8

Exotropia Kenneth W. Wright

xodeviations are quite common, and they are not necessa- Erily pathological. A small intermittent exotropia is normal in most newborns, as 70% of normal newborns have a transient exodeviation that resolves by 2 to 4 months of age.1 Another type of exodeviation that is considered normal is a small , usually less than 10 prism diopters (PD). Most normal adults have a small exophoria when fully dissociated. Exodeviations are controlled with our innate strong fusional convergence, typi- cally measuring 30 PD or more. The most common form of divergent is intermittent exotropia, which probably accounts for more than 90% of all exodeviations. Table 8-1 lists the different categories of pathological exodeviations, with the one most frequently occurring listed first.

INTERMITTENT EXOTROPIA

Intermittent exotropia is a large phoria that is intermittently controlled by fusional convergence. Unlike a phoria, intermit- tent exotropia spontaneously breaks down into a manifest exotropia (Fig. 8-1).

Clinical Features Intermittent exotropia is usually first observed by the parents in early childhood or late infancy as an infrequent drifting or squinting of one eye. 12 Patients with intermittent exotropia tend to manifest their deviation when they are tired, have a cold or the flu, or when they are daydreaming. Adult patients will often become exotropic after imbibing alcoholic beverages or taking sedatives.

266 chapter 8: exotropia 267

TABLE 14.1. Classifications of Exodeviations. Intermittent exotropia (common) Convergence insufficiency (common) Sensory exotropia (common) Congenital exotropia (rare)

Signs of intermittent exotropia include blurred vision, asthenopia, visual fatigue, and, rarely, in older children and adults. Many patients with intermittent exotropia have pho- tophobia (squinting to bright light). was originally thought to be a way for eliminating diplopia or confusion, but Wiggins and von Noorden have shown that the photophobia may not be related to diplopia avoidance.39 As a rule, during the phoric phase of intermittent exotropia, the eyes are perfectly aligned and the patient has bifoveal fusion with excellent stereoacuity ranging between 40 and 50 s arc. This excellent bifoveal fusion develops because the eyes are well aligned in early infancy when the critical binocular cortical con- nections are being established. A minority of patients with inter- mittent exotropia are primary monofixators and do not develop normal bifoveal fusion with good . Rarely a patient will even have significant . The poor fusion in these cases is associated with a predominance of the tropia phase. During the tropia phase of intermittent exotropia, patients will show large hemiretinal or regional suppression of the temporal .26,30 Anomalous retinal correspondence in the tropia phase and normal retinal correspondence in the phoria phase have been demon- strated in some patients with intermittent exotropia.4,38

Natural History The natural history of intermittent exotropia remains obscure, as there are no longitudinal prospective studies and only a few retrospective studies of untreated intermittent exotropia. Von Noorden found that 75% of 51 untreated patients showed pro- gression over an average follow-up period of 3.5 years, whereas 9% worsened and 16% improved.36,38 Hiles et al.,20 in their study of 48 patients, found no significant change in the deviation after an average of 11 years follow-up, and 2 patients progressed to a constant tropia. The most we can say about the natural history is that, in the majority of cases, intermittent exotropia does not get better; it either stays the same or progresses. If the tropic phase increases, patients may develop dense suppression and, 268 handbook of pediatric strabismus and amblyopia

A

B FIGURE 8-1A–B. (A) Patient with intermittent exotropia and straight eyes in the phoric phase. Patient has 40 s arc stereoacuity. (B) Occlusion of the right eye disrupts fusion and manifests the exotropia. Under the occluder, the right eye is deviated temporally. over time, may progress to a constant exotropia with loss of fusional potential.

Classifications Intermittent exotropia has been classically categorized into three subtypes based on the difference between the distance chapter 8: exotropia 269

C FIGURE 8-1C. (C) Occluder is removed and the right eye is deviated temporally, showing the exotropia. Patient is in the tropic phase and suppresses right eye.

deviation and the near deviation. These three “older” classic cat- egories are (1) basic, (2) pseudodivergence excess, and (3) true divergence excess. It is important to note that the older termi- nology uses the term divergence excess, and pseudodivergence excess is only descriptive as to the difference of the deviation distance versus near; it is not meant to imply a mechanism for the distance–near disparities. The mechanism for the dis- tance–near disparities seen in patients with intermittent exotropia is most likely caused by superimposed overconver- gence on the basic exodeviation. These convergence mecha- nisms include tonic fusional convergence (tenacious proximal fusion),22 accommodative convergence (AC/A ratio), and proxi- mal convergence (instrument convergence).

