Strabismus Surgery Kenneth W

Strabismus Surgery Kenneth W. Wright and Pauline Hong

his chapter discusses various strabismus surgery procedures Tand how they work. When a muscle contracts, it produces a force that rotates the globe. The rotational force that moves an eye is directly proportional to the length of the moment arm (m) (Fig. 11-1A) and the force of the muscle contraction (F) (Fig. 11-1B). Rotational force m F where m moment arm and F muscle force. Strabismus surgery corrects ocular misalignment by at least four different mechanisms: slackening a muscle (i.e., recession), tightening a muscle (i.e., resection or plication), reducing the length of the moment arm (i.e., Faden), or changing the vector of the muscle force by moving the muscle’s insertion site (i.e., transposition).

MUSCLE RECESSION

A muscle recession moves the muscle insertion closer to the muscle’s origin (Fig. 11-2), creating muscle slack. This muscle slack reduces muscle strength per Starling’s length–tension curve but does not signiﬁcantly change the moment arm when the eye is in primary position (Fig. 11-3). The arc of contact of the rectus muscles wrapping around the globe to insert anterior to the equator of the eye allows for large recessions of the rectus muscles without signiﬁcantly changing the moment arm. Figure 11-3 shows a 7.0-mm recession of the medial and lateral rectus muscles. Note there is no change in the moment arm with these large recessions. Thus, the effect of a recession on eye position is determined by the amount of muscle slack created.1a The

388 chapter 11: strabismus surgery 389

FIGURE 11-1A,B. (A) Diagram of the horizontal rectus muscles shows the relationship of the moment arm (m) to the muscle axis and center of rotation. The moment arm intersects the center of rotation and is per- pendicular to the muscle axis. The longer the moment arm, the greater the rotational force. (B) Starling’s length–tension curve. The relationship of a muscle’s force is proportional to the tension on the muscle. More tension on a muscle increases muscle force and slackening a muscle reduces its force. Note that the relationship is exponential, not linear: toward the end of the curve, a small amount of slackening produces a dis- proportionately large amount of muscle weakening.

AB C FIGURE 11-2A–C. Drawing of rectus muscle recession (shaded muscle). The effect of the recession is greatest when the eye rotates toward the recessed muscle. (A) The eye rotates toward the recessed muscle, causing the recessed muscle to tighten, therefore reducing muscle slack. (B) A rectus muscle resection resulting in muscle slack. (C) The eye rotates toward the recessed muscle, and the muscle and the muscle slack increase. 390 handbook of pediatric strabismus and amblyopia

5.5

7.0 m

FIGURE 11-3. Medial rectus muscle recession. Diagram shows normal insertion at 5.5 mm posterior to the limbus and a 7.0-mm medial rectus recession. In primary position, the moment arm (m) has not changed, so the effect of the recession is to create muscle slack rather than to change the moment arm. amount of muscle slack is most accurately determined by measuring the recession from the muscle insertion.8 Note the exponential character of the length–tension curve, as there is a precipitous loss of muscle force at the end of the curve when muscle slack is increased (see Fig. 11-1B); this is why even small, inadvertent inaccuracies of large recessions (6– 7mm) can cause dramatic changes in muscle force and result in an unfavorable outcome. Technical mistakes, such as allowing central muscle sag and not properly securing the muscle, can lead to large overcorrections. For example, each 0.5mm of bilateral medial rectus recessions up to a recession of 5.5mm chapter 11: strabismus surgery 391 will correct approximately 5 prism diopters (PD) of esotropia. However, for recessions greater than 5.5 mm, each additional 0.5mm of recession results in 10 prism diopters of correction (see chart on inside cover). Thus, an overrecession of only 1.0mm on a planned 6.0-mm bilateral medial rectus recession would result in a 20-prism diopter overcorrection. Figure 11-4 shows the proper rectus muscle recession, with the muscle well secured and no central muscle sag. The best way to prevent central muscle sag is to broadly splay the new insertion so it is approximately the same width as the original insertion. A rectus muscle recession has its greatest effect in the ﬁeld of action of the muscle. Figure 11-2 shows that muscle slack increases when the eye rotates toward the recessed muscle, thus reducing the rotational force on gaze toward the recessed muscle. In contrast, eye rotation away from the recessed muscle causes muscle slack to be reduced. In addition, on rotation away from the recessed muscle, the recessed muscle is inhibited (Sherrington’s law), minimizing the effect of the recession in this gaze. For example, a right medial rectus recession will produce an incomitant strabismus, with an exodeviation in primary position and a larger exodeviation in leftgaze with very little exodeviation in rightgaze. Induced incomitance can correct incomitant strabismus. If a patient has a small esotropia in primary position and a large esotropia in leftgaze, a right medial rectus recession would reduce the incomitance. Comitant strabismus, on the other hand, is best treated with bilateral symmetrical surgery. Recessions are routinely performed on rectus muscles but can also be performed on oblique muscles. Inferior oblique muscle recession is a popular procedure for weakening the inferior oblique muscle. Recession of the superior oblique tendon has also been described. It not only slackens the superior oblique tendon but also changes the function of the muscle. A recession of the superior oblique tendon collapses the normally broad insertion and moves the new insertion nasal and anterior to the globe’s equator. This alteration changes the function of the superior oblique muscle and can result in unpredictable outcomes, including postoperative limitation of depression. A more con- trolled way of slackening the superior oblique tendon without changing the functional mechanics of the tendon insertion is a tendon-lengthening procedure, such as the Wright silicone tendon expander. 392 handbook of pediatric strabismus and amblyopia

FIGURE 11-4A,B. (A) Drawing of rectus muscle recession with the muscle secured to sclera at the recession point posterior to the original insertion. Note that the new insertion is almost as wide as the original scleral insertion, and the new insertion is parallel to the original insertion. There is no central muscle sag. (B) Companion photograph shows a rectus muscle recession with no central sag because the new insertion is splayed as wide as the original insertion. chapter 11: strabismus surgery 393

