Surgical Treatment of Exotropia
John D. Baker
John D. Roarty
Surgical management of exotropia encompasses indications for surgery, choice of surgical procedure, specific surgical techniques, results to be expected, and potential complications.
Three types of exodeviations are not associated with neurologic problems: infantile, intermittent, and sensory. Surgical indications and contraindications for each type are discussed in this chapter. The surgical technique for horizontal rectus muscle recession and resection is summarized in the section on intermittent exotropia. Complications in management and expected results are given.
Infantile Exotropia
The presence of an exodeviation in the first few months of life follows one of the three courses. These must be differentiated to provide proper treatment.
Transient Exotropia of Infancy
Intermittent exotropia of small and variable magnitude is a common finding in the first months of life. Nixon and colleagues1 and Sondhi and associates2 reported that many infants have an exotropic eye position that may persist until the age of 4 months. In a few such children, the exodeviation persists until the child is 6 to 9 months of age. These children demonstrate an intermittent exotropia of up to 20 prism diopters (PD). These deviations usually are measured at near because attention for distance fixation often is not developed at this age. Children usually gain control of exodeviations that are present early in life, and associated drifting almost always disappears by the age of 1 year. We have prescribed some limited part-time occlusion of the dominant eye in a few of these children and the exodeviation has disappeared, although it might have resolved without patching. Surgery is not indicated for these patients.
Exotropia Associated with Delayed Development
The second category of exotropia observed in the fist few months of life is a manifest variable exotropia that is associated with a delay in neurologic or physical development. Occlusion therapy for suspected amblyopia is prescribed. If the deviation persists and becomes stable and an angle of deviation greater than 15 PD is measured repeatedly, surgery is indicated.
True Infantile Exotropia
This third very small group of patients includes those most correctly labeled as having infantile exotropia. These are healthy children younger than 12 months of age with exotropia and no other ocular, craniofacial, or neurologic defects. Moore and Cohen reported 10 cases, Rubin et al. reported 13 cases pooling four pediatric ophthalmology practices, and Biglan et al. reported 12 patients with the diagnosis confirmed by a pediatric ophthalmologist before 12 months..3–5 The exotropia is usually constant or a poorly controlled intermittent deviation of 25 PD or greater.
The first treatment is occlusion therapy for amblyopia and also for antisuppression to make a constant exotropia into an intermittent one and if intermittent, then onto a better-controlled intermittent state. Spectacles for myopia or significant hyperopia or anisometropia should be prescribed. If a constant or poorly controlled intermittent exotropia persists and the deviation can be reproducibly measured with alternate prism and cover test, then surgical correction is indicated. These criteria are similar to those that must be satisfied before surgical correction of infantile esotropia is performed. The goals of surgery are also the same: establishing and maintaining equal vision, straight eyes, full extraocular movements, and facilitating the development of binocularity.
The type of surgery performed is either a bilateral lateral rectus muscle recession especially if the deviation at 6 m is significantly more than 1/3m, or a lateral rectus recession and resection of the medial rectus on the same eye. The recess-resect operation is preferred when the distance and near measurements are equal or nearly so and the 6 m deviation is greater than 30 PD. The selection of specific surgical plans is covered more completely in the section on intermittent exotropia. In this group of infantile exotropia patients, however, the deviation is usually more than 30 PD and the same at 6 m and 1/3 m, so that a recess-resect operation is often our choice.
The results of treatment are somewhat similar to those in infantile esotropia. Second surgeries for overaction of the inferior oblique muscles and dissociated vertical divergence (DVD) occur in about one third of patients in addition to the need of additional horizontal alignment surgery. In the report by Biglan et al., 10 patients required 18 surgeries to maintain alignment.5 Hunter demonstrated a good surgical alignment in nine of 13 patients. Dissociated vertical deviations occurred in six of 13 patients with eight of 13 patients showing gross stereopsis.6 Limited binocularity is the usual sensory outcome as it is with infantile esotropia.