BASIC INTERMITTENT EXOTROPIA With this type of exotropia, there is no significant distance–near disparity, and the distance deviation is within 10 PD of the near deviation. Patients with a basic deviation have normal conver- gence, so their deviation is essentially the same for distance and near. 270 handbook of pediatric strabismus and amblyopia

PSEUDODIVERGENCE EXCESS This is an exodeviation that is measured larger for distance fix- ation than near fixation by brief alternate cover testing (distance 10 PD greater than near); however, with prolonged monocular occlusion (patch test for 30–60 min), the near deviation increases and becomes similar to the distance deviation (within 10 PD). For example, an exodeviation measures 30 PD in the distance and 10 PD at near to alternate cover testing. One eye is patched for 30 min, and now the patient measures 30 PD in the distance and 25 PD at near. This change occurs because patients with pseudodivergence excess have increased tonic near fusional convergence that dissipates slowly after monocu- lar occlusion. Prolonged monocular occlusion of 30 to 60 min is required in these patients to dissipate tonic near fusional con- vergence and disclose the full latent deviation (see Patch Test, below). The relatively brief period of monocular occlusion that occurs with alternate cover testing is not enough to break up the tonic near fusional convergence and disclose the full deviation at near. Surgery is performed for the full distance deviation. Pseudodivergence excess is quite common. More than 80% of patients with an apparent divergence excess actually have pseudodivergence excess, as the near deviation will increase to within 10 PD of the distance deviation after the patch test.5,22,37

PATCH TEST (OCCLUSION TEST) The patch test consists of placing an occlusive patch over one eye for at least 30 to 60 min, then measuring the deviation without letting the patient restore binocular fusion. The idea is to totally suspend all tonic fusional convergence by occluding one eye, forcing the full latent deviation to become manifest. When performing the patch test, be sure the patient does not peek around the patch and regain fusion before the deviation is measured. To measure the deviation, first cover the unpatched eye with a paddle occluder, then remove the patch and measure the deviation with alternate cover testing. This method ensures the patient will not sneak a peek with both eyes and reestablish fusion before the deviation is measured.

TRUE DIVERGENCE EXCESS In these cases, the distance deviation is greater than the near deviation by more than 10 PD, even after performing the patch chapter 8: exotropia 271 test. For example, the distance deviation would measure 30 PD, near deviation 10 PD and, after a 30-min patch test, the distance deviation would be 30 PD and the near deviation 15 PD. This author and Eugene De Juan (Los Angeles, CA) studied the cause of true divergence excess at the Wilmer Clinic at Johns Hopkins Hospital in Baltimore in 1981. They found that most of the patients with true divergence excess had a high AC/A (accom- modative convergence/) ratio as determined by a 3.00 add after a 60-min patch test. The patch test relaxes tonic fusional convergence, and the 3.00 add relaxes accom- modation. The high AC/A ratio patients do not show an increase in the near exotropia to the patch test, but the near deviation increases dramatically with a 3.00 near add.40 In a similar study, Kushner22 found approximately 60% of patients with a true divergence excess had a high AC/A ratio and 40% had a normal AC/A ratio. The group with a high AC/A ratio was prone to postoperative overcorrection (75% overcorrection) at near if the distance measurement is used as the surgical target angle. The 40% of true divergence excess patients with a normal AC/A ratio had relatively good results using the distance measure- ment. Patients with true divergence excess are a difficult group to surgically correct as they are prone to having a consecutive at near, and some will require bifocals or additional surgery. Following is a summary of the causes of overconver- gence that produce true divergence excess.

CAUSES OF TRUE DIVERGENCE EXCESS

HIGH AC/A RATIO This condition occurs when the distance deviation is larger than the near deviation even after the patch test, but the near devia- tion increases close to the distance deviation with a 3.00 add. High AC/A ratio intermittent exotropia has normal tonic fusional convergence but has a high AC/A ratio that causes the distance–near disparity. Surgery is usually performed for a deviation somewhere between the distance and near deviation measured without near add. Some of these patients require bifocals after surgery if there is an esotropia at near.

