Inferior Oblique Muscle Surgery

Chapter 87: Inferior Oblique Muscle Surgery

Cynthia L. Beauchamp, David R. Stager, Sr., and Paul R. Mitchell

Marshall M. Parks, M.D., once described the inferior oblique muscle surgery as the last bastion of motility disorders to be conquered. He and Leonard Apt, M.D., have inspired our interest in surgery of this muscle and its nerve. When describing his technique for locating the nerve to the inferior oblique, Dr. Parks stated that it could be located easily following disinsertion and anterior traction on the insertion of the muscle by “strumming” the inferior oblique nerve as it approached the inferior oblique near the temporal border of the inferior rectus. His description has led many to be curious about the nature of this nerve and has led to a great deal of research on the nerve and muscle over the last two decades.

The history of inferior oblique muscle surgery has been reviewed in detail by Dyer,¹ Parks,² and Weakley and Stager.⁹³ Various inferior oblique strengthening procedures have been described,^4–8 but they are no longer useful or recommended. Therapy for inferior oblique underaction includes techniques that involve the antagonist superior oblique muscle or the contralateral yoke muscle,^9,10 and they are not discussed here.

This chapter discusses surgery on the inferior oblique muscles for primary inferior oblique overaction, secondary inferior oblique overaction associated with palsy of the ipsilateral superior oblique or contralateral superior rectus, A and V patterns, and dissociated vertical deviations. More detailed and thorough descriptions of the etiology, diagnosis, and management of these conditions are provided in other chapters.

ANATOMY

In discussing surgery to the inferior oblique, one needs to first address the anatomy of the inferior oblique and the nerve to the inferior oblique (Fig. 1). The inferior oblique muscle measures 37 mm in length. It originates just posterior to the nasal aspect of the inferior orbital rim where the muscle is narrow, measuring about 5 mm in its anterior posterior diameter. The nasal aspect measures about 13 mm in length. About 3 mm nasal to the inferior rectus, the muscle expands in its anterior posterior diameter to become about 10 mm wide. The mid portion of the muscle crosses the underside of the inferior rectus muscle for the next 10 mm. The temporal portion of the inferior oblique measures about 14 mm in length and inserts on the sclera near the lower border of the lateral rectus muscle in the inferior temporal quadrant of the globe. The nerve to the inferior oblique, a branch of the inferior division of the third cranial nerve, enters the muscle approximately 2 mm temporal to the lateral border of the inferior rectus at its posterior border.

<COMP: Insert Figure 1 here>

Some variability in the muscle may exist. De Angelis et al¹¹ performed a cadaver study of 100 eyes. Multiple insertions were found in 17% of inferior oblique muscles examined; duplications of the inferior oblique muscle belly located at 10 to 12 mm from the insertion were found in 8%. The mean muscle width among these eight specimens was larger than normal muscles.

It is important to be aware of the surrounding layers of the inferior oblique. Nasally, the muscle is surrounded by a rather significant capsule and is located outside the intermuscular septum. Outside the capsule is the fat pad. In the midportion, the inferior oblique is surrounded by both its capsule, a thickened portion of Lockwood ligament, and Demers ligament on its superior surface. On the inferior surface, it is covered by the Tenon capsule. Temporally, the muscle is surrounded by a thin muscle capsule, the intermuscular septum, and the Tenon capsule. External to that is the orbital fat pad.

Much study has been done on the nerve to the inferior oblique¹² (Fig. 2). It lies in a straight line to the apex of the orbit just temporal to the lateral border of the inferior rectus. The stiffness of the nerve is derived from the capsule of the nerve. It is composed of collagen fibers that are aligned parallel to the axis of the nerve, giving the muscle the stiffness of which Dr. Parks originally spoke. Tension studies show that it is five times as stiff as the superior oblique tendon. This provides an ancillary origin for the posterior temporal fibers of the inferior oblique muscle when the insertion is transposed anteriorly.¹²

<COMP: Insert Figure 2 here>

EXPOSURE TECHNIQUE

<fl>All inferior oblique surgery requires the same initial steps to the point of isolation of the inferior oblique muscle (Figs. 3–13). At that time, any one of several procedures can be performed (Figs. 12–41). These procedures are described individually. The figures are presented from the view of the surgeon seated at the head of the patient.

<COMP: Insert Figures 3 to 41 here>

The inferior oblique is exposed through an inferior temporal cul–de–sac incision down to bare sclera (Fig. 3). The lateral rectus is isolated on a muscle hook and the eye is rotated nasally and superiorly (Fig. 4). The lateral portion of the incision is retracted inferior temporally with a double hook. A Bishop-Harman forceps grasps the anterior part of the inferior oblique temporally and pulls it forward. A Von Graefe hook is then passed posteriorly along the sclera, which is retracted nasally, as the muscle is pulled laterally and anteriorly. The posterior border of the inferior oblique can then be identified where the white intermuscular septum contrasts with the red inferior oblique muscle (Fig. 5). The hook is rotated under the inferior oblique with the point temporally and this then is pulled forward, taking care to isolate all the fibers of the muscle (Fig. 6). Excessive intermuscular septum and Tenon capsule must be separated and lifted off, and then an incision is made in the tissue to expose the tip of the hook. A second hook is placed through that opening and the two hooks are separated. The posterior portion of the intermuscular septum and Tenon capsule are retracted temporally with a Jameson hook while the muscle is retracted nasally. The second hook under the inferior oblique is repositioned under the lower border of the lateral rectus posteriorly, near the inferior oblique insertion. The tissue between the muscle and the fibrous tissue is cut with scissors (Figs. 7–10). Care must be taken to ensure there is no evidence of inferior oblique muscle that has been missed. If that does occur, one needs to go back and repeat the procedure to be certain that the entire inferior oblique is properly identified and isolated on the hook. The inferior oblique is thus exposed and ready for a myotomy, myectomy, disinsertion, recession, or extirpation (Fig. 11). A Beauchamp clamp, which has the advantage of holding the muscle without crushing the tissue, is then placed near the insertion of the inferior oblique and the muscle is then disinserted close to the sclera¹³ (Figs. 12 and 13).