Hang-Back Technique A hang-back recession suspends the muscle back, posterior to the scleral insertion, with a suture (Fig. 11-5). This technique has the advantage of excellent exposure and relatively easy needle passes through the thick anterior sclera. On the other hand, hang-back recessions are potentially less accurate than a ﬁxed recession. Small to medium-sized hang-back recessions of 3 to 6 mm tend to result in overcorrections because they have inherent central muscle sag (Fig. 11.5). On the other hand, large hang-back recessions, over 6mm, tend to produce undercorrec- tions because an otherwise normal muscle will not consistently retract more than 6 to 7mm posterior to the insertion. The surgeon experienced with adjustable suture surgery knows it is difﬁcult to recess a rectus muscle more than 6mm using an adjustable hang-back suture. Large hang-back recessions are

FIGURE 11-5. Hang-back recession. The suture is passed through sclera at the original insertion and the muscle is suspended posteriorly to achieve the recession. Inset: Note the caliper is measuring the planned recession; however, the muscle is overrecessed because of central sag. Central sag occurs because the new insertion is lax and not splayed as widely as the original insertion. 394 handbook of pediatric strabismus and amblyopia possible if the muscle is tight and contracted, as in the case of thyroid-associated strabismus, congenital fibrosis syndrome, or a slipped muscle. Indications for hang-back recessions include a recession over a retinal buckle, recession over an area of scleral ectasia, or large recessions, of a tight contracted muscle, if posterior exposure is difficult. However, for routine strabismus surgery, the author (K.W.W.) prefers the fixed recession so the muscle is secured at the desired recession point.

Adjustable Suture Technique Adjustable suture techniques allow movement of the muscle position after surgery when the patient is fully awake and the anesthesia has dissipated (Fig. 11-6). Unlike fixed sutures, the adjustable suture technique allows for fine-tuning of ocular alignment in the immediate postoperative period. The adjustable suture procedure is usually performed on recessions in two stages: in the first stage, surgery is performed under either local or general anesthesia, and the muscle is placed on a suture in such a way that the muscle position can be adjusted later. The second stage, or adjustment phase, is performed when the patient is fully awake or after the local anesthetic has worn off (5h for lidocaine) and the muscle function has returned to normal. In this phase, the muscle is adjusted to properly align the eyes and then permanently tied in place. The adjustment procedure must be performed within 24 to 48h after the initial surgery while the muscle is still freely mobile. Later adjust- ments have not been recommended because the muscle rapidly adheres to the globe. However, successful in-office reoperation within the first week of surgery has been described.5 The muscle is sutured like a hang-back recession, but the suture is tied in a bowknot or secured by a sliding noose so the position of the

FIGURE 11-6A–C. (A) Bow tie adjustable suture technique. After the sutures have been passed through the scleral insertion, they are tied together in a single-loop bow tie. This bow tie can be untied postoperatively to adjust the muscle. (B) Noose adjustable suture. Sutures suspend the muscle posteriorly, and a noose around the sutures slides up and down to secure the muscle at the desired position. The ocular alignment is ﬁne- tuned with the patient awake. The muscle placement is ﬁnalized by tying off the pole sutures, then trimming all loose sutures. (C) Companion photograph of (B) shows adjustable suture through fornix, with scleral traction suture holding the conjunctiva superiorly and exposing the adjustable suture. chapter 11: strabismus surgery 395

C 396 handbook of pediatric strabismus and amblyopia muscle can be easily changed (Fig. 11-6A–C). Adjustable sutures have limitations similar to hang-back recessions, with the maximum recession approximately 6 to 7mm. Central sag occurs but, because the muscle position can be changed after surgery, this is usually not an issue. Plan on a slight overcorrection, as advancing an over-recessed muscle is easier than trying to increase the recession, especially if the recession is greater than 6 to 7mm. The most important indication for an adjustable suture is complicated strabismus, including paralytic strabismus, large- angle strabismus, reoperations, and thyroid myopathy. In these situations, the standard tables for surgical measurements do not apply, and results with the fixed-suture technique are unpredictable. In addition to the more complicated strabismus cases, many surgeons routinely use adjustable sutures on most cooperative adult patients, even those undergoing uncomplicated, horizontal surgery. Adjustable sutures are usually used with recession procedures, as adjustable tightening procedures are difficult to perform. Patient selection is crucial for successful implementation of the adjustable suture technique. The adjustment procedure is somewhat uncomfortable and can evoke substantial anxiety. There is no specific age limitation for the use of adjustable sutures, but patients younger than 15 years of age are often too anxious about medical procedures. Unless a child is exception- ally calm and cooperative, adjustable sutures should be limited to cooperative adult patients. Strong sedatives before adjustment should be avoided because sedation influences eye position. The patient should wear full optical correction when ocular alignment is being assessed during the adjustment procedure to ensure proper image clarity and control of accommodation.

MUSCLE SHORTENING PROCEDURES

Muscle shortening procedures include muscle resections, tucks, and plications. These procedures tighten the muscle, but they do not actually strengthen the muscle. For the most part, they correct strabismus by creating a tight muscle that acts like a leash or tether. These procedures produce incomitance, as the tightened muscle restricts rotation away from the shortened muscle (Fig. 11-7). For example, a right medial rectus shortening procedure limits abduction of the right eye and creates an chapter 11: strabismus surgery 397

ACB FIGURE 11-7A–C. Effect of a rectus muscle resection (shaded muscle). The resection has its greatest effect on gaze away from the resection. (A) The muscle tightens on gaze away from the resected muscle. (B) A resected rectus muscle. (C) The muscle slackens on gaze to the resected muscle. esodeviation shift that increases in rightgaze. Right medial rectus tightening would be indicated to correct an incomitant exotropia that is greater in rightgaze. Note that tightening the medial rectus muscle does not strengthen adduction but instead limits abduction. Bilateral medial rectus resections limit diver- gence and induce an esodeviation greater for distance ﬁxation; therefore, it is not the answer for convergence insufﬁciency.

Resection A muscle resection consists of tightening a muscle by removing the anterior part of the muscle and reattaching the shortened muscle to the original insertion site. The muscle resection is the most popular tightening procedure and is performed on rectus muscles.

