through the temporal margins of the vertical recti at a distance of 8 to 10 mm behind the insertion points of the vertical recti. The sutures can be tied there to avoid muscle breakage due to tension during transposition. The same suture is then passed through each scleral wall at a distance of 10 to 12 mm behind the supero-temporal and infero-temporal limbus. The temporal margin of each vertical rectus is then transposed and anchored onto the sclera. (Figure7)
Superior Rectus transposition (SRT)
SRT was described by Jhonston et al. The procedure involves transposing only the superior rectus muscle to the insertion of lateral rectus. This helps to preserve ciliary circulation and simplifies the procedure. The procedure has been found to improve the mean angle of esotropia, abduction limitation, and head turn, with recovery of stereopsis in 80% of patients with no clinically significant induced vertical or torsional diplopia. Although theoretical concerns with the procedure include vertical misalignment and induced torsion, in practice these complications are infrequent.(Figure 8)
Inferior Rectus Transposition (IRT)
IRT is a logical alternative to SRT. When combined with an augmentation suture, either with or without simultaneous adjustable medial rectus recession, IRT was shown to significantly improve the esotropia, abduction, and head turn in a small group of patients who were deemed at risk of hypothetical exacerbation by SRT due to preoperative hypertropia or intorsion of the operative eye. IRT can also achieve a small downward shift in the operative eye that can help correct a preexisting hypertropia.
Transposition Procedures for Third Nerve Palsy
Superior oblique tendon transfer
The procedure was initially described by Peter in 1934. A limbal based peritomy is performed in the superonasal quadrant which is sufficient to provide exposure to both medial and superior recti. The superior oblique is isolated and dissected free of surrounding fascial attachments along the tendon, following underneath and along the nasal side of the superior rectus. The superior oblique tendon is engaged on a small muscle hook on the nasal side of the superior rectus muscle and is secured with a double-armed 6-0 absorbable suture The tendon is disinserted and brought down toward the superior edge of the medial rectus insertion. The tendon is then sutured to the sclera at the superior border of the medial rectus insertion. (Figure 9)
Lateral rectus to Medial Rectus transposition
A limbal based peritomy is performed over the lateral, superior, and medial rectus muscles. The attachments and intermuscular septa of lateral rectus muscle are dissected. The lateral rectus muscle is then secured with 6.0 vicryl sutures and disinserted This suture is then passed beneath the superior rectus muscle in the direction of the medial rectus muscle, thus pulling the lateral rectus muscle in that direction. The lateral rectus is then sutured near the superior margin of the medial rectus insertion. (Figure 10)
Full tendon medial transposition of lateral rectus with augmentation sutures
Saxena R et al introduced a modification of the full lateral rectus transposition, for cases with hypertropia along with exotropia. In this modification, the disinserted lateral rectus is passed under the inferior rectus and inferior oblique. It is then reattached just below the insertion of the medial rectus muscle. Augmentation sutures with 5–0 Ethibond are then placed 8 mm behind the new insertion of the transposed lateral rectus muscle, including at least a third of the muscle fibres in each bite. In their series of patients with hypotropia, the lateral rectus muscle was passed under the superior rectus and superior oblique and reattached at the upper border of the medial rectus along with augmentation sutures.
Medial transposition with Y splitting of lateral rectus
A limbal based peritomy is performed and the lateral rectus is hooked and the intermuscular septa is dissected up to 15mm posterior to the insertion. the lateral rectus muscle is then split half tendon width to approximately 15 mm posterior to the insertion. The split portions of the lateral rectus are then secured with a 6-0 polyester suture, and disinserted. The upper half of the muscle is then passed beneath the superior rectus muscle and superior oblique, and inferior half is moved beneath the inferior rectus muscle and inferior oblique. The two halves of lateral rectus are then inserted near the insertion of medial rectus. (Figure11)
Effectiveness of the procedure
Gokyigit et al, inserted the lateral rectus 1 mm posterior to medial rectus found a success rate of 90 percent with the procedure with the mean pre-op deviation being 73.7Δ ± 8.9Δ and post-op deviation being 11.8Δ ± 1.0Δ.
Augmentation of the split lateral rectus
The procedure has been used in third nerve palsy and synergistic divergence.