SURGICAL PROCEDURES

<fl>Currently, tightening the inferior oblique surgically has not received any significant acceptance as a surgical option although some work had been done combining resection with anterior transposition with promising results.^14,15 In the future, this may change as newer options become available to chemically tighten the muscle fibers. Chemically weakening the inferior oblique muscle fibers with botulinum toxin (Botox) has not become an acceptable alternative, perhaps due to the thinness and close proximity to the sclera, which has made this more hazardous. However, early studies have suggested promising results. Garnham et al¹⁶ retrospectively reviewed 20 patients who received botulinum injections in either the inferior oblique or inferior rectus for superior oblique palsy. They found the greatest benefit to patients with residual deviations following previous surgery, particularly when the inferior rectus was injected. Eighty-three percent of the patients receiving botulinum as a primary therapy required further surgery. In a prospective case series, Bagheri and Eshaghi¹⁷ injected botulinum toxin into the inferior oblique in 18 eyes with acute superior oblique palsies. At 6 months, they found a reduction in hyperdeviation, cyclodeviation, head tilt, and inferior oblique overaction. They concluded that botulinum toxin is a viable option for acute superior oblique palsies as patients await recovery.

In the future, other substances may also become available to inject into the muscle under direct vision to weaken the action. In a prospective study of 20 patients, Nemet¹⁸ found success with injecting acetylcholine following myectomy to prevent reattachment and recurrence.

Currently, surgical procedures that are performed on the inferior oblique muscles include recession, temporal myotomy, myectomy, nasal myectomy, disinsertion, denervation, denervation and extirpation, and anterior transposition.¹⁹ The decision on surgical options will depend to a great extent on the activity of its antagonist, the superior oblique muscle and its tendon. The effect of the superior oblique on the globe may vary from a mild underaction to a severe underaction, or perhaps even an absence or transposition of the superior oblique tendon to an ineffective location. Weakness may be permanent or temporary due to involvement of the fourth cranial nerve. Therefore, it is important in making a decision to weaken the inferior oblique that one has a complete history of the permanent nature of this imbalance between the opposing superior and inferior oblique muscle complex.

Recession

<fl>In comparing the various weakening procedures, Parks^2,20 concluded that the recession procedure was superior (Figs. 14–18). The major advantage of the recession is that it allows the weakening procedure to be titrated according to the severity of the overaction (Fig. 19). For 1+ or 2+ overaction, the inferior oblique muscle is recessed 10 mm for 3+ overaction, 12 mm; and 4+ overaction, 14 mm, which is the maximum recession. A double-armed 6–0 synthetic suture, such as polyglactin (Vicryl [J-562], Ethicon, Somerville, NJ) with a half-circle spatula needle (S-28, Ethicon, Somerville, NJ), is placed within the insertion of the inferior oblique with a locked bite at the anterior and posterior border after the muscle is tenotomized, as described previously. The muscle is recessed 10 mm by placing the anterior suture 6 mm posteriorly to the lateral border of the inferior rectus muscle insertion and 4 mm temporal to the lateral border of the inferior rectus (approximately at the vortex vein) (Figs. 15 and 17). Recessions of 12 and 14 mm are placed in relationship as depicted by the Figure, farther along the course of the inferior oblique muscle (Fig. 19). Metten et al²¹ studied the dose–response curve for recession and confirmed a dose-response relationship, with large vertical deviations with small excyclodeviation benefitting from increased anteropositioning. The study of Apt and Call²² on the pathway of the temporal portion of the inferior oblique has shown differences in what constitutes a 6– to 14-mm recession along the path of the muscle.

Parks^2,20 found that 15% of patients had a return of overaction with the recession procedure, compared with 79% with myectomy at the origin, 53% with disinsertion, and 37% with myectomy at the insertion. Short-term follow-up is not helpful with oblique muscle surgery because inferior oblique overaction can return gradually over 2 years.¹⁹ Therefore, long-term follow-up is necessary for proper evaluation of success. In a more recent report, Wilson and Parks²³ found a 25% recurrence rate of inferior oblique overaction after surgery, with an average follow-up of 3 years. Only 6% of patients required repeat inferior oblique surgery, however.

Myotomy

<fl>An inferior oblique myotomy, including Z myotomy, is rarely performed and is primarily of historical interest. The myotomized inferior oblique muscle tends to heal, however, and the preoperative state may be resumed within several months.^2,19 Lee et al²⁴ investigated Z myotomy, preformed 6 mm along the muscle. They found improved hypertropia and cyclodeviation when used in 2+ or less overacting inferior oblique muscles.

Myectomy

<fl>An inferior oblique myectomy includes removal of a section of the muscle to reduce the tendency of the myotomized ends to reunite. Two hemostats are used, as with the myotomy, except that a space of 5 mm or more is maintained between the hemostats so that the segment of muscle between the hemostats can be removed. Cautery, ligature, or both are used for hemostasis. This procedure offers the same advantages as the myectomy, but the cut ends still tend to reunite.¹⁹ There is a role for myectomy in treating recurrent inferior oblique overaction. A retrospective review by Squirrell et al²⁵ of re–exploration and temporal myectomy to treat recurrent inferior oblique overaction following recession or standard myectomy found success in decreasing overaction, improving alignment, and improving superior oblique underaction.

Alternatively, a nasal myectomy may be performed, particularly in the case of recurrent inferior oblique overaction. Stager et al²⁶and Stager and Weakley²⁷ described a new surgical approach for management of recurrent inferior oblique overaction after recession or anterior transposition. A myectomy of 1 cm or more of a portion of the inferior oblique, nasal to the inferior rectus, is performed using a temporal incision and approach. This preserves the distal inferior oblique fibers attached to the neurovascular bundle, which provide the depressor function of the inferior oblique in its anterior transposed position. Myectomy was found to be rather unsuccessful by Parks^2,20 when done through the nasal cul–de–sac or through the nasal aspect of the lower lid to approach the inferior oblique muscle. However, those procedures violated the fat pad nasally. A different approach avoids the fat pad (Figs. 20–24). The inferior oblique is isolated temporally at its recessed position and retracted temporally as the capsule of the inferior oblique is incised (Fig. 20) and, with a double hook, is retracted nasally (Fig. 21). The midportion of the inferior oblique is then grasped with a Bishop-Harman and retracted farther temporally as the capsule is retracted farther nasally (Fig. 22). Just nasal to the inferior rectus, there is a notch in the inferior oblique muscle as it narrows from its 10 mm diameter down to 5 mm in diameter on the origin side of the muscle. Two hooks are placed nasally under the thinner portion of the inferior oblique, taking care to avoid any penetration of the fat pad surrounding the inferior oblique capsule. A hemostat is placed across the inferior oblique muscle near the nasal border of the inferior rectus (Fig. 23), and the muscle is then disinserted as close to the origin as possible (Fig. 24). There are no vessels coming from the origin. However, the temporal portion can bleed rather profusely from the vascular supply near the neurofibrovascular bundle. Therefore, the inferior oblique muscle tissue is cut near the hemostat and heavily cauterized on both sides of the hemostat and then along the track of the hemostat to eliminate as much bleeding as possible from that segment of the muscle.

