• Strabismus

    Introduction

    Intermittent exotropia is the most common form of strabismus, characterized by an intermittent outward deviation of the eyes, affecting as much as 1% of the population.1,2 This condition most often presents in childhood and affects females more than males. Control of the intermittent deviation can vary throughout the day.3,4 

    Etiology

    The etiology of intermittent exotropia is largely unknown,5 and the deviating movement shares features with normal divergent eye movements, but divergence tone may exceed convergence tone in these patients, correlating with hypometric adduction saccades.6,7

    Patient History

    Patients with intermittent exotropia are often asymptomatic, although children with the condition will occasionally close either eye, a symptom that may be exacerbated by bright light, giving the condition its alternate moniker, “squint.” Patients can also rarely report intermittent binocular horizontal diplopia or a sense of discomfort associated with eye deviation. The angle or control of the deviation may be exacerbated by systemic states including fatigue, systemic illness, or in adults, ingestion of alcohol or other sedatives.2 Additionally, symptoms can be elicited regarding quality of life and difficulty with eye contact and social interactions, as intermittent exotropia has been associated with a decrease in health-related quality of life in children with intermittent exotropia, as well as in their parents.8 Both adults and children with this condition can be perceived negatively by their peers, and it can even affect employability.9,10

    Diagnostic Evaluation and Classification

    Patients with intermittent exotropia typically have equal visual acuity in both eyes, although a fixation preference can be associated with strabismic amblyopia. Ocular motility is typically full and the deviation is comitant. A full sensorimotor evaluation should be completed with strabismus measurements taken at distance and near and in all directions of gaze, which can provide insight into the subtype of intermittent exotropia, first described by Burian in 1966.11,12

    Basic Exotropia

    The angle of the exodeviation measures the same at distance and at near.

    Convergence Insufficiency

    The angle of the exodeviation measures greater at near than distance by ≥ 10 prism diopters.

    True divergence Excess

    The angle of the exodeviation measures greater at distance than near by ≥ 10 prism diopters.

    Pseudo-divergence Excess

    The angle of the exodeviation measures greater at distance than near by ≥ 10 prism diopters but this difference becomes minimal after 60 minutes of monocular occlusion or measurement of near angle with +3.00 diopter lenses.

    Although this traditional Burian classification has been widely cited, Kushner has proposed a reclassification or expansion to include categories such as tenacious proximal fusion, high AC/A ratio, low AC/A ratio, proximal convergence, and pseudo-convergence insufficiency.13

    There have been multiple methods described to quantify control of patients with intermittent exotropia. The clinic control score is described below, scored 0 to 5 with 5 being the worst control. Scores of 5 to 3 are based on a 30-second observation period. If no exotropia is observed during this period, scores of 2 and below are based on the worst of 3 10-second observation periods with occlusion.4,14

    Exotropia Control Score

    Control Score

    Description

    5

    Constant exotropia during 30-second observation before dissociation

    4

    Exotropia >50% of the time during 30-second observation before dissociation

    3

    Exotropia <50% of the time during 30-second observation before dissociation

    2

    No exotropia noted unless dissociated, recovers in > 5 seconds

    1

    No exotropia noted unless dissociated, recovers in 1-5 seconds

    0

    No exotropia noted unless dissociated, recovers in < 1 seconds (exophoria)

     

    The Newcastle score and the updated Newcastle score incorporate data from both the clinic visit and from symptoms experienced at home and are scored from 0 to 9, with 9 being the worst control. Scores from home, distance, and near measurements are added to calculate the final score.15,16

    The Newcastle Score

    Home Control

     

    Exotropia seen:

     

        Never

    0

        <50% of time fixating at distance

    1

        >50% of time fixating at distance

    2

        <50% of time fixating at distance, 

         also seen at near

    3

    Clinic Control

     

    Exotropia seen at near:

     

        Immediate realignment after dissociation

    0

        Realignment with aid of blink or re-fixation

    1

        Remains manifest after dissociation/prolonged fixation

    2

        Manifests spontaneously

    3

    Exotropia seen at distance:

     

        Immediate realignment after dissociation

    0

        Realignment with aid of blink or re-fixation

    1

        Remains manifest after dissociation/prolonged fixation

    2

        Manifests spontaneously

    3

    Total score (home + near + distance) = 

     

     

    Generally, near stereoacuity is normal in patients with intermittent exotropia, although distance stereopsis can often be worse than normal controls.17 Decreased distance stereoacuity likely represents poor control of the deviation, and is often considered in determining who should undergo surgical correction (see below).

    Additional workup

    Intermittent exotropia is not typically associated with neurologic disease. It can be seen more commonly in children with hypotonia or developmental delay, but no specific workup is recommended when the sensorimotor examination is consistent with intermittent exotropia.