The surgery is undertaken through four fornix conjunctival incisions in each quadrant after performing a forced duction test to confirm a free lateral rectus muscle. The lateral rectus is hooked, split in half up to at least 15 mm posteriorly after placing 6-0 Vicryl sutures. Superior oblique is hooked and posterior tenectomy is done as in conventional surgeries. Posterior tenectomy of superior oblique helps in free movements of the transposed muscle under it, reduces the abduction effect and hence brings out greater correction. The upper half of the muscle is then passed beneath the superior rectus muscle, and inferior half is moved beneath the inferior rectus muscle and inferior oblique. The two halves of lateral rectus are then inserted near the insertion of medial rectus. In this modification, in addition to the above surgical steps, equatorial fixation sutures are placed 8 mm behind the new insertion of the split muscles with 5-0 Ethibond sutures, incorporating 25℅ of the muscle.
Transposition Procedures for Duane's Retraction Syndrome
Indications of surgery for Duane's Retraction Syndrome
- Presence of a compensatory head posture towards the affected side causing neck discomfort
- Cosmesis in primary gaze due to the horizontal deviation
- Severe co-contraction
Transposition procedures help improve abduction in cases of Duane's retraction syndrome, although there has always been a concern about the increase in severity of co-contraction after transposition surgery.
Various transposition procedures advocated for Duane's Retraction Syndrome include:
- Full Vertical Rectus transposition
- Full Vertical Rectus transposition with cross suture approach
- Jensen's procedure
- Nishida's procedure
- Superior Rectus Transposition
- Inferior Rectus Transposition
Vertical Muscle Palsy
Knapp’s procedure is indicated in patients of monocular elevation deficit. In this procedure the horizontal recti (both medial and lateral recti) are disinserted full width and transposed superiorly.(Figure 12)
In modified Knapp’s procedure, both the horizontal recti are split into two halves. The lower half is used for correction of horizontal deviation and the superior half is transposed superiorly.(Figure 13)
The augmented Knapp’s procedure is similar to Knapp’s with the addition of a posterior fixation or foster suture
The procedure is relatively less commonly performed, the common indications include double depressor palsy and congenital absence of inferior rectus. In this procedure, the horizontal recti are transposed to the level of inferior rectus.  (Figure10) V.Maurino et al in their study found Inverse Knapp's to correct the vertical deviation in primary position and down gaze from 16.06 prism dioptres (PD) to 7.35 PD and 26.45 PD to 6.66 PD respectively.
- Anterior segment ischemia: Circulation to the anterior segment is provided by seven anterior ciliary arteries and two posterior ciliary arteries. One anterior ciliary artery supplies the lateral rectus muscle and two supply each of the other extraocular muscles. The vertical rectus muscle in particular has a major impact on anterior segment circulation. Disinsertion of the recti leads to disruption of blood supply to the anterior segment increasing the risk of anterior segment ischemia (ASI). Old age, atherosclerotic changes in the blood vessels, hyperviscosity, buckle surgery for retinal detachment and simultaneous disinsertion of two or more recti increase the risk of anterior segment ischemia. ASI is associated with lid edema, conjunctival congestion and chemosis, cataractous changes in the lens, corneal edema, anterior chamber cells and flare. Vertical transposition surgeries along with recession of a horizontal muscle carry the highest risk for ASI. Jensen's procedure, although theoretically safer, can also cause ASI. The risk of ASI is maximum in the first six months, hence a repeat surgery should be planned after 6 months of the initial surgery, providing sufficient time for the development of collateral vessels.
- Undercorrection: Undercorrection in strabismus surgeries is far more common than overcorrection. The second surgery to be done depends upon the cause of residual deviation. If the forced duction test (FDT) is still tight then further recession of the antagonist can be planned, but if FDT is free then further addition of foster sutures can be helpful. In third nerve palsy medial periosteal fixation of globe can be planned.
- Induced Vertical deviation: Induced vertical deviations have been observed in 6-30% patients undergoing vertical rectus transposition for lateral rectus palsy. Vertical deviations can be minimized by meticulous surgical dissection of the attachments between inferior rectus muscle, capsulopalpebral fascia, and Lockwood’s ligament. Induced vertical or cyclotropias from altered tension or direction of action of the superior oblique muscle can also occur; thus dissection of the attachments between the superior rectus muscle and the superior oblique tendon are also important. Intraoperative forced ductions can help identify patients at risk for a new-onset hypotropia
- Other complications of squint surgery include tenon's prolapse, conjunctival prolapse, conjunctival cyst, corneal dellen formation, scleral perforation, slipped muscle, bradycardia and secondary infections.
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