The nasal myectomy has a significant effect on the inferior oblique overaction while preserving any antielevation effect from a previous anterior transposition procedure. This technique may also be used as a primary procedure for severe inferior oblique overaction. Following this technique, the insertion of the inferior oblique and the nerve remain intact but without an origin against which to pull. This leaves the option for a future anterior transposition in the setting of ongoing dissociated vertical deviation. The long-term results of these studies are promising with 95% of patients having either a reduction in, or in many cases elimination of, inferior oblique overaction.²⁶ The temporal approach within the capsule and to the nasal portion of the inferior oblique may explain the better outcome than Parks found.

The temporal portion of the inferior oblique can also be transposed to the inferior orbital wall,²⁸ maintaining the normal origin and innervation, but eliminating the chance for reattachment to the globe. This can profoundly weaken inferior oblique muscle overaction while preserving reversibility. However, it does not control dissociated vertical action.

Parks^2,20 performed a controlled prospective study comparing the various methods of weakening the inferior oblique muscle. The study included 638 consecutive patients who had a minimum follow-up of 2 years. Parks performed 19 myectomies at the origin of the inferior oblique, 86 myectomies at the insertion, 89 disinsertions, and 444 recessions of inferior oblique muscles. The overaction of the inferior oblique muscle returned in 79% of the 19 eyes that underwent a myectomy at the origin through the lower lid or the nasal cul-de-sac. Because the recurrence rate is high and the procedure is difficult to perform, this procedure is no longer performed. Myectomy at the insertion produced a 37% return on overaction, also causing that procedure to be abandoned by Parks. Many authors, however, still prefer this procedure and advocate its use. Dyer¹ reported a 91% success rate in reducing the hypertropia to 10 diopters or less, and Davis et al²⁹ reported a 93% success rate.

Disinsertion

<fl>Disinsertion occurs at the scleral attachment of the inferior oblique muscle. Assuming proper exposure of the inferior oblique, this procedure is performed rapidly. The reattachment rate of the inferior oblique tendon is not predictable, however. It has a tendency to attach at or near the original insertion site or to the inferior border of the lateral rectus muscle.¹⁹ Parks^2,20 found a 53% rate of return of overaction, worse than that for the myectomy procedure at the insertion end. Jones et al,(Citation not #29 but should be inserted: Jones TW, Lee DA, Dyer JA: Inferior Oblique Surgery: Experience at the Mayo Clinic from 1962 to 1981. Arch Ophthalmology 102: 714, 1984.²⁹ however, reported a success rate of 88% with inferior oblique disinsertion. Suturing the intramuscular membrane over the global side of the cut end of the inferior oblique may help prevent its reattachment to the globe.

Denervation

<fl>Gonzalez³⁰ first described denervation of the inferior oblique muscle as a weakening procedure but also reported the return of overaction within the first postoperative year.³¹ The nerve to the inferior oblique is attached tightly to the posterior belly of the inferior oblique muscle, lateral to the inferior rectus muscle border. At this site is a fusiform enlargement of the belly. After the nerve is hooked posteriorly with a Stevens hook, cautery is used to sever the nerve and the accompanying artery and vein. The procedure of denervation is no longer used alone because of the likelihood of recurrence of the inferior oblique overaction, but it is used as the foundation for the denervation and extirpation procedure.

Denervation and Extirpation

<fl>The denervation procedure (Figs. 25–36) allows the inferior oblique muscle to be released farther into the operative field once the nerve has been sectioned (Figs. 30 and 31). A 3–0 Vicryl suture ligature is placed around the inferior oblique muscle (Fig. 32) as close to the Tenon capsule penetration as possible. The inferior oblique is sectioned by cautery (Fig. 33), just distal to the suture ligature. The cauterized stump is pushed through the opening in the Tenon capsule (Figs. 34 and 35) and a 6-0 or 7-0 Vicryl suture is used to close the opening, with either a running or a purse-string suture (Fig. 36). The purse-string suture can be preplaced before the inferior oblique muscle is sectioned and closed after the muscle is sectioned. The advantage of the denervation and extirpation procedure is its ability to permanently correct 4+ overaction of the inferior oblique.^19,32 In addition, it may be the best operative procedure to eliminate recurring inferior oblique overaction after disinsertion, myectomy, or recession.¹⁹ Disadvantages include permanent underaction of the inferior oblique muscle in some patients. Pupil dilation can occur, but the dilation usually is transient, clearing in 3 to 6 months.^19,32 Also, one loses the option of a future anterior transposition procedure to control dissociative vertical deviation

Anterior Transposition

<fl>Elliott and Nankin³³ modified the standard recession procedure by transposing the inferior oblique muscle anteriorly toward the insertion of the inferior rectus muscle (Figs. 37–39). This procedure reduced the persistent inferior oblique overaction that frequently is noted after recession surgery and reduced marked inferior oblique overaction when performed as the initial surgical procedure. Seventy-three percent of patients, however, had a postoperative deficiency in primary position elevation, compared with only 25% who underwent the usual inferior oblique recession surgery. Bremer et al³⁴ performed a recession and anteriorization of the inferior oblique muscle on three patients who had fourth nerve palsy. They suggested that this procedure would benefit patients with dissociated vertical deviations because the inferior oblique is converted to a depressor muscle by the anterior transposition. Ziffer et al³⁵ also suggested that the anterior transposition converts the inferior oblique muscle from an elevator to a depressor on attempted elevation. Because of its powerful weakening ability, the anterior transposition should be reserved for patients with moderate to severe inferior oblique muscle overaction and dissociative vertical deviation and was historically performed only on both eyes to avoid postoperative hypotropia in upgaze. Gonzalez and Cinciripini³⁶ proposed the anterior transposition procedure for treatment of unilateral superior oblique palsy. None of their three patients developed primary position hypotropia, but all demonstrated some elevation deficiency with elevation of the lower lid in upgaze. Chang et al³⁷ found confirming results. A retrospective review of 33 patients who underwent unilateral anterior transposition of the inferior oblique for unilateral superior oblique palsy found unilateral anterior transposition to be safe and effective. None of their patients noticed elevation deficiency or lower lid elevation postoperatively.