INCREASED PROXIMAL CONVERGENCE This situation arises when the distance deviation is larger than the near deviation after the patch test and a 3.00 add OU. These patients have a normal tonic fusional convergence and a 272 handbook of pediatric strabismus and amblyopia normal AC/A ratio, but have increased proximal convergence that reduces the near deviation. Proximal convergence is inde- pendent of binocular fusion. Surgery should be performed for an angle between the distance and near deviation.

MIXED CONVERGENCE MECHANISM There are many cases where more than one mechanism of con- vergence causes the distance–near disparity. This group with mixed convergence mechanism explains the patients that do not specifically fit into the pure categories listed previously. An example of a mixed convergence mechanism is when the dis- tance deviation is 45 PD and the near deviation is 20 PD. After the patch test, the near deviation increases to 30 PD and, with a 3.00 near add and the patch test, the near deviation equals the distance. In this example, there is a component of increased tonic fusional convergence brought out by the patch test and a slightly high AC/A ratio (AC/A ratio 5; i.e., 45 30/3.00) disclosed by the 3.00 add. Both the increased tonic fusional convergence and the slightly high AC/A ratio contribute to the distance–near disparity. Surgery is performed for the angle measured between the distance deviation and near the deviation (after the patch test).

Measuring the Exodeviation Obtaining reproducible measurements in a patient with inter- mittent exotropia can be difficult, as the angle of deviation is often variable when measured by routine alternate cover prism testing. If it is late in the day and the patient is tired, fusional convergence will be weak and a large deviation will be easily manifest. On the other hand, if the patient is wide awake and alert, strong fusional convergence will keep the deviation small and difficult to elicit. The patch test reduces variability second- ary to fusional convergence because prolonged monocular occlu- sion disrupts fusion and discloses the full latent deviation. Because most patients with intermittent exotropia often have strong tonic fusional convergence, they should be measured using prolonged alternate cover testing, making sure that one eye is always covered. If there is significant angle variability or a significant distance–near discrepancy after prolonged alternate cover testing, then a patch test is indicated. In contrast, patients who show consistent measurements and no significant distance near disparity do not need the patch test. chapter 8: exotropia 273

FAR DISTANCE TEST Another technique that reduces measurement variability by dis- closing the full distance deviation is the far distance test. This test is performed by simply having the patient fixate on an object well past 20 feet to relax all proximal convergence. It is prefer- able to measure the deviation while the patient fixates out a window to a far distant target. Combining the patch test and the far distance test has greatly reduced undercorrections and has improved overall results.

Treatment of Intermittent Exotropia

NONSURGICAL TREATMENT In general, nonsurgical treatments for intermittent exotropia are not very effective. One method is to prescribe 2 to 3 diopters of myopic correction over what is required by cycloplegic refraction.8 Overminusing induces accommodative convergence, thus reducing the exodeviation. Another method is part-time monocular occlusion therapy.15,18 By occluding the dominant eye, the patient is forced to use the nonpreferred eye, thus pro- viding antisuppression therapy. Although others have found success with this procedure, in this author’s experience, only a few patients have responded to this therapy. In virtually every case, the intermittent exotropia returns when the patching is stopped. Part-time occlusion therapy may be tried in younger patients as a method for delaying surgery, but it is only a tem- porary measure. Convergence exercises are useful for conver- gence insufficiency but not for most cases of intermittent exotropia. The use of antisuppression orthoptic therapy and diplopia awareness are not indicated, as this practice may lead to intractable diplopia and is detrimental to the patient.