Tuck A muscle tuck shortens the muscle by folding the muscle and suturing the folded muscle to muscle. The muscle tuck has, for the most part, fallen out of favor partially because the muscle- to-muscle suturing does not hold well and tends to become cheese-wire loose over time. A superior oblique tendon tuck or plication, however, is used for some cases of superior oblique 398 handbook of pediatric strabismus and amblyopia palsy, either as a full-tendon plication or plication of the anterior tendon ﬁbers (i.e., Harada–Ito procedure).

Wright Plication The author (K.W.W.) developed a rectus muscle plication procedure that tightens the muscle by folding the muscle and suturing it to sclera (Fig. 11-8).14,18 With the plication, the muscle is sutured to the scleral insertion, in contrast to a tuck, where muscle is sutured to muscle. The muscle–scleral attachment of

B FIGURE 11-8A,B. Wright rectus muscle plication. (A) The muscle is secured with the suture placed posterior to the insertion at the desired plication point (usually 6 mm or less). Once the posterior muscle is secured, the suture ends are passed through the scleral insertion. The drawing shows the suture secured to the posterior muscle and the double- armed needles being passed at the scleral insertion. (B) The plication is completed with the posterior muscle advanced to the insertion. There is a small roll of redundant tendon that will ﬂatten and disappear 3 to 4 weeks after surgery. chapter 11: strabismus surgery 399 the plication is more secure than the muscle-to-muscle union of a tuck. The plication can be used in place of a standard resection. Because there is a fold of tendon associated with the plication, a small lump is present immediately after surgery but disappears within 3 to 4 weeks. Important advantages of the plication procedure over resection include reversibility. A plication can be removed by simply cutting and removing the suture within 2 days of the surgery, before the muscle heals to sclera. Another advantage is safety against a lost muscle. Because the muscle is not disinserted, there is little risk of a lost muscle. The plication procedure also preserves the anterior ciliary vessels and reduces the risk of anterior segment ischemia. These advantages have made the Wright plication popular for small or medium- sized rectus muscle tightening surgeries.

RECESSION AND RESECTION

Resections (or plications) of rectus muscles can be teamed with a recession of the antagonist muscle same eye to correct strabismus. This monocular surgery is called a recession–resection, or “R & R,” procedure. The effect of the recession–resection of agonist and antagonist induces incomitance and limits ocular rotation in one direction. For example, a right lateral rectus muscle recession reduces ocular rotation to the right, and a resection of the right medial rectus muscle also restricts rotation to the right. Limited rotations after an R & R procedure may improve over several months to years, but some residual incomitance often persists. Because the R & R procedure induces incomitance, it can be used to treat incomitant strabismus. It is also useful in treating sensory strabismus, allowing monocular surgery to be performed only on the amblyopic eye and sparing surgery to the good eye.

FADEN

The Faden procedure is performed by suturing the rectus muscle to sclera, 12 to 14 mm posterior to the rectus muscle insertion. This technique pins the rectus muscle to the sclera so, when the eye rotates toward the fadened muscle, the arc of contact cannot unravel. As a result, the moment arm shortens, thus reducing 400 handbook of pediatric strabismus and amblyopia the rotational force. The Faden, however, does not significantly change the moment arm when the eye is in primary position, and it has no effect when the eye is turned away from the muscle with the Faden (Fig. 11-9). Thus, a Faden reduces ocular rotational force when the eye rotates toward the fadened muscle and is used to correct incomitant strabismus. The weakening effect of the Faden operation by itself is relatively small, so the fadened muscle is usually also recessed as part of the Faden procedure. The Faden operation works best on the medial rectus muscle because the medial rectus muscle has the shortest arc of contact (approximately 6mm), and a 12- to 14-mm Faden significantly changes its arc of contact. Alter- nately, a Faden of the lateral rectus muscle has little effect because the arc of contact is 10mm, and pinning the muscle at 12mm does not significantly change this naturally long arc of contact. For the most part, the Faden operation is indicated to correct incomitant esotropia by enhancing the effect of a medial rectus recession, such as in the case of sixth nerve paresis or high AC/A esotropia. The following case is an example where a

A FIGURE 11-9A. Faden of rectus muscle. (A) In primary position, the Faden does not signiﬁcantly change the moment arm (m). FIGURE 11-9B–C. (B) Ocular rotation toward the Faden results in shortening of the moment arm (m) as the muscle is pinned to sclera. (C) On rotation away from the Faden, the moment arm (m) is normal and the faden has no signiﬁcant effect. Thus, the Faden weakens the muscle on rotation toward the fadened muscle.

401 402 handbook of pediatric strabismus and amblyopia

Faden and recession of the right medial rectus muscle is indicated. One may rightfully argue, however, that a large (5-mm) right medial rectus muscle recession would also work without the difﬁculty of performing the Faden. As you will see, there is often more than one way to approach a strabismus.

Rightgaze Primary position Leftgaze Orthotropia E 4 ET 10

ET, esotropia. Surgery: recess the right medial rectus muscle 3 mm with a Faden.

Sixth Nerve Paresis An example where the Faden may be effective is a partial sixth nerve paresis and good lateral rectus function. The standard surgery has historically been a recession of the medial rectus muscle and resection of the lateral rectus muscle of the paretic eye, which helps correct the esodeviation in primary position but does not address the large esotropia that occurs with gaze to the side of the paretic lateral rectus muscle. Incomitance can be improved with a recession and a Faden operation of the contralateral medial rectus muscle. A Faden to the contralateral medial rectus muscle helps correct the esotropia that increases in the side of the paretic lateral rectus muscle by decreasing the rotational force of the yoke medial rectus, thus matching the paretic lateral rectus muscle. Matching yoke muscles only works if there is good lateral rectus function with no more than 1 limitation of abduction.

High AC/A Ratio Esotropia Theoretically, the Faden operation reduces convergence at near, thus lowering the AC/A ratio. Experience with this procedure indicates that most patients still require a bifocal add to obtain fusion at near. Augmented bilateral medial rectus recessions probably work just as well.7 The use of a Faden operation with a medial rectus recession in high AC/A ratio esotropia patients remains controversial.