Stager et al³⁸ described clinical, radiologic, and histologic evidence that the neurovascular bundle supplying the inferior oblique muscle serves as an ancillary functional origin of the inferior oblique after anterior transposition of the insertion and converts the distal portion of the inferior oblique muscle from an elevator to a depressor (Fig. 40). One can use the ancillary origin from the neurofibrovascular bundle on the posterior fibers of the inferior oblique muscle to create a downward vector force and change or even reverse the torsional vector forces.¹² Placing these posterior muscular fibers on stretch alters the vector force of the inferior oblique from an elevator to a depressor and possibly maintains its extorsion vector force. These posterior fibers are then placed very tightly against the sclera and may, over time, form an adhesion of the fibers extending from the neurofibrovascular bundle to its new origin 16 mm anteriorly.³⁹ This may then represent a new insertion point for the mid and nasal portion of the inferior oblique that is properly innervated and along the normal axis of the inferior oblique. This then may contribute to a recurrent inferior oblique overaction in cases that have been anteriorly transposed temporal to the lateral border of the inferior rectus. If these posterior fibers are reattached temporal to the inferior rectus insertion, it can also continue to produce an extorsion of the globe, producing a hypotropia in side gaze and a V pattern, duplicating the original pattern that prompted the surgical intervention. As those temporal fibers are placed more nasally, closer to the Y-axis, there is less extorsion produced.³⁹ When the muscle is sutured just anterior to the lateral border of the inferior rectus insertion, the fibers are closer to the Y-axis, reducing extorsion while preserving the downward vector forces to control dissociated vertical deviation. This may, however, cause a limitation of upgaze and thickening of the inferior lid, particularly when done unilaterally. The use of a permanent suture may prevent detachment and posterior pull of the fibers caused by prolonged traction.³⁹

Stager et al^39,40 described an approach of anterior nasal transposition (Fig. 41). Reattaching those posterior fibers 3 mm posterior and 2 mm nasal to the nasal border of the inferior rectus muscle can reverse the torsional action of the posterior fibers of the inferior oblique muscle. The anterior pole of the inferior oblique should be reattached approximately 3 mm further nasally. This technique can intort the eye as demonstrated by fundus photography and by a change in the high-resolution photograph of the iris crypts⁴¹

Fard⁴² found success in using the anterior nasal transposition to address dissociative vertical deviation. He found that 60% of 20 eyes had an excellent outcome, with higher rates of success for patients with less than 15 diopters of dissociative vertical deviation.

Mims⁴³ performed an anterior transposition on 61 children who had bilateral overaction of the inferior oblique muscle with concurrent or previous infantile esotropia. A substantial reduction in the dissociated vertical deviation occurred in all cases when it was present, and only one child required subsequent surgery for dissociated vertical deviation.

In a prospective study, Elliott and Parks⁴⁴ compared the effectiveness of denervation and extirpation with that of anterior transposition in the management of patients who had maximum inferior oblique overaction. They concluded that the anterior transposition procedure was effective for eliminating bilateral inferior oblique overaction but that it should be performed identically on both eyes because of its tendency to restrict elevation. The anterior transposition procedure was recommended for the treatment of patients with overactive inferior oblique muscles in association with dissociated vertical deviation. In a series of 21 patients receiving an anterior transposition of the inferior oblique with overaction of the inferior oblique and dissociated vertical deviation, Seawright and Gole⁴⁵ reduced the preoperative incidence of inferior oblique overaction from 84% to 16% postoperatively. Inferior oblique overaction was absent in 43% and improved in 86% of eyes. At the latest postoperative visit, 57% did not demonstrate any evidence of dissociated vertical deviation and 68% of eyes had no dissociated vertical deviation or improved dissociated vertical deviation. There was no evidence of primary position hypotropia in any patient who had unilateral anterior transposition, and there was no evidence of inferior oblique underaction. Three patients required repeat inferior oblique surgery.

Burke et al⁴⁶ also found the anterior transposition of the inferior oblique to be an effective treatment for dissociated vertical deviation with inferior oblique overaction, but the long-term results may be less stable if the preoperative dissociated vertical deviation is in excess of 15 prism diopters.

PREFERRED TECHNIQUES

<fl>Although myectomy and disinsertion procedures have their advocates, Parks preferred recession of the inferior oblique muscle for 1+, 2+, or 3+ overaction and denervation and extirpation for 4+ overaction of the inferior oblique muscles. Because the anterior transposition limits elevation, the use of this procedure should be limited. When doing an anterior transposition, the posterior suture at the temporal end of the inferior oblique (IO) should be reattached near the temporal border of the inferior rectus (IR) and not spread out farther temporally. Anterior transposition is recommended for patients who have bilateral marked inferior oblique overaction with dissociated vertical deviation. The role of a nasal myectomy from a temporal approach has not been adequately studied to determine its role in these cases. It may be more helpful with recurrent inferior oblique overaction following a recession procedure. An anterior and nasal transposition procedure, especially when the superior oblique muscle is missing as in Apert syndrome, may convert the inferior oblique from an extorter to an intorter and an elevator to a depressor, significantly improving a V pattern.⁴⁷

Whether the inferior oblique overaction is primary, secondary, or associated with V pattern, these guidelines should be observed consistently. A 1+ overaction of an inferior oblique muscle is not a justification for surgical intervention unless a more severe overaction is present in the fellow eye. A 2+ overaction merits a 10-mm recession of the inferior oblique, a 3+ overaction merits a 14-mm recession, and a 4+ overaction merits a denervation and extirpation procedure. A 6-mm recession would be performed for a 1+ overaction. In the presence of a 1+ overaction, this type of bilateral surgery would be indicated for treatment of a V pattern or in anticipation that the 1+ overactive eye may become more overactive in time.

INFERIOR OBLIQUE SURGERY

Indications

<fl>Whether inferior oblique overaction (Fig. 42) is primary, secondary, or associated with the V pattern, surgery is indicated to treat hypertropia, diplopia, binocular vision compromise in the field of adduction, torticollis, and to improve cosmetic appearance. These indications are not repeated for the subsequent sections on primary inferior oblique overaction, secondary inferior oblique overaction, V pattern, and dissociated vertical deviations.