INDICATIONS FOR SURGERY As with any strabismus, the indications for surgery include preservation or restoration of binocular function and cosmesis. In intermittent exotropia, one of the most important indications for therapeutic intervention is an increasing tropia phase. If the frequency or duration of the tropia phase increases, this indi- cates diminished fusional control and a potential for loss of binocularity. Progression should be monitored by recording size, frequency, duration of the exotropia, and the ease of dissociation 274 handbook of pediatric strabismus and amblyopia after brief monocular occlusion. Documentation of deteriorating fusional control is an indication for treatment. Additionally, if the exotropia is manifest more than 50% of waking hours, surgery is probably indicated. In most cases, surgery should be delayed until 4 years of age. A study compar- ing surgery performed at various ages showed a significant increase in the incidence of amblyopia and loss of stereopsis when a consecutive esotropia occurred in children under 4 years of age.14 Because the desired result is an initial consecutive esotropia, younger children who have surgery for intermittent exotropia are at risk for developing amblyopia and losing bino- cularity. If, however, the exotropia is present more than 50% of waking hours, and is increasing in size, frequency, or duration, then early surgery may be indicated even in children under 4 years of age. Richard and Parks32 found no significant difference in results between early or late surgery, and Pratt-Johnson et al.29 actually had better results when surgery was performed under 4 years of age. The take-home message is that patients can be oper- ated safely under 4 to 6 years of age for intermittent exotropia, but they must be followed closely, because a persistent consec- utive esotropia can cause loss of stereopsis and amblyopia in this age group.

SURGICAL TREATMENT

CHOICE OF PROCEDURE For all three classic types of intermittent exotropia (i.e., basic, pseudodivergence excess, and true divergence excess), bilateral lateral rectus recessions work well. Symmetrical surgery is usually preferred over a monocular resect/recess procedure, as recession/resection procedures produce lateral incomitance with a significant esotropia in the side of the operated eye. This incomitance can produce diplopia in sidegaze that may persist for months or even years. In patients with amblyopia of 20/50 or worse, a recession/resection procedure on the amblyopic eye is preferred, avoiding surgery on the “good” eye.

ROLE OF THE PATCH TEST Historically, the patch test was important to distinguish among the three subgroups of intermittent exotropia because patients with basic or pseudodivergence intermittent exotropia would receive a monocular recess/resect procedure, whereas patients with true divergence excess would undergo bilateral lateral chapter 8: exotropia 275 rectus recessions.7,37 Parks28 has shown that bilateral lateral rectus recessions work well for all three types of intermittent exotropias, so the patch test is probably not very important for determining whether a recess/resect or a bilateral recession should be performed. The patch test is, however, very useful in patients with a distance–near disparity to bring out the full devi- ation. Use the patch test in divergence excess cases to determine if there is pseudo- or true divergence excess.

AMOUNT OF SURGERY Surgical parameters for patients with basic or pseudodivergence excess intermittent exotropia should be based on the full dis- tance deviation as determined by alternate cover testing or the patch test. Patients with true divergence excess, however, should be treated more conservatively, especially if there is an associated high AC/A ratio. These patients are difficult to manage, because totally correcting the distance deviation often leads to a persistent esotropia at near that may require post- operative bifocal glasses.22 If a true divergence excess associated with a high AC/A ratio is present, it is best to operate for a devi- ation somewhere between the distance and near deviations. These patients with true divergence excess and a high AC/A ratio should be told they have a significant risk for a persistent overcorrection and, postoperatively, may require a reoperation, a bifocal add, or miotic drops. Moore25 suggested reducing the amount of recession in patients with lateral incomitance. It is this author’s experience that even moderate amounts of lateral incomitance are not important.

A- AND V-PATTERNS: OBLIQUE OVERACTION Intermittent exotropia may be associated with A- and V-patterns and inferior and superior oblique overaction (see Chapter 9). In these cases, it is appropriate to simultaneously operate on the obliques if dysfunction is present, or vertically offset the hori- zontal muscles for A- and V-patterns. Inferior oblique weaken- ing procedures are safe in patients with bifoveal fusion and intermittent exotropia, but beware of performing superior oblique tenotomies or tenectomies, as this may result in a con- secutive superior oblique paresis with intractable cyclovertical diplopia.41 If significant superior oblique overaction and an “A” pattern is present, consider an infraplacement of the lateral 276 handbook of pediatric strabismus and amblyopia rectus muscles or the Wright superior oblique tendon expander procedure, rather than a tenotomy or tenectomy of the superior oblique muscle. Do not significantly alter the amount of hori- zontal surgery just because simultaneous oblique surgery is also being performed. Small vertical deviations associated with intermittent exotropia should be ignored, as these small vertical deviations usually disappear after surgery. Patients with large-angle inter- mittent exotropia may have an X-pattern, with the exotropia increasing in upgaze and downgaze relative to the deviation in primary position. In some cases, there is true overaction of all four oblique muscles; however, usually this pattern is due to tight lateral rectus muscles causing a leash effect similar to Duane’s syndrome upshoot and downshoot. The X-pattern is usually small, and it is best to address the pattern by simply per- forming bilateral lateral rectus recessions for the deviation in primary position.