MUSCLE TRANSPOSITION PROCEDURES

Transposition surgery is based on changing the location of the muscle insertion so the muscle pulls the eye in a different direction (i.e., changes the vector of force). Transposition surgeries chapter 11: strabismus surgery 403 can be used to treat A- and V-patterns, small vertical tropias, rectus muscle paresis, and torsion.

Horizontal Muscle Transposition for A- and V-Patterns See Chapter 9: A- and V-Patterns and Oblique Dysfunction.

Transposition for Small Vertical Deviations Transposition surgery can correct small vertical deviations by vertically offsetting the horizontal rectus muscles. A patient with an esotropia and a small right hypertropia, for example, can be corrected by a recession–resection procedure of the right eye with inferior infraplacement of the horizontal rectus muscles. By transposing the horizontal rectus muscles inferiorly, they act to pull the eye down, thus correcting the hypertropia. Each horizontal muscle is recessed or resected as speciﬁed by the mag- nitude of the horizontal deviation. There is approximately 1 prism diopter of improvement in the vertical deviation per 1mm of displacement; this is true when two muscles in the same eye are transposed in the same direction. Vertical muscle displacements as large as 6 to 7mm may be readily performed with this technique. It is most useful when the surgeon is performing monocular recession–resection surgery in which both muscles are moved in the same direction (Fig. 11-10). This surgery, however, is not effective if restriction is present (e.g., thyroid orbitopathy).

FIGURE 11-10. Full-tendon-width inferior transposition of both horizontal rectus muscles. The muscle on the left has been resected and infraplaced; the muscle on the right has been recessed and infraplaced. This technique would be used with a recession/resection procedure to correct a hypertropia and horizontal strabismus. 404 handbook of pediatric strabismus and amblyopia

Transposition Procedures for Rectus Muscle Palsy Three transposition procedures used to correct severe rectus muscle palsies are described here: Knapp, Jensen, and Hummelsheim. In a right lateral rectus palsy, there is limited abduction and a large esotropia that increases in rightgaze. If there is less than 50% lateral rectus function, the treatment should be a lateral transposition of all or part of the superior and inferior rectus muscles. Because the vertical muscles do not con- tract on attempted abduction, the amount of abduction would relate to the elasticity or tonic contraction of the transposed muscles, rather than the active contraction of the transposed muscles.

KNAPP PROCEDURE A full-tendon transfer, or Knapp procedure, was originally described for the management of double elevator palsy. This procedure, however, can also be used for a sixth nerve palsy. The key for successful surgery is symmetrical transposition to avoid induced vertical or horizontal deviations. A large posterior dissection to free the muscle of the intermuscular septum and check ligaments is necessary to mobilize the muscle for the tendon transfer (Fig. 11-11).

JENSEN PROCEDURE The Jensen procedure is a split-tendon transfer with the adjacent muscle tied together but not disinserted (Fig. 11-12). This procedure has the advantage of leaving the anterior ciliary arteries intact, diminishing the risk of anterior segment ischemia. Even with the Jensen procedure, however, some vascular compromise occurs, and anterior segment ischemia has been associated with this procedure.

HUMMELSHEIM PROCEDURE The Hummelsheim procedure is a split-tendon transposition technique designed to preserve anterior ciliary artery perfusion. Half of each of the two rectus muscles adjacent to the weak muscle is mobilized. The halves are then transposed and inserted at the insertion of the weak or lost muscle (Fig. 11-13). In contrast to the Jensen procedure, the Hummelsheim procedure can be used for a lost muscle, as it does not require the FIGURE 11-11. Knapp procedure. The medial rectus (MR) and lateral rectus (LR) muscles are transposed superiorly to the insertion of the superior rectus (SR) muscle.

FIGURE 11-12. Jensen procedure. Nonabsorbable sutures tie muscle halves from adjacent muscles. The ﬁnal result shows the tendon unions of superior rectus to lateral rectus and inferior rectus to lateral rectus muscles. The posterior location of the union is important, and sutures should be at least 12 mm posterior to the insertions. Anterior union sutures will reduce the effect of the transposition.

FIGURE 11-13. Hummelsheim procedure. Half of each of the superior and inferior rectus muscles is transposed to the lateral rectus insertion. Note that the transposed muscle halves touch the lateral rectus insertion, and the muscles are sutured together 3 mm posterior to the insertion (Foster modiﬁcation). 406 handbook of pediatric strabismus and amblyopia presence of the weak muscle. The Hummelsheim procedure is the author’s procedure of choice for a muscle palsy.

MODIFICATION OF THE HUMMELSHEIM Two modifications of the Hummelsheim procedure, which increase the effect of the transposition, are described here. Augmented Hummelsheim Brooks of Augusta, Georgia, has augmented the Hummelsheim by resecting 4 to 6 mm of the transposed rectus muscle halves. Resecting some of the transposed muscle halves tighten the transposition, increasing the leash effect. Muscle Union Modification (Foster modification) Increased effect of the Hummelsheim has been suggested if the transposed muscle is sutured to the paretic muscle. The transposed and paretic muscles are sutured together and then to sclera, 4 mm posterior the insertion.

Complications of Transposition Surgery Transposition procedures for rectus muscle palsies can induce unwanted deviations if there is asymmetrical muscle placement. In split-tendon procedures, it is important to split and transpose the muscle equally to prevent inadvertent deviations. Anterior segment ischemia is always an important consideration. Split-tendon procedures such as the Jensen and Hum- melsheim lessen the risks, but even these procedures have been associated with anterior segment ischemia. The best strategy is to preserve as many anterior ciliary arteries as possible. A limbal conjunctival incision disrupts local vessels and may increase the risk of anterior segment ischemia, suggesting that a fornix incision may be preferable.