<COMP: Insert Figure 42 here>

Contraindications

<fl>In asymmetric inferior oblique overaction, an inferior oblique without overaction should not be weakened surgically.^2,10,48 It is not possible to predict which inferior oblique muscle will become overactive at a later date. Weakening a normal inferior oblique muscle leads to subsequent vertical deviation in the primary position, and further corrective surgery may be needed.

In a V pattern, the normal inferior oblique muscle should not be weakened, and surgery should be confined to vertical displacement of the horizontal rectus muscles.^2,10,48

It is not prudent to perform an anterior transposition procedure unless the patient has both markedly overactive inferior oblique muscles and dissociated vertical deviation. In the absence of dissociated vertical deviation, the anterior transposition procedure can lead to limitation in upgaze movements, which is not a desired effect if the patient had only overaction of the inferior oblique muscle preoperatively.

PRIMARY INFERIOR OBLIQUE OVERACTION

<fl>There are two clinical types of inferior oblique overaction: primary, without ipsilateral superior oblique palsy or contralateral superior rectus palsy, and secondary, with palsy of a cyclovertical muscle.^2,20,48 In primary overaction of the inferior oblique, there is overelevation of the adducting eye, which increases with increasing adduction. Usually, there is no vertical deviation in the primary position and no cyclodeviation. Therefore, there is no torticollis. The Bielschowsky head-tilt test result is negative.^49,50 Primary inferior oblique overactions are not congenital and rarely are noted in patients younger than 1 year.^23,50 The patient may or may not have an associated horizontal deviation of the eyes. In a series of 50 patients who had congenital esotropia and underwent surgical alignment before the age of 1 year, Parks and Mitchell^2,48 found 65% with inferior oblique overaction. Hiles et al⁵¹ reported a 78% incidence of overaction of the inferior oblique muscle in one or both eyes in a series of 54 patients who had congenital esotropia. Wilson and Parks²³ found primary overaction of the inferior oblique muscle in 72% of patients with congenital esotropia at an average age of 3.6 years, 34% of patients with accommodative esotropia at an average age of 5.2 years, and 32% of patients with intermittent exotropia—also at an average age of 5.2 years. When detected, inferior oblique overaction was asymmetric in 44% of patients and unilateral in 23%.²³ If one inferior oblique muscle is overactive, the second may or may not overact.⁴⁸ When the second inferior oblique muscle overacts, the onset usually is evident 2 to 6 months after the onset of overaction in the first muscle, regardless of whether inferior oblique surgery has been performed.⁴⁸ Overaction may occur many years later, however.²³

Indications

<fl>Surgical weakening is the treatment of choice for the overactive inferior oblique muscle. Excyclodeviation is not associated with primary overaction, and weakening the inferior oblique produces neither significant incyclodeviation nor a vertical deviation in the primary position. No significant change in the horizontal alignment of the eyes in primary position is produced by weakening the primary overacting inferior oblique muscles.⁵² The previous terminology for overaction—mild, moderate, marked, and supermarked—should be classified as 1+, 2+, 3+, and 4+ overaction, respectively.²³

Eustis and Nussdorf⁵³ photographed the posterior pole at the time of surgery in 27 eyes of 14 patients with infantile esotropia. A masked observer graded each photo for the presence or absence of fundus torsion. Of the 27 eyes, 15 developed inferior oblique overaction, and 6 of the 15 demonstrated fundus excyclotorsion before the inferior oblique overaction was clinically recognized. Therefore, the presence of fundus torsion with infantile esotropia may serve as a marker or a predictor of subsequent overt inferior oblique overaction.

When unilateral inferior oblique overaction occurs, only the overactive muscle is weakened. If asymmetric inferior oblique overaction is present, asymmetric surgery is performed. Weakening procedures on the overactive inferior oblique muscles are performed simultaneously with surgery on the horizontal muscles if the misalignment is a combined horizontal tropia with the overactive inferior obliques.

Weakley et al⁵⁴ studied 368 patients with infantile esotropia, acquired accommodative or partially accommodative esotropia, or acquired nonaccommodative esotropia. Of the 126 patients (34%) with amblyopia, 93 patients (74%) had symmetric inferior oblique overaction. Thirty-one patients (25%) had more inferior oblique overaction in the eye with the amblyopia, however. The authors stress that asymmetric inferior oblique overaction in esotropia correlates highly with amblyopia in the eye with greater inferior oblique overaction, regardless of the subtype of esotropia, the duration of the esotropia, or the angle of deviation.

Contraindications

<fl>An inferior oblique that is not overactive should not be weakened because it is not possible to predict if and when an inferior oblique muscle will become overactive. Bilateral weakening in the presence of unilateral overaction produces a vertical deviation in the primary position. This deviation can persist and become symptomatic,⁵⁰ necessitating further surgery to alleviate the symptoms.

SECONDARY INFERIOR OBLIQUE OVERACTION

<fl>A secondary inferior oblique overaction is an early finding in patients with contralateral superior rectus palsy or a late finding in patients with ipsilateral superior oblique palsy. Overelevation of the adducting eye is seen, as in primary overaction of the inferior oblique. In addition, there is a significant vertical and cyclovertical deviation in the primary position, regardless of whether there is hypertrophy or contracture of the secondarily overactive inferior oblique.^2,48 The V pattern is associated with the secondary overaction of the inferior oblique muscle. Secondary overaction usually occurs 6 months or more after the onset of superior rectus or superior oblique muscle palsy. Hypertrophy or contracture can occur within several weeks of the onset of the palsy, however.^2,48 Torticollis tends to develop to maintain single binocular vision, and the Bielschowsky head-tilt test result is positive.

Treatment

<fl>The ideal treatment is weakening of the inferior oblique muscle, preferably by a recession procedure. Usually, the recession procedure precedes a tuck of the ipsilateral palsied superior oblique tendon or resection of the palsied superior rectus, or a weakening of the yoke muscle.⁴⁸ Improvement will occur in the overelevation of the adducting eye in the vertical position and in the cyclodeviation in the primary position, and the torticollis should be reduced or eliminated. The result of the Bielschowsky head-tilt test should be improved, but the positive finding will not be eliminated. Hatz el al⁵⁵ confirmed this in a retrospective review, finding that isolated inferior oblique muscle weakening is an effective treatment option for superior oblique palsy up to 15 prism diopters of vertical deviation in primary position. For larger vertical deviations, a recession of the contralateral inferior rectus should be considered.