POSTOPERATIVE CARE Immediately after surgery, a small consecutive esotropia of 8 to 10 PD is desirable, as even a large consecutive esotropia up to 20 PD may resolve without further surgery.31,33 Be sure to warn the parents and patients before surgery that postoperative diplopia may occur so they are not surprised. Postoperative diplopia associated with the initial overcorrection usually resolves by 1 to 2 weeks. In children under 4 years of age, alter- nate part-time patching of each eye helps prevent suppression and amblyopia and may facilitate straightening of the eyes. If a residual esotropia persists past 2 to 3 weeks, then the patient should be treated with prism glasses to neutralize the esotropia and re-establish fusion.17 Prescribe just enough prism to allevi- ate the diplopia, but leave a small residual to encour- age divergence. If after 6 to 8 weeks the esotropia persists, then a reoperation should be considered. Advancement of the lateral rectus muscle is indicated if there is limited adduction or lateral incomitance that is consistent with a slipped muscle. Other- wise, bimedial recessions are usually the procedure of choice for a consecutive esotropia, especially if the esotropia is greater at near. If the consecutive esotropia is present only at near, one may consider a bifocal add, miotics, or even a base-out prism to correct the near esotropia while creating a small exodeviation in chapter 8: exotropia 277 the distance. Failing this, small bimedial rectus recessions is the next option, with or without a Faden procedure. Patients with a residual exotropia greater than 10 PD after the first postoperative week will probably not improve and most will require additional surgery. It is best to wait 8 weeks before reoperating on the residual exotropia. Rerecess both lateral rectus muscles if the primary surgery was bilateral recessions of 6.0 mm or less. If the primary recessions were greater than 6.0 mm, perform bilateral resections but be conservative, as over- corrections are common after resecting against a large recession.

PROGNOSIS The success rate, as in most types of strabismus, is dependent on the length of follow-up and, the longer the follow-up, the higher the incidence of undercorrection. Richard and Parks,32 in one of the longest follow-up studies, found a 56% success rate with one surgery, defining success as a postoperative deviation less than 10 PD, with a follow-up period of 2 to 8 years (mean, 4 years). Thirty-eight percent (38%) of their patients were under- corrected and 6% were overcorrected. An additional surgery improved their success rate to just over 80%. Hardesty16 reported an 80% success rate after no more than two surgeries with a 10-year follow-up. Hardesty attributed the long-term success to the aggressive use of postoperative prisms for both over- and undercorrections to maintain constant fusion to prevent suppression.

CONVERGENCE INSUFFICIENCY

Convergence insufficiency is the inability to maintain conver- gence on objects as they approach from distance to near. Symp- toms usually first occur during the teenage years and include asthenopia, reading difficulty, blurred near vision, and diplopia. Alternate cover testing will disclose a near exophoria with essentially no distance deviation. The exophoria at near inter- mittently breaks down into a tropia, especially after prolonged near work such as reading. When tropic, most patients will see double while some will not, as they have learned to suppress. Even patients with suppression can experience asthenopia and are often symptomatic. 278 handbook of pediatric strabismus and amblyopia