Intermittent Exotropia
Classification
The classic classification of intermittent exotropia is based on distance/near differences. In 1987, Duane7 first presented a classification, which was modified by Burian into the accepted standard that is used today.8,9,10,11 The four types are:
Basic exotropia, in which the deviation at distance fixation (6 m) is the same as near fixation (one third m)
Divergence excess, in which the deviation at distance is more than 10 PD greater than at near and remains so even when the near deviation is measured following one hour of monocular occlusion
Pseudo divergence excess, in which the distance deviation is more than 10 PD greater than the near deviation, but the near deviation becomes close to the distance deviation in magnitude when measured following 1 hour of monocular occlusion
Convergence insufficiency, in which the near deviation is more than 10 PD greater than the distance deviation
The method of identifying patients in the pseudo divergence or simulated divergence excess group has elicited controversy. Brown12 suggested that these patients could be identified if they showed an increase in their near deviation when measured through a +2.50 or +3.00 (D) lens placed in front of each eye and that this was the same as 1 hour occlusion. Helveston13 reported that the same as 1-hour monocular occlusion and the +3.00 (D) lenses were affecting different convergence mechanisms and should be considered interchangeable. Cooper et al.14 pointed out the need to eliminate tonic proximal fusion with 1-hour occlusion and then not let the patient refuse before obtaining near measurements with +3.00 (D) lenses for the purpose of calculating the AC/A ratio. Thus, the proper method to identify a patient with pseudo- or simulated divergence excess exotropia is to measure the near deviation following 1 hour of monocular occlusion and then measure the near deviation with +3.00 (D) lenses to look for the presence of a high AC/A ratio. This subject is beautifully reviewed and discussed in an excellent paper by Kushner and Morton.15
Intermittent exotropia is also classified secondarily based on fusion. Does the deviation remain intermittent by fusion at both near and distance fixation, is the deviation phoric at one fixation distance and intermittent at the other, or has the deviation become constant at one of these fixation distances? Some authors and strabismologists will declare an exodeviation intermittent only if the deviation is intermittent or phoric at both near and far fixation distances. If the ocular alignment seen is constantly exotropic at 6 m, but intermittent at one third m, some label this presentation intermittent exotropia, whereas others classify it as constant exotropia. In this chapter, the term intermittent exotropia includes those patients whose deviation is intermittent at both far and near fixation distances or intermittent at one and phoric at the other. If a constant deviation initially is present at one fixation distance but is made intermittent by occlusion therapy before surgery, then this patient is classified as having intermittent exotropia. If the patient has constant exotropia at one or both fixation distances even following occlusion therapy, then that patient is considered as having constant exotropia.
Patients with third cranial nerve palsy, bilateral, and unilateral internuclear ophthalmoplegia, and exotropic Duane’s syndrome are excluded from consideration in this discussion.
Treatment Goals
Treatment goals for intermittent exotropia are the same as for all types of strabismus: to establish and maintain good and equal vision, fusion and stereopsis, full ductions and versions, relief of asthenopic symptoms and diplopia, functionally and cosmetically straight eyes, and elimination of any abnormal head position. Amblyopia is not a common finding in intermittent exotropia, and when it occurs, the difference in acuity between the eyes is usually small. When seen, it often is related to anisometropia. Fusion is present in all patients with intermittent exotropia, and excellent stereopsis is present in at least two thirds. The remaining one third has monofixation.16 Asthenopic symptoms are rarely expressed or determined to be present before patients reach their mid-teen years, and face turns or head postures are rare in intermittent exotropia.
The goals of treatment of intermittent exotropia may be achieved through nonsurgical management in some patients. If this treatment fails, surgery is recommended.
Hardesty et al.17,18 and Pratt-Johnson et al.19 presented the results of treatment and discussed the criteria or parameters to be considered in evaluating the treatment of intermittent exotropia and determining whether a functional cure has been obtained. Their results and the follow up are excellent.
Nonsurgical Management
Natural History of Intermittent Exotropia
Many ophthalmologists feel exotropia is a lifelong problem and requires surgery for poor control. Romanchuk followed 109 patients with intermittent exotropia for an average of 9 years. Amblyopia proved to be a mild problem and stereoacuity remained good. Over the study the mean angle of deviation did not vary but in 19% the angle decreased and in 23% the angle increased. Control improved in 26% and deteriorated in 23%.20 It appears exotropia does not change dramatically with time.
Amblyopia
Significant amblyopia is not a common finding in intermittent exotropia, but when detected, it must be treated first. We prefer occlusion therapy to pharmacologic penalization in patients with intermittent exotropia. When occlusion is made, there is the effect of antisuppression as well as that of amblyopia therapy. Penalization probably does not have an antisuppression effect but may provide adequate amblyopia therapy.
Spectacles
Appropriate spectacles must be prescribed to allow good and equal vision and enhance fusion. Accommodative convergence may also be stimulated by altering the spectacle prescription. If the patient is myopic or has minimal hyperopia, up to -3.00 diopters (D) may be added to the cycloplegic refraction. This “overminusing” stimulates accommodation and accommodative convergence. The goal in such therapy is to reduce the amount of frequency of the intermittent exotropia. Kushner21 considered whether overcorrecting minus lens therapy in intermittent exotropia causes myopia and concluded that it does not.