Rectus Muscle Transposition for Torsion Torsional strabismus can be improved by moving vertical rectus muscles nasally or temporally. Nasal placement of the superior rectus causes extorsion (corrects intorsion) whereas temporal placement causes intorsion (corrects extorsion). The opposite is true for the inferior rectus muscle, with nasal transposition induces intorsion (corrects extorsion) and temporal transposition induces extorsion (corrects intorsion). Transposition of a chapter 11: strabismus surgery 407 tendon width (approximately 7mm) will induce about 4° to 5° of torsion. Most of the torsional effect is seen in the ﬁeld of action of the transposed muscle. If the superior rectus muscle is nasally transposed 7mm and the inferior rectus muscle temporally transposed 7mm, a total of 8° to 10° of extorsion would be induced, thus correcting 8° to 10° of intorsion. Horizontal rectus muscle transposition will also produce some torsional changes, but less than vertical rectus muscle transpositions. Supraplace- ment of the medial rectus muscle induces intorsion; infraplacement induces extorsion. The opposite is true for the lateral rectus muscle. It is unusual for a vertical transposition of a horizontal muscle to induce signiﬁcant torsion. Most cases of torsional strabismus are caused by oblique dysfunction and are best treated with oblique muscle surgery to correct the torsion. For example, extorsion associated with bilateral superior oblique paresis is usually best handled with a bilateral Harada–Ito procedure, not a rectus muscle transposition.

INFERIOR OBLIQUE MUSCLE WEAKENING PROCEDURES

Surgical management of inferior oblique muscle overaction is based on weakening or changing the function of the inferior oblique muscle. Techniques include myectomy, recession, and anterior transposition. Inferior oblique myotomy is not effective because the cut ends of the muscle inevitably reunite or scar to sclera; this causes residual inferior oblique overaction and an unacceptably high reoperation rate. Myectomy weakens the inferior oblique, as removing a portion of muscle reduces the chance of local reattachment. A very large myectomy with surgical transection of the neurovascular bundle virtually elimi- nates inferior oblique overaction and is termed inferior oblique extirpation–denervation. Extirpation–denervation may be indicated for severe residual inferior oblique overaction after previ- ous inferior oblique surgery. An inferior oblique recession places the insertion closer to the origin and induces muscle slack, thus reducing muscle tension (Fig. 11-14). Apt1 and Elliot4 were the ﬁrst to describe the inferior oblique anterior transposition. It is similar to a recession, but the inferior oblique muscle insertion is moved anterior to its origin, thus changing the function of the inferior oblique muscle from an elevator to more of a depressor 408 handbook of pediatric strabismus and amblyopia

FIGURE 11-14. Inferior oblique recession. The muscle is reattached along the path of the inferior oblique, but closer to its origin, thus slackening the muscle.

(Fig. 11-15). The more anterior the placement of the inferior oblique muscle insertion, the more the muscle becomes a depressor. This procedure has been shown to be very effective for treating both primary inferior oblique overaction and inferior oblique overaction secondary to superior oblique palsy.6 One possible complication of the anteriorization procedure is postoperative limited elevation. Limited elevation usually occurs from three possible mechanisms: (1) the new insertion is too anterior (i.e., anterior to the inferior rectus insertion); (2) resection of too much muscle (3mm) at the time of securing

FIGURE 11-15. Inferior oblique anterior transposition. The diagram shows placement of the inferior oblique (IO) muscle in relationship to the inferior rectus (IR) insertion. The inferior oblique muscle is placed 1 mm posterior to the inferior rectus insertion. Note that the posterior inferior oblique muscle fibers are placed posterior to the anterior fibers and parallel to the inferior rectus muscle (no J-deformity). chapter 11: strabismus surgery 409 and disinserting the inferior oblique muscle; and (3) anterior placement of the posterior fibers of the inferior oblique muscle. Stager described this last mechanism as a common cause for limited elevation after the anterior transposition procedure. The posterior fibers of the inferior oblique muscle are important, as the neurovascular bundle of the muscle inserts into these muscle fibers. Because the neurovascular bundle is inelastic, large anteriorizations of the posterior muscle fibers will create a J-deformity of the muscle, with the neurovascular bundle tethering the inferior oblique muscle and limiting elevation of the eye (Fig. 11-16).12a To prevent postoperative limitation of elevation, the author (K.W.W.) recommends:

FIGURE 11-16. Full anteriorization of the inferior oblique muscle including the posterior fibers with J-deformity. Anteriorization of the posterior fibers creates the J-deformity, as the neurofibrovascular bundle tethers the posterior muscle fibers; this can limit elevation of the eye. Because of this complication, the author (K.W.W.) does not perform the “J” deformity anteriorization, except if performed bilaterally for severe dissociated vertical deviation (DVD) and inferior oblique overaction. 410 handbook of pediatric strabismus and amblyopia

1. Keep the new insertion at or behind the inferior rectus insertion. 2. Secure the muscle close to its insertion to avoid resecting too much muscle (this would shorten the muscle). 3. Avoid the “J” deformity by keeping the posterior muscle ﬁbers posterior to the anterior muscle ﬁbers and posterior to the inferior rectus muscle insertion by at least 3mm.8,14 The full anteriorization with a “J” deformity has been used for the treatment of bilateral dissociated vertical deviation (DVD) with inferior oblique overaction. If performed, the full anteriorization with “J” deformity should be performed bilaterally to avoid asymmetrical elevation of the eyes.

Graded Recession–Anteriorization The author (K.W.W.) has reported on a graded recession– anteriorization approach for the management of inferior oblique overaction.8,14 This procedure tailors the amount of anteriorization according to the amount of inferior oblique overaction. The basis of the graded anteriorization procedure is that the more anterior the inferior oblique insertion, the greater the weakening affect. Table 11-1 lists the inferior oblique placement for a speciﬁc amount of inferior oblique overaction and represents only a guideline for the management of inferior oblique overaction. The ﬁnal surgical decision must be based on a combina- tion of factors, including the amount of V-pattern and the presence of a vertical deviation in primary position. Asymmet- rical graded anteriorization is indicated if a hypertropia is present in primary position; otherwise, consider symmetrical surgery. More anteriorization of the inferior oblique should be done on the side of the hyperdeviation. A full anteriorization (without J-deformity) on the side of the hypertropia and 4 mm anteriorization on the opposite side will correct approximately 6 prism diopters (PD) of hypertropia. In the case of a unilateral

TABLE 11-1. Graded Recession–Anteriorization of Inferior Oblique Muscle. Overaction Inferior oblique placement 1 4 mm posterior and 2 mm lateral to inferior rectus (IR) insertion 2 3 mm posterior to IR insertion 3 1–2 mm posterior to IR insertion 4 At the IR insertion chapter 11: strabismus surgery 411 inferior oblique overaction (e.g., associated with congenital superior oblique paresis), a unilateral anteriorization of 1 mm will correct approximately 8 to 12 PD of hypertropia.