In a retrospective review, Ghazawy et al⁵⁶ demonstrated that myectomy is more effective than anterior transposition in improving superior oblique underaction associated with primary and secondary inferior oblique overaction. Both approaches adequately addressed the inferior oblique overaction. Min et al disagree,⁵⁷ based on the findings of a prospective randomized study comparing anterior transposition to myectomy. They found an 85% success rate for the anterior transposition and 25% for the myectomy in eliminating inferior oblique overaction.

Stager et al^39,40 prefer the use of the anterior nasal transposition with severe or recurrent congenital and acquired superior oblique palsies, particularly as a secondary procedure.

The trochlear nerve palsy may be bilateral or unilateral. If it is bilateral, the involvement may be asymmetric, with the minimally involved side being masked by the maximally involved side. This effect may become apparent only after the maximally involved eye is corrected surgically. Bilateral superior oblique palsy shows 10 degreres or more of excyclotorsion on double Maddox rod testing, with left hypertropia in right gaze, right hypertropia in left gaze, V pattern, chin depression, and a preferred upgaze posture. Bilateral superior oblique palsy occasionally may be present when less than 10 degrees of excyclotorsion is found on the double Maddox rod test. The treatment of choice is not inferior oblique weakening but a Harada-Ito procedure or a modification. This procedure requires sagittalization and advancement of the anterior half of the superior oblique tendons toward or adjacent to the superior border of the lateral rectus muscles.^50,58

V PATTERN

<fl>Overaction of the inferior oblique muscle frequently is associated with the V pattern, regardless of whether the primary position measurements include orthophoria, esotropia, or exotropia.² The deviation is measured at distance in the primary position, 30-degree chin elevation, and chin depression. A difference of 10 prism diopters in the horizontal alignment between the upgaze and downgaze position is considered diagnostic of A or V pattern. Clinical evidence supports the association of abnormality in A and V patterns with abnormalities of the cyclovertical muscles.

Treatment

<fl>Procedures to weaken the inferior oblique muscles improve the V pattern but must be used only when the oblique muscles are overactive. When a V pattern is present without overaction of the inferior obliques, vertical displacement of the horizontal muscles is indicated,^{2,10,48,59,60} rather than weakening of normal inferior oblique muscles. Weakening of normal muscles leads to underaction and an undesirable surgical result.¹⁰

DISSOCIATED VERTICAL DEVIATIONS

<fl>The usual surgical correction of dissociated vertical deviations includes recession of the superior rectus muscle, resection of the inferior rectus muscle, placement of superior rectus posterior fixation sutures, supermaximum recession, or a hang-back technique with a superior rectus weakening.

The child who has overaction of the inferior oblique muscle in addition to dissociated vertical deviation can benefit from an anterior transposition or anterior nasal transposition of the inferior oblique muscle. The tethering effect of the anterior transposition is the result of the physiologic conversion of the inferior oblique to a depressor muscle.^{27,35,36,38,40,42}

INFERIOR OBLIQUE SURGERY COMPLICATIONS

<fl>Inferior oblique surgery has several complications, all of which can be avoided by the application of proper and judicious surgical technique under direct vision.

Ecchymosis of the lower lid and bulbar conjunctiva is the most common complication. Regardless of which procedure is selected, adequate hemostasis is essential to avoid postoperative bleeding, which can lead to a hematoma in the orbit or within the eyelid tissue.

This can be can be particularly severe if the vortex vein is severed. Following myotomy or myectomy, without adequate cautery, the temporal end of the inferior oblique can bleed. The closer one is to the neurofibrovascular bundle, the greater that problem can become. A common approach in the past was the blind sweeping of the eye-muscle hook in an attempt to grasp the inferior oblique without proper exposure of the operative field. The blind sweeping technique can cause the inferior rectus or lateral rectus muscle to be hooked inadvertently and torn or severed by vigorous “fishing” for the inferior oblique muscle. The inferior temporal vortex vein can be hooked accidentally and severed, causing extensive orbital hemorrhage. Accidental severing of the vortex vein is not vision threatening, and cautery should be applied to stop the bleeding and reduce the possibility of postoperative hematoma. The inferior oblique can be torn partially from repeated attempts to hook the muscle without direct vision.² Therefore, if posterior fibers of the inferior oblique muscle remain intact, the recession of only the anterior fibers will not reduce the overaction. Postoperatively, full overaction will be present and reoperation will be necessary. The entire inferior oblique insertion must be seen before surgery is performed.

The adherence syndrome² is an iatrogenic syndrome caused by rupturing the Tenon capsule and allowing orbital fat to protrude from the normal position into the operative field. The result is fibrofatty proliferation and adherence to the sclera and the inferior rectus muscle.

Treatment requires extensive, careful dissection of fibrofatty scarring from the sclera and inferior rectus muscle capsule. The goal of this treatment is to halt the relentless progression of the hypotropia in the affected eye. In the best of hands, the surgical results are limited. Residual hypotropia consistently is present, despite the most aggressive approach in removing scar tissue and attempting to elevate the affected eye. Parks² reported that the average preoperative hypotropia in 23 patients was 19 diopters, ranging from 4 to 40 diopters. The average postoperative hypotropia was 8 diopters, even though an average of 2.6 vertical muscles per patient required surgery to reduce the hypotropia.

These conditions can be avoided with the use of proper illumination, such as that provided by a fiberoptic headlight, and with adequate surgical direct vision assistance that allows direct vision of the vortex vein and posterior border of the inferior oblique muscle.

Grasping only a portion of the inferior oblique muscle leads to residual overaction if most posterior fibers are left intact. Proper placement of hooks reduces the likelihood that the inferior rectus or lateral rectus would be grasped and severed in a myotomy or myectomy procedure if the inferior oblique is identified correctly before the muscle is sectioned.

Pupillary dilation can result form trauma to the ciliary ganglion. This often resolves after several months but may be permanent.⁶¹

Internal ophthalmoplegia has been reported. Usually, it is transient^19,32,62; however, it may last many years.⁶³ Cellulitis is usually avoided with adequate preoperative prep. Endophthalmitis is exceedingly rare and may be secondary to sinusitis that spreads into the globe and has an entry into the eye through the suture tracks.

Other orbital inflammation problems can develop such as acute orbital myopathy following eye muscle surgery in Graves disease.