Patients with convergence insufficiency will show a remote near point of convergence. The near point of convergence (NPC) is how close one can bring a fixation target to the nose and still maintain fusion. The break point is when the target is too close, fusion breaks, and an exotropia becomes manifest. The normal NPC is between 5 to 10 cm from the bridge of the nose. Patients with convergence insufficiency will have a remote break point ranging from 10 to 30 cm or more. Convergence insufficiency may also be associated with reduced fusional convergence amplitudes. Normal fusional convergence amplitudes for near are between 30 to 35 PD, but patients with convergence insufficiency usually break with less than 20 PD base-out. Some patients with convergence insufficiency will initially show a fairly good near point of convergence and convergence fusion amplitudes at near; however, on repeat testing, they are easily fatigued. The diagnosis of convergence insufficiency should not be based solely on one test trial but, instead, on repeat testing. The best treatment for convergence insufficiency is orthop- tic convergence exercises.23 The two most useful convergence exercises are near point exercises (pencil pushups) and prism convergence exercises. Near point exercises consist of present- ing a target at a remote distance where it is easily fused, then slowly bringing the target in toward the eyes until break point is achieved (Fig. 8-2). With prism convergence exercises, a prism bar oriented base-out is presented to one eye to induce fusional convergence (Fig. 8-3). First, use a small prism that can be easily fused while the patient fixates on a near target. Increase the base- out prism until the patient notes blurred vision (blur point). Then, increase prism until fusion breaks (break point). Both con- vergence exercises should be repeated 15 to 20 times during each session and repeated 2 to 3 times per day. Convergence exercises stimulate fusional convergence only if the patient appreciates diplopia and the break point. Patients who do not appreciate diplopia can be treated with red glass convergence exercises. A red filter is placed over the dominant eye and a light is used as the fixation target. The red filter and light will help the patient recognize diplopia. Convergence exercises have been found to be extremely helpful and curative in patients with convergence insufficiency so long as these exercises are diligently performed. Improvement of symptoms usually occurs after a few weeks of exercises, but in some cases several months are needed before symptoms are relieved. In this author’s experience, almost all chapter 8: exotropia 279

FIGURE 8-2. Near point convergence exercise showing accommodative target at near. Patient starts with the target at arm’s length, and then brings the target toward the nose, converging on the accommodative target. patients with convergence insufficiency can be managed by exer- cises alone; it is the rare case that requires surgery. Always try orthoptic exercises first and, if they fail to alleviate the symp- toms, then surgery may be considered. The standard surgery for

FIGURE 8-3. Photograph of child with congenital exotropia. 280 handbook of pediatric strabismus and amblyopia convergence insufficiency is a small medial rectus resection of one or both medial rectus muscles. In this author’s experience, surgery is not effective in most cases of convergence insuffi- ciency.19,35

Accommodative Insufficiency A common cause of asthenopia and blurred near vision is convergence insufficiency, but occasionally patients have a combination of convergence insufficiency and accommodative insufficiency.24,35 Even more rare is isolated accommodative insufficiency without convergence insufficiency.11 Obviously, is the most common type of accommodative insuf- ficiency, but primary accommodative insufficiency can occur in children and young adults as well. Accommodative insufficiency can be secondary to a systemic disorder such as Parkinson’s disease, oral lithium, or local dysfunction associ- ated with Adie’s .2 According to Duane’s standard curve of accommodation, normal patients under 20 years of age should be able to accom- modate at least 10 diopters, or read the 20/40 line on the near card at 10 cm.13 Patients with accommodative insufficiency will have a remote near point of accommodation. There are no beneficial exercises for treating accommodative insufficiency; however, accommodative exercises can be tried. Mazow et al.23 found modest improvement with pretreatment accommodation averaging 7.1 diopters and posttreatment 11.4 diopters. A reading add can also be prescribed, but prescribe the lowest power that relieves the symptoms and still stimulate some accommodation. Prescribing a strong reading add only weakens the patient’s remaining accommodation.

SENSORY EXOTROPIA

If a patient loses vision in one eye, that eye may drift out (sensory exotropia). Patients with dense amblyopia may also develop a sensory exodeviation. It is often said that if the visual loss occurs before 4 years of age, an esotropia develops. If vision loss occurs after 4 years of age, an exodeviation results. This rule, however, is violated as often as it is followed. Studies of patients with unilateral congenital show an even dis- tribution between esodeviations and exodeviations.10 Treatment chapter 8: exotropia 281 for sensory exotropia is performing a recession/resection proce- dure of the eye with the decreased vision.

CONGENITAL EXOTROPIA

Congenital exotropia is extremely rare, and most ophthalmolo- gists will see only one or two cases during their career (Fig. 8- 3). Congenital exotropia may occur in patients with systemic disease, craniofacial anomalies, ocular albinism, or .21 The treatment for congenital exotropia is bilateral lateral rectus recessions, which should be performed after 6 months of age. This syndrome should not be confused with the normal, variable, small-angle exodeviation seen in 70% of normal new- borns. Instead, congenital exotropia is a large-angle constant exodeviation, with a relatively poor prognosis for fusion. It has a much higher incidence of amblyopia than intermittent exotropia, with the incidence of amblyopia being similar to congenital esotropia (20 to 40%).

References

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