COMPLICATIONS Limited elevation after inferior oblique anteriorization has been discussed previously, but another problem of inferior oblique surgery is persistence or recurrence of the overaction. A common cause of residual overaction is incomplete isolation of the inferior oblique muscle, leaving posterior fibers intact. It is important to explore posteriorly along the globe for bridg- ing muscle fibers that would indicate missed inferior oblique fibers. Weakening procedures of the inferior oblique muscle for primary overaction only rarely produce a postoperative torsional diplopia. Even so, an adult patient may complain of a transient excyclodiplopia after weakening of the inferior oblique muscle. An important anatomic consideration is the proximity of the inferior oblique muscle insertion to the macula. A misad- venture with a stray needle in this area can cause the loss of central vision. Another consideration is the course of the inferior temporal vortex vein, which lies underneath the inferior oblique and can be inadvertently traumatized during surgery. The proximity of extraconal fat to the inferior oblique muscle is also an important concern, and fat adherence syndrome should be kept in mind; this may occur when the inferior oblique muscle is approached blindly and posterior Tenon’s capsule is violated. Other possible complications of inferior oblique surgery include orbital hemorrhage, pupillary dilation, endophthalmitis, and inadvertent surgery or damage to the lateral rectus muscle.11 Paramount in avoiding these complications is the clear and direct visualization of the inferior oblique muscle during its isolation. Blind hooking procedures must be avoided. Meticulous surgical dissection and hemostasis are the key to proper exposure and visualization of the anatomy.

SUPERIOR OBLIQUE MUSCLE TIGHTENING PROCEDURES

The superior oblique tendon can be functionally divided into the anterior third, responsible for intorsion, and posterior two- thirds, responsible for depression and abduction (Fig. 11-17). 412 handbook of pediatric strabismus and amblyopia

FIGURE 11-17. Diagram of superior oblique tendon insertion. The anterior ﬁbers are responsible for intorsion and the posterior ﬁbers for abduction and depression.

Tightening the anterior ﬁbers will induce intorsion without too much change in the depression and abduction functions of the superior oblique muscle; this is the basis of the Harada–Ito procedure, which is used for correcting extorsion. Tightening the full tendon is termed a superior oblique tuck or plication.

Harada–Ito Procedure The Harada–Ito procedure is commonly used to treat extorsion associated with a partially recovered acquired superior oblique palsy, where the residual strabismus is only extorsion. Tighten- ing the entire tendon will result in depression and abduction and often produces an iatrogenic Brown’s syndrome. Therefore, the Harada–Ito has the advantage of correcting extorsion without causing a signiﬁcant Brown’s syndrome. Figure 11-18 shows two techniques for tightening the anterior ﬁbers: Figure 11-18A chapter 11: strabismus surgery 413 shows the disinsertion technique and Figure 11-18B shows a classic Harada–Ito procedure. The author prefers the classic Harada–Ito procedure because it is reversible by simply cutting the pullover suture.

Full-Tendon Tuck or Plication The superior oblique tuck or plication is reserved for severe bilateral superior oblique underaction where the tendon is lax, usually associated with either a congenital or trauma-induced palsy. A full-tendon tuck or plication tightens both anterior and posterior ﬁbers and enhances all three functions of the superior oblique muscle (Fig. 11-19). Tightening of the entire superior oblique tendon may improve its function slightly, but this will consistently cause an iatrogenic Brown’s syndrome or limited elevation in adduction. Care must be taken to balance the superior oblique tightening against the induced Brown’s syndrome by performing intraoperative forced ductions of the superior oblique after tucking or plicating. The amount of tuck or plication should be readjusted appropriately. This author (K.W.W.)

A B FIGURE 11-18A,B. Harada–Ito procedure: (A) With the disinsertion technique, the anterior fibers of the superior oblique tendon are sutured, then disinserted, and moved anteriorly and laterally to be secured to sclera at a point 8 mm posterior to the superior border of the lateral rectus insertion. Lateralizing the anterior fibers intorts the eye, thus correcting extorsion. (B) In the classic Harada–Ito procedure, the anterior superior oblique tendon fibers are looped with a suture and displaced laterally without disinsertion. The anterior superior oblique tendon fibers are sutured to sclera 8 mm posterior to the superior border of the lateral rectus muscle. 414 handbook of pediatric strabismus and amblyopia

FIGURE 11-19. Superior rectus tuck or plication. Inset—Sutures are placed in the nasal tendon, then passed through sclera at the insertion. The tendon is pulled to plicate the tendon.

reserves the superior oblique plication for those rare cases of congenital superior palsy caused by a lax superior oblique tendon, or severe bilateral traumatic superior oblique palsy with severe extorsion and esotropia in downgaze. Bilateral medial rectus recessions with infraplacement usually accompany the plications.

