CHAPTER 9
Exodeviations
An exodeviation is a manifest or latent divergent strabismus. The frequency of exodeviations varies among different ethnic groups. In Europeans and North and South Americans, exodeviations are the second most common form of strabismus, occurring approximately one-third as often as esodeviations, whereas in Asians, exodeviations are more common than esodeviations. Risk factors for exotropia include maternal smoking during pregnancy, premature birth, family history of strabismus, and uncorrected refractive errors.
Pseudoexotropia
The term pseudoexotropia refers to an appearance of exodeviation when in fact the eyes are properly aligned. Pseudoexotropia is much less common than pseudoesotropia (see Chapter 8). It may result from the following:
wide interpupillary distance
positive angle kappa without other ocular abnormalities (See the discussions of angle kappa in Chapter 7 and in BCSC Section 3, Clinical Optics.)
positive angle kappa together with ocular abnormalities such as temporal dragging of the macula in retinopathy of prematurity
Exophoria
Exophoria is an exodeviation controlled by fusion under normal viewing conditions. An exophoria is detected when binocular vision is interrupted, as during an alternate cover test. Exophoria is fairly common and is often asymptomatic if the angle of strabismus is small and fusional convergence amplitudes are adequate. Prolonged, detailed visual work may bring about asthenopia. Breakdown of an exophoria to an exotropia may occur transiently during a serious illness or after ingestion of sedatives or alcohol. Treatment is usually not necessary unless an exophoria progresses to intermittent exotropia or it causes asthenopic symptoms.
Intermittent Exotropia
Intermittent exotropia is the most common type of exodeviation that clinicians encounter.
Clinical Characteristics
The onset of intermittent exotropia usually occurs before age 5 years. Intermittent exotropia may develop during the first year of life, in which case it must be differentiated from (a) the intermittent
strabismus that is common in the first 1–2 months of life and that spontaneously resolves; and (b) constant infantile exotropia (discussed later in this chapter). Because proper eye alignment with intermittent exotropia requires that compensatory fusional factors be active, the deviation often becomes manifest during times of visual inattention, fatigue, or stress. Parents of affected children often report that the exotropia occurs late in the day or during illness, daydreaming, or drowsiness on awakening. Exposure to bright light often causes a reflex closure of 1 eye (which is why strabismus is sometimes referred to as a “squint”).
Exodeviations are usually larger when the patient views distant targets, and they may be difficult to elicit at near. Because most parental interactions with young children occur at near, parents of a child with an exodeviation may not notice it initially. Intermittent exotropias can be associated with small hypertropias, A and V patterns, and overelevation and underelevation in adduction.
Untreated intermittent exotropia may progress toward constant exotropia. During this progression, tropic episodes occur at lower levels of fatigue and last for longer periods. Children younger than 10 years usually do not experience diplopia when they are tropic, because of suppression. However, normal retinal correspondence and good binocular function remain when the eyes are straight. Amblyopia is uncommon unless the intermittent exotropia progresses to constant or nearly constant exotropia at an early age or unless another amblyogenic factor, such as anisometropia, is present.
Evaluation
The clinical evaluation begins with a history, including the age of onset of the strabismus, whether the exotropia is becoming more frequent, and the circumstances under which the deviation is manifest. During the examination, an assessment is made of the patient’s control of the exodeviation, which can be categorized as follows:
Good control: Exotropia manifests only after cover testing, and the patient resumes fusion rapidly without blinking or refixating.
Fair control: Exotropia manifests after fusion is disrupted by cover testing, and the patient resumes fusion only after blinking or refixating.
Poor control: Exotropia manifests spontaneously and may remain manifest for an extended time.
Some ophthalmologists use the Newcastle Control Score to quantitatively grade the control exhibited by patients with intermittent exotropia.
Prism and alternate cover testing should be used to evaluate the exodeviation at fixation distances of 6 m and 33 cm. A far distance measurement at 30 m or greater (eg, at the end of a long hallway or out a window) may uncover a latent deviation or bring out an even larger one. The deviation at near fixation is often less than the deviation at distance fixation. This difference is usually due to tenacious proximal fusion, a slow-to-dissipate fusion mechanism at near, but it may also be due to a high accommodative convergence/accommodation (AC/A) ratio; however, a high AC/A ratio occurs much less commonly in exotropia than in esotropia (see Chapter 8). For patients with significantly more exodeviation at distance than at near, a near alternate cover test, administered after 30–60 minutes of monocular occlusion to eliminate the effects of tenacious proximal fusion, may help distinguish between a truly high AC/A ratio and a distance–near disparity due to tenacious proximal fusion (a pseudo-high AC/A ratio). A patient with a pseudo-high AC/A ratio has roughly equal distance and near measurements after occlusion, whereas a patient with a truly high AC/A ratio continues to have significantly less exodeviation at near. Testing with +3.00 D lenses at near or –2.00 D lenses at distance can confirm the abnormality of the AC/A ratio.
Haggerty H, Richardson S, Hrisos S, Strong NP, Clarke MP. The Newcastle Control Score: a new method of grading the severity of intermittent distance exotropia. Br J Ophthalmol. 2004;88(2):233–235.
Kushner BJ, Morton GV. Distance/near differences in intermittent exotropia. Arch Ophthalmol. 1998;116(4):478–486.