Overcorrections are less common but certainly can be a problem when the surgery involves an anterior transposition procedure. This can cause an antielevation syndrome and has been reported to cause a primary position hypotropia. When done unilaterally, the opposite eye can develop an inferior oblique overaction.

ALTERNATIVES

<fl>When faced with an apparent inferior oblique overaction, one needs to be certain of the proper diagnosis and possible alternatives as discussed previously. A good history of the deviation along with measurements in all fields of gaze, a Maddox rod measurement of torsion, and an adequate fundus exam to evaluate for torsion are very important in making certain that your choice of inferior oblique surgery is the proper choice. Alternative surgery includes superior oblique tuck or Harada-Ito, or recession of the contralateral inferior rectus.

ACKNOWLEDGMENT

<fl>The authors and editors wish to gratefully acknowledge the contributions of the late Marshall M. Parks, M.D., former author of this chapter. Many of the surgical illustrations originated with Marshall.

REFERENCES

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8. McLean JM: Direct surgery of underacting oblique muscles. Trans Am Ophthalmol Soc 46:622, 1948

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27. Stager DR, Weakley DR Jr: A new temporal surgical approach to the nasal portion of the inferior oblique muscle. ~~Binocul Vis Eye Muscle Surg~~Binocular Vision and Eye Muscle Surgery Q 7:211, 1992

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31. Gonzalez C: Reinnervation of the nerve to the inferior oblique after iatrogenic denervation. J Pediatr Ophthalmol Strabismus 18:21, 1981

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33. Elliott RL, Nankin SJ: Anterior transposition of the inferior oblique. J Pediatr Ophthalmol Strabismus 18:35, 1981

34. Bremer DL, Rogers GL, Quick LD: Primary position hypotropia after anterior transposition of the inferior oblique. Arch Ophthalmol 104:229, 1986

35. Ziffer AJ, Isenberg SJ, Elliott RL et al: The effect of anterior transposition of the inferior oblique muscle. Am J Ophthalmol 116:224, 1993

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39. Stager DR Sr: Anatomy and surgery of the inferior oblique muscle: Recent findings. J AAPOS 5:203–208, 2001

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41.Felius J, Locke KG, Stager DR Jr et al: Photographic assessment of change in torsional strabismus. J AAPOS 13(6):593–595, 2009

42 Fard M: Deviation associated with inferior oblique muscle overaction. J AAPOS 14:35–38, 2010

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55. Hatz KB, Brodsky MC, Killer HE: When is isolated inferior oblique muscle surgery an appropriate treatment for superior oblique palsy? Eur J Ophthalmol 16(1):10–16, 2006

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62. Damanakis AG, Theodossiadis GP: Internal ophthalmoplegia caused by inferior oblique myectomy. J Pediatr Ophthalmol Strabismus 22:117, 1985

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<Figure 1>Figure 1. Cadaver specimen of the inferior oblique muscle and neurofibrovascular bundle. Reprinted with permission from Stager DR: The neurofibrovascular bundle of the inferior oblique muscle as its ancillary origin. Trans Am Ophthalmol Soc 94:1080, 1996, with permission.

<Figure 2>Figure 2. Cadaver dissection of the neurofibrovascular bundle with fibrous bands that extend to the inferior rectus capsule. Reprinted with permission from Stager DR: The neurofibrovascular bundle of the inferior oblique muscle as its ancillary origin. Trans Am Ophthalmol Soc 94:1080, 1996, with permission.

<Figure 3>Figure 3. The standard inferior-temporal fornix incision is made through the conjunctiva and Tenon capsule with a Westcott scissors. The incision is placed about 8 mm posterior to the limbus, anterior to the fat pad.

<Figure 4>Figure 4. After the lateral rectus muscle is hooked with a Stevens and then a Green muscle hook, the lateral rectus muscle is tented to allow placement of a 4–0 silk suture beneath the insertion of the lateral rectus muscle. The needle tip should glide along the inferior surface of the muscle hook to avoid laceration of the lateral rectus muscle or laceration or penetration of the globe. Alternatively, a muscle hook may be used to stabilize the lateral rectus muscle, thereby eliminating risk of globe penetration. This method is discussed in the text.

<Figure 5>Figure 5. A stiff iris repositor can be placed against the sclera to depress the sclera and enhance the view of the inferior oblique muscle. The inferior temporal vortex vein adjacent to the Stevens hook is retracting tissue along the inferior border of the lateral rectus muscle. The second Stevens hook is pointing to the inferior oblique muscle. Using a double hook to retract conjunctiva and Tenon enhances visibility of the inferior temporal quadrant. The inferior oblique muscle may then be lifted with Bishop–Harman forceps to visualize the posterior border.

<Figure 6>Figure 6. The Stevens hook is placed adjacent to the posterior edge of the inferior oblique muscle and rotated as the inferior oblique muscle is drawn into the operative field. A forceps is used to retract the redundant Tenon capsule, encapsulated fat, and intermuscular septum as the muscle is drawn further into the operative field with the Stevens muscle hook.

<Figure 7>Figure 7. The Westcott scissor is used to open the intermuscular septum along the posterior border of the inferior oblique muscle. Care is taken to cut adjacent to the tip of the Stevens muscle hook. This incision is important because it helps to avoid damaging the Tenon capsule and releasing fat, which leads to the adherence syndrome and postoperative hypotropia.

<Figure 8>Figure 8. A Westcott scissor is used gently to open a space beneath the elevated inferior oblique muscle and allow placement of a Green muscle hook.

<Figure 9>Figure 9. The Green muscle hook is placed beneath the belly of the inferior oblique muscle, and the eye is retracted nasally.

<Figure 10>Figure 10. A Westcott scissor is used to dissect the intermuscular septum and expose the insertion of the inferior oblique muscle.

<Figure 11>Figure 11. The inferior oblique muscle is placed on the Green muscle hook. The exposed insertion is ready for myotomy, myectomy, disinsertion, placement of sutures for recession, or removal for the extirpation procedure.

<Figure 12>Figure 12. An Aebli scissor is placed at the inferior oblique muscle insertion and it is tenotomized from the globe. In this photo, a suture has been preplaced. We prefer the use of the Beauchamp clamp near the insertion allowing the suture to be placed once the muscle has been tenotomized from the globe. The placement of a vascular clamp along the insertion is seen in Figures 25 and 26.

<Figure 13>Figure 13. The oblique is sectioned from the globe with the 6-0 Vicryl suture attached to the insertion.