SUPERIOR OBLIQUE MUSCLE WEAKENING PROCEDURES

Superior oblique weakening procedures are used in the management of superior oblique overaction and Brown’s syndrome.19 Various weakening procedures have been described including tenotomy, tenectomy, recession, split-tendon lengthening, and Z-lengthening of the superior oblique tendon. The split-tendon lengthening procedure works well but is difﬁcult to perform and has the disadvantage of causing tendon scarring. The superior chapter 11: strabismus surgery 415 oblique recession also creates a new insertion site nasal to the superior rectus muscle, changing the superior oblique muscle function from a depressor to an elevator. Limited depression has been described as a complication of the recession procedure. The superior oblique tenotomy has been popular, but it is an uncon- trolled procedure and the tendon ends can separate, resulting in palsy, or grow back together, causing an undercorrection. A suture bridge has been used to prevent separation of the tendon ends, but the suture can act as scaffolding, allowing the tendon to grow back together. The author (K.W.W.) has developed a procedure to lengthen the superior oblique tendon, the Wright superior oblique tendon expander. This procedure has been very effective in treating superior oblique overaction and especially treating Brown’s syndrome.17

Superior Oblique Tenotomy Superior oblique tenotomy should be performed nasal to the superior rectus muscle (Fig. 11-20). Guyton’s exaggerated forced ductions should be performed after tenotomy to verify that the full tendon was found and tenotomized. Temporal tenotomies usually have minimal effect, as the superior oblique tendon is sandwiched between the superior rectus and the sclera. When the

FIGURE 11-20. Berk superior oblique tenotomy performed at the nasal tendon. (From Ref. 2, with permission.) 416 handbook of pediatric strabismus and amblyopia temporal fibers are removed from the sclera, they do not retract but, instead, scar down to sclera under the superior rectus muscle. Another disadvantage of the temporal tenotomy is that the tendon is extremely splayed out at its insertion; thus, it is difficult to hook and tenotomize all the posterior superior oblique fibers. The preferred procedure, developed by Marshall Parks, is to perform the superior oblique tenotomy nasal to the superior rectus muscle through a temporal conjunctival incision. By placing the conjunctival incision temporal to the superior rectus muscle and reflecting the incision nasally, the surgeon can keep the nasal intermuscular septum intact and minimize scleral–tendon scarring. Intact nasal intermuscular septum is vital to maintain the anatomic relationship of the superior oblique tendon and helps reduce the incidence of postoperative superior oblique palsy.

Wright Superior Oblique Tendon Expander This procedure controls the separation of the ends of the tendon, allowing quantification of tendon separation.16 A segment of a silicone 240 retinal band is inserted between the cut ends of the superior oblique tendon (Fig. 11-21). The length of silicone is determined by the degree of superior oblique overaction, as well as the amount of A-pattern and downshoot. The maximum length of silicone is 7mm, but most significant Brown’s syn- dromes can be surgically managed with a segment of 5 to 6mm.17 Perform the superior oblique expander through a temporal conjunctival incision, even though the silicone is placed in the nasal tendon. By placing the conjunctival incision temporal to the superior rectus muscle, then reflecting the incision nasally, the surgeon can keep the nasal superior oblique tendon capsule floor and intermuscular septum intact and prevent adhe- sion of the silicone implant to sclera. This maneuver is analo- gous to cataract surgery and placing an intraocular lens (IOL) in the capsular bag. An intact nasal tendon capsule floor is important to maintain the anatomic relationships of the superior

FIGURE 11-21A,B. Wright superior oblique tendon expander. (A) A segment of 240 silicone retinal band is sutured between the cut ends of the superior oblique tendon. (B) The silicone segment elongates the tendon.

418 handbook of pediatric strabismus and amblyopia oblique tendon insertion and not create a new insertion site nasal to the superior rectus muscle. Scarring of the silicone to nasal sclera or the nasal aspect of the superior rectus muscle can cause limitation of depression postoperatively.

SLIPPED OR LOST RECTUS MUSCLE

An important complication of strabismus surgery is a slipped or lost muscle. The medial rectus muscle is the muscle most commonly lost or slipped after strabismus surgery and is the most difficult to retrieve, as there are no fascial connections to oblique muscles that keep the muscle from retracting posteriorly. In contrast, the inferior, superior, and the lateral recti have check ligaments that connect to adjacent oblique muscles. A slipped rectus muscle occurs when a muscle retracts posterior to the intended recession or resection point but there is some tissue still attached to the intended scleral insertion. A slipped muscle after strabismus surgery is caused by inadvertently suturing the muscle capsule or anterior Tenon’s capsule instead of true muscle tendon. Anterior Tenon’s capsule and muscle capsule are then secured to sclera, so the muscle slips posteriorly while a “pseudotendon” of connective tissue remains attached to sclera. A lost muscle occurs when the muscle retracts posteriorly and there is no connection of the muscle to sclera. Orbital trauma or hemorrhage can also result in a lost or damaged muscle.3 Typical signs of a slipped or lost muscle include decreased muscle function with limited ductions and lid fissure widening in the field of action of the lost muscle. On occasion, the presentation may be subtle, with slight limitation of ductions as the only finding. The key observation is an incomitant deviation with underaction of the slipped muscle. Initial eye alignment during the first postoperative week may be fairly good in primary position, with only a mild limitation of ductions. Over several weeks to months, however, ductions become pro- gressively more limited. This progression probably represents muscle slippage in addition to secondary contracture of the antagonist muscle against a weakened slipped muscle. Management of a slipped or lost muscle is to find the muscle and surgically advance it to anterior sclera if possible. Full- thickness locking bites through muscle fibers must be obtained, because partial-thickness locking bites may result in slippage of chapter 11: strabismus surgery 419 the posterior tendon fibers. If a lost muscle cannot be retrieved, then a transposition procedure, such as the Hummelsheim, should be performed.

STRETCHED INSERTION SCAR

In contrast to a slipped or lost muscle that results in an immediate overcorrection, there are many cases where an overcorrection occurs 4 to 6 weeks, and some-times years, after muscle surgery (Fig. 11-22). When this overcorrection is associated with minimal underaction of the operated muscle, consider a stretched or elongated scar, with the operated muscle migrating posteriorly. Late overcorrection is particularly common after inferior rectus recession for a hypotropia associated with thyroid disease, as it occurs in approximately 50% of cases.12 There has been much speculation about the cause for this late overcorrection,15 but work by Ludwig probably provides the best explana- tion.9,10 This theory states that the new insertion scar of the muscle to sclera stretches after the suture dissolves. The 6-0 vicryl suture used by most ophthalmologists lasts about 3 to 6 weeks, thus explaining the timing of the overcorrection. In this author’s (K.W.W.) experience, the use of a nonabsorbable suture reduces the problem of late overcorrection of the inferior rectus muscle. Any rectus muscle can have a stretched scar and a late overcorrection including, in order of frequency, inferior rectus, medial rectus, and superior rectus muscles. The likelihood of stretched scar formation may be inversely related to the length of the muscle’s arc of contact.4