Classification
Intermittent exotropia may be classified based on the difference between prism and alternate cover test measurements at distance and at near and the change in near measurement produced by monocular occlusion or +3.00 D lenses:
Pseudodivergence excess exotropia is the most common form of intermittent exotropia. Patients initially have larger deviations at distance than at near fixation, but this difference becomes minimal after monocular occlusion or with +3.00 D lenses at near.
Basic exotropia is present when the exodeviation is approximately the same at distance and near fixation.
True divergence excess exotropia is the least common form of intermittent exotropia. It is present when the distance deviation is greater than the near deviation, and the deviation does not equalize after monocular occlusion or with +3.00 D lenses at near.
Convergence weakness exotropia is present when the exodeviation is greater at near than at distance. This type is discussed later in the chapter.
Sensory testing usually reveals excellent stereopsis with normal retinal correspondence when the eyes are aligned and suppression when the exodeviation is manifest. Uncommonly, patients may manifest diplopia when the eyes are exotropic.
Treatment
There are no firm guidelines for determining when patients with intermittent exotropia require treatment. Opinions vary widely regarding the timing of surgery and the use of nonsurgical methods to delay or possibly prevent the need for surgical intervention.
Nonsurgical management
Correction of refractive errors Corrective lenses should be prescribed for significant myopic, astigmatic, and hyperopic refractive errors. Correction of even mild myopia may improve control of the exodeviation. Mild to moderate degrees of hyperopia are not routinely corrected in children with intermittent exotropia because of concern about worsening the deviation. However, children with marked hyperopia (>+4.00 D or >+1.50 D hyperopic anisometropia) may be unable to sustain accommodation, and this results in a blurred retinal image and manifest exotropia. Optical correction may improve retinal image clarity and help control the exodeviation in these patients.
Some ophthalmologists use additional minus lenses, usually 2.00–4.00 D beyond the actual refractive error, to stimulate accommodative convergence to help control the exodeviation. This therapy may cause asthenopia in school-aged children, however. It can be effective as a temporizing measure to promote fusion and delay surgery while the visual system is immature. For patients in whom the initial overrcorrection results in control, the prescription can be gradually tapered and surgery may be avoided.
Occlusion therapy Patching of patients with amblyopia may improve control of exotropic deviations. For patients without amblyopia, part-time patching of the dominant (nondeviating) eye or alternating daily patching in the absence of a strong ocular preference can be an effective treatment for small-to moderate-sized deviations, particularly in young children. The improvement is often temporary, however, and many patients eventually require surgery.
Active orthoptic treatments Antisuppression therapy/diplopia awareness and fusional convergence training can be used alone or in combination with patching, minus lenses, and surgery. For deviations of 20 prism diopters (Δ) or less, orthoptic treatment alone has been reported by some authors to have a long-term success rate comparable to that of surgery. Others, finding this treatment to be of no benefit, recommend surgery for any poorly controlled deviation. A potential risk of antisuppression orthoptic therapy is the loss of ability to suppress diplopia, which may be quite bothersome.
Prisms Although they can be used to promote fusion in intermittent exotropia, base-in prisms are seldom chosen for long-term management because they can cause a reduction in fusional vergence amplitudes.
Surgical treatment
Surgery is customarily performed when there is documented progression toward constant exotropia, as evidenced by a manifest deviation occurring more frequently, decreased control, or decreased distance stereoacuity. No consensus exists regarding specific indications; however, the best sensory outcomes are probably achieved with motor alignment before age 7 or with strabismus duration of fewer than 5 years, or while the deviation is still intermittent. Many surgeons use a manifest deviation occurring more than 50% of the time as a criterion for surgery.
For pseudodivergence excess exotropia, symmetric recession of both lateral rectus muscles is the most common surgical procedure. Patients with basic intermittent exotropia may do better with combined lateral rectus muscle recession and medial rectus muscle resection, or larger lateral rectus muscle recessions than those used for patients with pseudodivergence excess. For patients with smaller exodeviations, unilateral lateral rectus muscle recessions may be performed.
Some surgeons believe that the usual quantity of surgery (see Chapter 14) may result in overcorrection if the exotropia in each side gaze is less than that in primary position by at least 10Δ. Thus, if lateral rectus muscle recessions are planned for such patients, these surgeons might reduce the amount of surgery on each side by 1 mm. Other surgeons consider such alignment measurements to be an artifact.
Patients with true divergence excess exotropia have an increased risk of developing near esodeviations after lateral rectus muscle recessions and may require bifocal glasses if the near esodeviation is associated with a high AC/A ratio (see the Evaluation section).
Botulinum toxin may also be used to treat intermittent exotropia, although multiple injections are commonly needed.
Kushner BJ. Selective surgery for intermittent exotropia based on distance/near differences. Arch Ophthalmol. 1998;116(3):324–328.
Postoperative alignment A small-angle esotropia in the immediate postoperative period is desirable, because it is associated with a decreased risk of recurrent exotropia. Patients may experience diplopia during the time they are esotropic, and they should be advised of this possibility. An esotropia that persists beyond 3–4 weeks or that develops after 1–2 months in patients who were initially wellaligned postoperatively usually requires additional treatment such as base-out prisms or patching. Corrective lenses may be considered if significant hyperopia is present. Bifocals can be used for a high AC/A ratio. Unless deficient ductions suggest a slipped or lost muscle, a delay of a few months is recommended before reoperation, because spontaneous improvement is common. If the esotropia persists, surgical options include unilateral or bilateral recession of the medial rectus muscle, advancement of the recessed lateral rectus muscles, or injection of botulinum toxin.
Mild to moderate residual exodeviation is often treated with observation alone if fusional control is