<Figure 14>Figure 14. The inferior temporal vortex vein, 8 mm posterior to the temporal insertion of the inferior rectus muscle, is held on a Green muscle hook.

<Figure 15>Figure 15. Two sutures straddle the inferior temporal vortex vein insertion for a 10-mm recession procedure.

<Figure 16>Figure 16. Knots are tied to show the placement. The inferior oblique muscle is held on a Stevens muscle hook.

<Figure 17>Figure 17. A 10-mm inferior oblique muscle recession is placed over the inferior temporal vortex vein.

<Figure 18>Figure 18. Optional closure of the incision with a 6-0 Vicryl suture. The incision may be allowed to heal without a suture.

<Figure 19>Figure 19. A representation of the inferior globe to show the placement of the inferior oblique muscle in a graded recession. For 1+ or 2+ overaction, the muscle is recessed 10 mm; for 3+ overaction, 12 mm; for 4+ overaction, 14 mm, which is the maximum recession.

<Figure 20>Figure 20. A representation of the inferior aspect of the globe shows a previously anteriorly transposed inferior oblique muscle. The procedure for nasal myectomy is demonstrated. The muscle is isolated on a muscle hook. An opening is made in the capsule using Bishop-Harman forceps and Westcott scissors.

<Figure 21>Figure 21. A double hook is placed inside the opening of the capsule to expose the inferior oblique muscle belly.

<Figure 22>Figure 22. The inferior oblique muscle is grasped with Bishop-Harman forceps as the double hook is drawn nasally to expose the nasal portion of the muscle.

<Figure 23>Figure 23. A hemostat is placed across the inferior oblique muscle nasal to the inferior rectus muscle. Just nasal to the inferior rectus, there is a notch in the inferior oblique muscle as it narrows from its 10 mm diameter down to 5 mm in diameter on the origin side of the muscle. Care must be taken to avoid any penetration of the fat pad surrounding the inferior oblique capsule.

<Figure 24>Figure 24. The muscle tissue is cut near the hemostat and heavily cauterized on both sides of the hemostat and then along the track of the hemostat to eliminate as much bleeding as possible from that segment of the muscle.

<Figure 25>Figure 25. The following photographs depict a denervation and extirpation procedure (Figs. 25–36). A hemostat is placed adjacent to the insertion of the right inferior oblique muscle.

<Figure 26>Figure 26. An Aebli scissor is used to section the inferior oblique muscle from the globe.

<Figure 27>Figure 27. The inferior oblique muscle is drawn into the operative field with a hemostat. This step is commonly used for the recession procedure to replace the need for the placement of the 6-0 Vicryl suture while the muscle is attached to the globe in the proximity of the macula. The recession sequence would follow after the placement of the suture while the muscle is grasped in the hemostat.

<Figure 28>Figure 28. The eyelid speculum is removed and a Desmarres lid retractor is placed inferiorly. The inferior oblique muscle is drawn superiorly and nasally. The fusiform expansion of the inferior oblique muscle is evident on the posterior border, where the nerve to the inferior oblique muscle and artery and vein enter the inferior oblique muscle. The Stevens muscle hook is used to hook the neurovascular bundle.

<Figure 29>Figure 29. The hook is advanced farther for identification of the neurovascular bundle on the inferior oblique muscle.

<Figure 30>Figure 30. Cautery is applied to sever the neurovascular bundle, and a sponge is used to protect the sclera and lids from cautery.

<Figure 31>Figure 31. As the neurovascular bundle is severed completely, the inferior oblique muscle will be released of the normal tension provided by the neurovascular bundle on the posterior border of the inferior oblique muscle.

<Figure 32>Figure 32. The inferior oblique muscle is drawn farther into the operative field, and a 3-0 Vicryl suture ligature is applied about the belly of the muscle.

<Figure 33>Figure 33. Cautery is used to sever the inferior oblique muscle and to provide hemostasis.

<Figure 34>Figure 34. The stump of muscle is placed through the Tenon capsule opening. The surgeon must apply sufficient cautery to prevent postoperative bleeding.

<Figure 35>Figure 35. The inferior oblique muscle stump is visible at the opening in the Tenon capsule before closure with 6-0 or 7–0 Vicryl suture. The S-28 half-circle curved needle is preferred to the S-29 needle, which has a 3/8-circle curve.

<Figure 36>Figure 36. The Tenon capsule opening is closed with 6-0 or 7-0 Vicryl suture to place the inferior oblique muscle entirely outside Tenon capsule.

<Figure 37>Figure 37. The following photographs depict an anterior transposition procedure (Figs. 37–39). A 6-0 Vicryl suture is placed in the inferior oblique muscle in preparation for anterior transposition. A Green muscle hook is placed beneath the inferior rectus muscle.

<Figure 38>Figure 38. The eye is retracted nasally with a Green muscle hook. The anterior suture is placed just anterior and temporal to the insertion of the inferior rectus muscle. The temporal fibers should be placed adjacent to the lateral border of the inferior rectus muscle.

<Figure 39>Figure 39. The suture is tied, and the inferior oblique muscle is in the position for anterior transposition. The inferior rectus muscle is retracted nasally on the Green muscle hook, and the Desmarres lid retractor is placed inferior temporally.

<Figure 40>Figure 40. Representation of the inferior globe of a traditional anterior transposition procedure. The neurofibrovascular bundle serves as the functional origin following converting the inferior oblique from an elevator to a depressor. Reprinted from Stager DR. Costenbader lecture. Anatomy and Surgery of the inferior oblique muscle: Recent findings. J AAPOS 5(4):203, 2001, with permission.

<Figure 41>Figure 41. Inferior view of representation of eye showing the placement of the inferior oblique in anterior nasal transposition 2–3 mm posterior to and 2 mm nasal to the nasal border of the inferior rectus muscle. The neurofibrovascular bundle now serves as the functional origin. Reprinted from Stager DR Jr, Beauchamp GR, Wright WW et al: Anterior and nasal transposition of the inferior oblique muscles. J AAPOS 7(3):168, 2003, with permission.

<Figure 42>Figure 42. Representation of the clinical manifestation of bilateral inferior oblique overaction causing hypertropia of the adducting eye in side gaze. Reprinted from Stager DR: Costenbader lecture. Anatomy and Surgery of the inferior oblique muscle: Recent findings. J AAPOS 5:204, 2001, with permission.

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