If the patient is significantly hyperopic (greater than +4.00), the value of prescribing glasses may be looked at in two ways. The amount of their hyperopia can be reduced to stimulate accommodation. This approach is similar to the rationale of over-minusing to improve exodeviation. However, if the deviation is poorly controlled and surgery is indicated, wearing and measuring the deviation with the full plus cycloplegic refractive error may increase the deviation and provide a truer measure of the deviation as well as worsen the control. In some instances, however, giving the full plus actually improves the control. Iacobucci et al.22 reported resolution of exotropia after providing full or nearly full hyperopic correction in seven patients.
We use the following guidelines in prescribing spectacles for patients with intermittent exotropia. If the patient myopia is -0.75 D or more, spectacles are ordered. The prescription is written with an additional -0.25 D over the cycloplegic refraction to ensure that the patient is not underminused. We over-minus in the case of some postoperative patients who are drifting back to exotropia. The second situation for using overcorrecting minus lenses is the intermittent exotropia patient with a proven high AC/A ratio. This high AC/A ratio is identified with the use of +3.00 D lenses following 1 hour of monocular occlusion. Prescribing over minus lenses helps provide better control and actual reduction of the distance deviation. If an esotropia is produced at near fixation, then a bifocal is also prescribed. If not detected preoperatively, these patients will usually develop a near deviation of esotropia postoperatively and require a bifocal. Kushner23 reports successful treatment of 16 high AC/A intermittent exotropia patients with overcorrecting minus lenses and a bifocal. These patients are described in the section on treatment of undercorrection.
If the patient has hyperopia of +4.50 D or less, spectacles are not ordered. If the patient’s hyperopia measures +5.00 D or more on cycloplegic refraction, I prescribed spectacles and reduce the hyperopia by 2.50 D. The goal is to allow the patient to see clearly with maximum accommodative effort of +2.50 D at 6 m, which is within the normal range of accommodation for individuals younger than 20 years old. If larger amounts of hyperopia are not corrected, the patient may not accommodate and simply may be content with blurred vision. Blurred vision is a negative factor for fusion. If reducing the hyperopic prescription by 2.50 D produces blurred vision, particularly in older patients, then more hyperopia closer to the cycloplegic refraction must be given.
Astigmatism of 1.25 D in patients 5 years and older and 2.00 D in younger patients warrants spectacles.
For exotropia associated with anisometropia, Nemet showed good control of the exotropia with refractive surgery for the anisometropia. If the patient had good control of the exotropia with glasses, it was maintained after refractive surgery.24
Occlusion
In the absence of amblyopia, part-time occlusion is prescribed at the initial visit when intermittent exotropia is diagnosed. This occlusion is done for 2 hours daily and on an alternate basis if one eye is not markedly dominant. In the presence of marked dominance, the dominant eye is occluded for 2 hours daily. The patient is examined again in 6 to 8 weeks. If the exotropia is well controlled and especially if the control has improved with occlusion therapy, patching is continued for up to 6 months. If the control remains poor or deteriorates while occlusion is being performed, surgery is advised. The occlusion is continued up to the date of surgery to minimize suppression. These visits allow the physician to measure the deviation more than once, thereby increasing the accuracy of determining the true amount of deviation and surgical target. The visits also provide an opportunity for the patient, family, and physician to discuss the treatment and expected results of treatment of intermittent exotropia.
Prisms and Prism Adaptation
The use of base-in fusing prisms has not been successful in the treatment of intermittent exotropia. They compensate for the deviation but do not result in improvement of control or reduction in the amount of deviation.
Prism adaptation for exotropia is controversial as to its effectiveness. Kalmesh demonstrated an 89% surgical success in patients whose deviation built with prism adaptation and only 54% in patients that did not respond to prism adaptation.25 However, Ohtsuki found no difference in the surgical response to prism adaptation in a large series of 94 patients.26
Botulinum Toxin Injection
Botulinum toxin injection has not shown to be an effective primary treatment modality for exotropia.27
Indications for Surgery
In intermittent exotropia, fusion is present in the phoric phase and absent in the tropic phase. The most important reason for surgical intervention is to allow binocular vision (fusion) to occur as much of the time as possible and to reduce or eliminate the need for abnormal sensory adaptation. Thus, when exotropia occurs almost 50% of the time, then the eyes diverge frequently and spontaneously at one or both (far or near) fixation distances, or when the eyes remain tropic and the individual is unaware of the divergence, surgery should be considered. Each patient must be considered individually. How much the patient, the teacher, and the family and friends report frequent manifest exotropia, the decision to perform surgery is easily reached. If the patient is a child and the parents deny the child’s exotropia, although spontaneous exotropia is seen in the physician’s office, the process becomes more difficult. The exotropia should be demonstrated to the family, and they should be asked to watch for it. They may deny seeing exotropia because they want to avoid surgery, and the decision to perform surgery may come solely from the office examination findings.