BOTULINUM NEUROTOXIN

Botulinum is a cholinergic blocking agent. Blockage in a muscle occurs by binding sodium at the myoneural junction, causing the loss of acetylcholine activity that paralyzes the muscle. Minimal diffusion occurs through the nerve or the muscle because there is tight binding within the muscle. Injection of botulinum toxin into a rectus muscle results in paralysis that occurs after 24 to 48h and lasts from 3 to 6 months. The most common strabismus indication for use of botulinum is sixth nerve palsy. The treatment is to inject the ipsi- 420 handbook of pediatric strabismus and amblyopia

FIGURE 11-22A,B. Late overcorrection (4 weeks after strabismus surgery) after a left inferior rectus recession for thyroid-related, tight inferior rectus muscle. The left inferior rectus muscle was found to be posterior, caused by a stretched scar. (A) Note the left hypertropia and lower lid retraction. (B) Limited depression, left eye.

lateral medial rectus muscle (antagonist of the paretic lateral rectus muscle). The induced weakness of the medial rectus muscle from botulinum injection balances forces against the weak lateral rectus muscle (weakness with weakness), which chapter 11: strabismus surgery 421 theoretically allows the paretic muscle to regain its strength without secondary contracture of the antagonist. The use of botulinum is controversial, as studies have not shown an improvement in recovery rates for sixth nerve palsy (see Chapter 10). Botulinum has also been used for comitant strabismus. The rationale for using botulinum toxin in nonparalytic strabismus is twofold: to weaken and lengthen the injected muscle and to induce a mild secondary contracture in the injected muscle’s antagonist. Botulinum causes secondary muscle contracture by paralyzing the injected muscle, producing a large consecutive deviation in the opposite direction; this causes shortening and contracture of the antagonist to the injected muscle, theoretically leading to a permanent correction of the strabismus even after the botulinum wears off. In infantile strabismus, it is the- orized that the overacting muscle can be injected before the development of contracture. Because of the temporary large overcorrection associated with the initial paralysis and the need for multiple injections to correct strabismus, strabismus surgery is usually preferred for the treatment of comitant strabismus.

References

1. Apt L, Call NB. Inferior oblique muscle recession. Am J Ophthalmol 1978;95:95–100. 1a. Beisner DH. Reduction of ocular torque by medial rectus recession. Arch Ophthalmol 1971;85:13. 2. Berk RN. Tenotomy of the superior oblique for hypertropia. Arch Ophthalmol 1947;38:605. 3. Cates CA, et al. Slipped medial rectus muscle secondary to orbital hemorrhage following strabismus surgery. J Pediatr Ophthalmol Stra- bismus 2000;37:361–362. 4. Chatzistefanou KI, et al. Magnetic resonance imaging of the arc of contact of extraocular muscles: implications regarding the incidence of slipped muscles. J Am Assoc Pediatr Ophthalmol Strabismus 2000; 4:84–93. 4a. Elliot L, Nankin J. Anterior transposition of the inferior oblique. J Pediatr Ophthalmol Strabismus 1981;18:35. 5. Eustis HS, Leoni R. Early reoperation after vertical rectus muscle surgery. J Am Assoc Pediatr Ophthalmol Strabismus 2001;5:217–220. 6. Guemes A, Wright KW. Effect of graded anterior transposition of the inferior oblique muscle on versions and vertical deviation in primary position. J Am Assoc Pediatr Ophthalmol Strabismus 1998;2;201– 206. 7. Kushner BJ. Fifteen-year outcome of surgery for the near angle in patients with accommodative esotropia and a high accommodative 422 handbook of pediatric strabismus and amblyopia

convergence to accommodation ratio. Arch Ophthalmol 2001;119: 1150–1153. 8. Kushner BJ, et al. Should recessions of the medial recti be graded from the limbus or the insertion? Arch Ophthalmol 1989;107:1755– 1758. 9. Ludwig IH. Scar remodeling after strabismus surgery. Trans Am Oph- thalmol Soc 1999;97:583–651. 10. Ludwig IH, Chow AY. Scar remodeling after strabismus surgery. J Am Assoc Pediatr Ophthalmol Strabismus 2000;4:326–333. 10a. Mims JL, Wood RC. Bilateral anterior transposition of the inferior obliques. Arch Ophthalmol 1989;107:41. 11. Recchia FM, et al. Endophthalmitis after pediatric strabismus surgery. Arch Ophthalmol 2000;118:939–944. 12. Sprunger DT, Helveston EM. Progressive overcorrection after inferior rectus recession. J Pediatr Ophthalmol Strabismus 1993;30:145– 148. 12a. Stager DR. The neuroﬁbrovascular bundle of the inferior oblique muscle as its ancillary origin. Trans Am Ophthalmol Soc 1996;94: 1073–1094. 13. Wright KW. Brown’s syndrome: Diagnosis and management. Trans Am Ophthalmol Soc 1999;XCVII:1023–1109. 14. Wright KW. Color atlas of ophthalmic surgery: strabismus. Philadel- phia: Lippincott, 1991:173–193. 15. Wright KW. Late overcorrection after inferior rectus recession. Oph- thalmology 1996;103:1503–1507. 16. Wright KW. Superior oblique silicone expander for Brown’s syndrome and superior oblique overaction. J Pediatr Ophthalmol Strabismus 1991;28(2):101–107. 17. Wright KW. Surgical procedure for lengthening the superior oblique tendon. Investig Ophthalmol Vis Sci 1989;30(suppl):377. 18. Wright KW, Lanier AB. Effect of a modiﬁed rectus tuck on anterior segment circulation in monkeys. J Pediatr Ophthalmol Strabismus 1991;28:77–81. 19. Wright KW, Min BM, Park C. Comparison of superior oblique tendon expander to superior oblique tenotomy for the management of superior oblique overaction and Brown’s syndrome. J Pediatr Ophthalmol Strabismus 1992;29(2):92–97; discussion 98–99.