Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

normal AC/A ratio, but have increased proximal convergence that reduces the near deviation. Proximal convergence is independent 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 convergence 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 distance 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 intermittent 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 secondary to fusional convergence because prolonged monocular occlusion 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.

FAR DISTANCE TEST

Another technique that reduces measurement variability by disclosing 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 preferable 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 providing 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 temporary measure. Convergence exercises are useful for convergence 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 indicates 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

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 comparing 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 binocularity. 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 operated safely under 4 to 6 years of age for intermittent exotropia, but they must be followed closely, because a persistent consecutive 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

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 deviation. Use the patch test in divergence excess cases to determine if there is pseudoor true divergence excess.

AMOUNT OF SURGERY

Surgical parameters for patients with basic or pseudodivergence excess intermittent exotropia should be based on the full distance 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 postoperative bifocal glasses.22 If a true divergence excess associated with a high AC/A ratio is present, it is best to operate for a deviation 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 horizontal muscles for A- and V-patterns. Inferior oblique weakening 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 consecutive 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

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 horizontal 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 intermittent 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 performing 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, alternate 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 alleviate the diplopia, but leave a small residual esophoria to encourage 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. Otherwise, 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

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 overcorrections 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 undercorrected 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 overand undercorrections to maintain constant fusion to prevent suppression.

CONVERGENCE INSUFFICIENCY

Convergence insufficiency is the inability to maintain convergence on objects as they approach from distance to near. Symptoms 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 intermittently 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.

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 orthoptic convergence exercises.23 The two most useful convergence exercises are near point exercises (pencil pushups) and prism convergence exercises. Near point exercises consist of presenting 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 baseout prism until the patient notes blurred vision (blur point). Then, increase prism until fusion breaks (break point). Both convergence 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

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 insufficiency.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, presbyopia is the most common type of accommodative insufficiency, 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 ciliary body dysfunction associated with Adie’s pupil.2

According to Duane’s standard curve of accommodation, normal patients under 20 years of age should be able to accommodate 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 cataracts show an even distribution between esodeviations and exodeviations.10 Treatment

for sensory exotropia is performing a recession/resection procedure of the eye with the decreased vision.

CONGENITAL EXOTROPIA

Congenital exotropia is extremely rare, and most ophthalmologists 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 cerebral palsy.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 newborns. 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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4.Burian HM. The sensorial retinal relationship in comitant strabismus. Arch Ophthalmol 1947;337:336.

5.Burian HM, Franceschetti AT. Evaluation of diagnostic methods for the classification of exodeviations. Trans Am Ophthalmol Soc 1970;68:56.

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