    Treatment

    While generally intermittent exotropia is thought to be a straightforward form of strabismus with predictable surgical outcomes, many enigmas remain with regard to the treatment of this condition.18

    In considering treatment, it is helpful first to consider the natural history of intermittent exotropia. Though the natural history remains to be fully elucidated, a few observational studies have addressed this. In a prospective observational study of 183 children between 3 and 10 years old who had intermittent exotropia, but for whom surgery was not considered necessary, the probability of deterioration at 3 years (constant exotropia or decline in stereopsis) was 15%.19 Similarly, a retrospective study of patients aged 5 to 25 years found that without surgery, the angle remained stable in 58%, improved in 19%, and worsened in 23%;20 this was similar to previous reported values.21

    Apart from observation, many treatments have been described to address intermittent exotropia, although there is a lack of clarity regarding criteria for intervention and optimal intervention.22,23

    Surgery

    Before considering which type of surgery to perform for intermittent exotropia, it is useful to consider the surgical goals for the surgeon and the patient. A framework for this has been described, breaking goals of surgical treatment into the following categories: binocular potential, restoration of eye contact, diplopia control, and torticollis.24

    The 2 classic surgical procedures for intermittent exotropia are bilateral lateral rectus muscle recessions, or a unilateral medial rectus muscle resection combined with a lateral rectus muscle recession. In a multi-center, randomized clinical trial comparing these 2 surgical approaches in children aged 3 to 10 years with intermittent exotropia, no difference in surgical success was identified, 63% versus 54% in the recess/resect group.25 Similar results have been reported in younger children.26 A retrospective study found that recess/resect had slightly better outcomes, although it did concede that the surgeon’s preference likely influenced the results.27 The argument to support bilateral symmetric surgery is to avoid inducing horizontal incomitance and prevent the palpebral fissure narrowing sometimes associated with horizontal rectus muscle resections. In situations where patients perceive the deviation to only involve 1 eye, or there is unilateral ocular pathology, a recess/resect may be preferred.

    Another surgical technique that has been described is the “augmented bilateral lateral rectus recession.”28 This is an updated surgical table that proposes recessing an additional 1.5 – 2.5 mm per eye to Park’s formula based on the angle of the deviation. A comparison of this technique to standard lateral rectus recession showed lower rates of undercorrection in the augmented group.28

    Two common concerns that arise regarding surgical planning regardless of the approach are the appropriate preoperative deviation to target as well as the predictors of success. Regarding preoperative target, some have advocated for targeting the deviation after 1 hour of monocular occlusion or for the largest angle of exotropia ever measured.29 Kushner has recommended having the patient fixate in the far distance (eg, ¼ mile) to potentially uncover a larger angle of deviation and target that deviation.30

    Predictors of success following surgery for intermittent exotropia are less clear, although a small retrospective study suggested that performing surgery before a patient is 7 years old, or before the duration of the patient's exotropia has reached 5 years, may be associated with better long-term sensory outcome.31 Another retrospective study with long-term outcomes (10 years) found that patients with anisometropia, lateral incomitance, and immediate postoperative undercorrection were at highest risk for poor outcomes following surgery for intermittent exotropia.32 Approximately 1 in 5 children with intermittent exotropia have a second surgery, although as many as 50% had suboptimal sensorimotor outcomes.33

    Botulinum Toxin (Botox)

    Botulinum toxin has been used for strabismus since the 1990s. A nonrandomized, case-controlled study of children aged 3 to 144 months with intermittent exotropia showed similar results to those in prior surgical reports when they received injections of 2.5 units of Botox into each of the lateral rectus muscles.34 In this study, 69% of patients injected with Botox were orthophoric 12 to 44 months following the intervention. A more recent study corroborates these results.35

    Patching

    Patching has been described to treat intermittent exotropia with the idea of preserving binocularity and reducing the exodeviation. A multi-center, randomized, controlled trial assessed the role of patching in children aged 3 to 10 years with intermittent exotropia.36 Children were randomized to observation versus 6 months of 3 hours of daily patching (1 eye or alternating eyes, at the prescriber’s discretion). At 6 months, rate of deterioration in both groups was low, suggesting that both observation and patching are reasonable treatment options in this patient population. In a similar study addressing this same question in younger patients aged 12 to 35 months, the findings were similar, with insufficient evidence to recommend patching in this patient population.37

    Prisms

    Both ground-in prisms and temporary stick-on prisms have been described in the treatment of intermittent exotropia. Base-in prisms can be used to neutralize the deviation to stimulate fusion, or to overcorrect the deviation in the hope of reducing the deviation,38 but this treatment modality is not commonly used today.

    Over-Minus Glasses

    Prescribing 2.0 to 4.0 diopters of myopic correction to the refraction of children with intermittent exotropia is thought to induce convergence, promote fusion, and decrease the angle of the eye misalignment. This can be a useful tool in young children, with hopes of promoting bifoveal fixation, if deferring surgery. Success rates have been described as 46% experiencing improved quality of fusion and 26% demonstrating improved quality of fusion and quantitative decrease in the angle of deviation.39 There are concerns that over-minus therapy could lead to increasing myopia, and while a retrospective study found that this therapy is not associated with increasing myopia in intermittent exotropia,40 results of a prospective Pediatric Eye Disease Investigator Group (PEDIG) study evaluating the efficacy of this treatment is forthcoming.

    Conclusion

    Intermittent exotropia is a very common form of strabismus typically not associated with neurologic or systemic disease. The natural history of the disease is not fully understood. Multiple interventions, including strabismus surgery, patching, over-minus glasses, and prisms have been described; they have had varying success.41 Further advances in the etiology of this condition could lead to improved treatment options and outcomes.

    References

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