It is difficult to state how frequently exotropia must occur for surgery to be warranted. Most ophthalmologists would be comfortable with an occurrence of 50%. I begin to consider a surgical recommendation when the occurrence of exotropia is estimated at 25%, if the family sees it daily and for more than a few moments.
The frequency and duration of the occurrence of exotropia are more important factors than the size of the deviation in determining whether to proceed with surgery. The amount of the deviation only determines how much surgery should be done. A patient with a poorly controlled deviation of 13 to 15 PD (present in the tropic phase 50% of the time) is more of a candidate for surgery than a patient with a well-controlled intermittent exodeviation that measures 30 PD.
Surgery will not improve the degree of fusion (i.e., change monofixation to bifixation or improve the level of stereopsis as measured by the Titmus stereo test at near); however, it will improve the amount of time the individual is fusing.16,28 In a report by O’Neal et al., the distance stereo acuity as measured by the Mentor B-Vat II Video acuity tester and binocular system deteriorated as the control of the distance exotropia worsened and was improved following surgical alignment.29,30 On rare occasions, surgery is performed because a patient reports asthenopia or intermittent diplopia, even though little or no tropia is seen during the examination, and only exophoria is noted. Surgical success for motor control did not appear to differ if the deviation was an intermittent or constant exotropia. However, binocular function was found to be much better postoperatively in the intermittent exotropia patients.31
After 6 months of age, the timing of surgery is not age dependent. The decision to proceed with surgery should be based on the degree of control of the deviation and the amount of time that the patient demonstrates a tropia. If the eyes are frequently exotropic, surgery should not be delayed, particularly if the family also notes a decrease in control of ocular alignment. In general, most patients do not undergo surgical correction until they are 2 to 3 years of age or older. If part-time occlusion therapy helps maintain control and allows surgery to be delayed, it may be better to operate after the patient is 3 years old. More accurate measurements may be possible, affording a truer surgical target angle, and postoperative therapy may be applied more easily. A study by Pratt-Johnson et al. showed a better cure rate for patients who were operated on before age 4 than for those operated on later.19 Asjes et al. showed better stereopsis and control in patients corrected before age 7 years old.32 Abroms confirmed better surgical response if surgery done before 7 years of age and before 5 years duration of the strabismus.33 Baker et al. did not show a better sensory outcome resulting from early surgery.34
If surgery is considered, the patient should be measured at least twice before surgery is performed. The amount of deviation measured intermittent exotropia may vary from 1 day, or even 1 hour, to the next. Having a reproducible measurement of the exodeviation that serves as the surgical target angle is the first step in obtaining a good surgical result.
Contraindications
The most important consideration in performing surgery is the patient’s general medical condition. Most strabismus surgery, and all strabismus surgery that is performed on children, is done with the patient under general anesthesia. Therefore, the most significant contraindication to surgery for intermittent exotropia is a medical condition that would not allow the patient to undergo general anesthesia. It is also important that an anesthesiologist competent to deal with pediatric patients be available.
Surgery should be delayed if the physician cannot obtain an accurate measurement of the deviation, or if the deviation is variable and different measurements are obtained during different examinations. Stable, accurate, and reproducible measurements are necessary before surgery should be performed.
Surgical Treatment
Choosing the Operation
The surgical procedures used to treat intermittent exotropia are bilateral lateral rectus muscle recession (bilateral recession), unilateral lateral rectus recession and resection of the medial rectus on the same eye (recess-resect), resection of both medial rectus muscles (bimedial resection) or single muscle surgery (unilateral recession). If the deviation is large (greater than 45 PD), operating on three horizontal rectus muscles may be considered.
The surgeon determines the muscles on which to operate based on the amount of deviation. Another factor to consider is previous surgery. In general, a bilateral recession is chosen when the distance deviation is greater than the near deviation. When the deviation is the same at distance and near (basic deviation), either a bilateral lateral rectus recession or a recess-resect procedure may be performed. Kushner has pointed out that patients with basic deviations seem to have better results following a recess-resect procedure. There may be a tethering effect from the medial rectus resection in these patients who lack tonic proximal fusion. A recent study supports the recess-resect procedure as better than bilateral lateral rectus recessions for a basic exodeviation.35 However, a previous large study of 666 patients with surgery for exotropia described bilateral lateral rectus recessions as superior.36 Those with simulated divergence excess deviations with tonic proximal fusion as proposed by Kushner do well with bilateral lateral rectus recession procedures.37
In selected cases in which the distance deviation is greater than the near and 20 PD or less, a unilateral lateral rectus recession of 7 to 9 mm may be considered.38,39,40 Olitsky and Kalmesh demonstrated surgical success of 77% and 78% respectively when the deviation was less than 30 prism diopters.41,42
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