Exotropia is a condition in which, when one eye is fixated on a target, the other eye deviates outward (toward the ear). Most cases of exotropia without vertical or torsional deviation are comitant exotropia.
In Western countries, the ratio of esotropia to exotropia is 1.8:1, with esotropia being more common, whereas in Japan the ratio is 1:1.2–2.8, with exotropia being more frequent. A similar trend is observed in Southeast Asia. Intermittent exotropia is the most common type of strabismus in Japanese children, found in about 0.14% of elementary school students. The frequency in the United States is about 0.06%, lower than in Japan. Onset occurs from early childhood to around 8 years of age, most commonly at 3–4 years. Exotropia is estimated to be present in about 1% of the total population. The estimated prevalence of adult strabismus is about 4%, and the IRIS Registry reports 2.7% 5).
Intermittent exotropia (IXT)
Basic type: The difference in deviation angle between distance and near is within 10Δ. This is the most common type.
Divergence excess type: Distance > near by 10Δ or more. Differentiation between true and pseudo divergence excess is important.
Convergence insufficiency type: Near > distance by 10Δ or more.
Initially, fusional convergence can maintain orthophoria, but exotropia appears during fatigue, poor physical condition, or immediately after waking.
Constant, Sensory, and Special Types of Exotropia
Constant exotropia: Constant outward deviation. Often transitions from intermittent exotropia.
Infantile exotropia: Ocular alignment does not normalize by age 1. Binocular vision development is severely impaired. Since 60% of infantile-onset cases are associated with systemic diseases or central nervous system abnormalities, CT/MRI is essential.
Sensory exotropia: Caused by long-term blurred retinal image in one eye. If the primary disease occurs before age 4, esotropia tends to develop; after age 4, exotropia is more common.
Consecutive exotropia: Exotropia occurring after esotropia surgery. Frequency 2–29%. Incidence increases with longer postoperative follow-up.
Long-term observation shows spontaneous resolution in about 10%, no change in about 40%, and progression to constant exotropia in about 50%. A North American report following 138 cases for 20 years found that 74.0% eventually required surgery. Von Noorden followed 51 untreated patients for an average of 3.5 years and reported progression in 75%, no change in 9%, and improvement in 16%.
Long-term observation indicates that about 50% progress to constant exotropia, while spontaneous resolution occurs in only about 10%. In a 20-year follow-up of 138 cases, 74% eventually required surgery. However, the rate of progression varies greatly among individuals, and some cases remain unchanged for several years. Surgery is considered when control scores worsen or stereopsis declines.
In intermittent exotropia, binocular vision develops well even during exodeviation, and most cases show normal retinal correspondence (NRC) and normal stereoacuity (<60 arcseconds). The binocular visual field (BVF) during exodeviation is 20–30 degrees, narrower than the normal 40 degrees. Infantile-onset cases may be complicated by monofixation syndrome and mild amblyopia (approximately 5%).
Assesses severity of control of eye alignment (maximum 9 points; higher scores indicate poorer control). Calculated by summing home assessment (0–3 points) and clinic assessment (0–3 points):
| Score | Home Assessment | Clinic Assessment |
|---|---|---|
| 3 | Strabismus or monocular eye closure ≥50% of time at both distance and near | Exotropia without occlusion |
| 2 | Strabismus or eye closure at distance ≥50% of the time | Exotropia after cover test, does not return to phoria after cover removal |
| 1 | Strabismus or eye closure at distance <50% of the time | Exotropia after cover test, returns to phoria with blink after cover removal |
| 0 | No strabismus or eye closure at all | Exotropia after cover test, immediately returns to phoria after cover removal |
In bright outdoor environments, glare makes fusion difficult, and the outward deviation becomes apparent. When exotropia occurs, double vision results, and it is thought that one eye is closed to avoid this, but the exact mechanism is unknown. This “eye closure” is an important sign suggestive of intermittent exotropia.
The etiology of exotropia is multifactorial, and impairment of fusion and ocular alignment maintenance is considered the underlying mechanism.
In recent years, there have been reports of an association between prolonged near work such as smartphone use and convergence insufficiency-type exotropia.
Exclusion of organic disease and visual impairment is the first priority. In infantile exotropia, neurological examination (CT/MRI) is essential.
| Type | Features | Differential points |
|---|---|---|
| Basic type | Distance-near difference ≤10Δ | Normal AC/A ratio. Most common |
| Divergence excess type | Distance > near by 10Δ or more | Must differentiate from pseudo-divergence excess type |
| Convergence insufficiency type | Near > distance by 10Δ or more | Differentiate from convergence insufficiency. Symptoms tend to appear during near work. |
| Pseudo divergence excess type | Initially appears as divergence excess type | Changes in near deviation after monocular occlusion/prism adaptation/+3D lens wear |
Methods to differentiate pseudo divergence excess type (3 methods):
If any of these methods brings the near deviation angle close to the distance deviation (difference within 10Δ), it is diagnosed as basic type (pseudo divergence excess type).
Based on distance-near disparity, it is classified into three types: basic type (difference ≤10Δ), divergence excess type (distance > near by 10Δ or more), and convergence insufficiency type (near > distance by 10Δ or more). It is also important to differentiate from the “pseudo-divergence excess type (latent basic type)” where the near deviation angle increases with occlusion or +3D lens loading even if it appears as divergence excess type.
Conservative Management
Observation: If control is good and asymptomatic, observation is the first step.
Part-time occlusion: Occlude the healthy eye for 3 hours per day. Effective for IXT with distance control score ≥2 in children aged 3–10 years 1). At 3 months, distance control score improves by 0.4 points (95% CI 0.1–0.7) and distance deviation improves by 2.1 PD 1).
Optical treatment: Fresnel membrane prisms, or overcorrecting glasses with +2 to +3 D minus lenses added to the cycloplegic refraction. Indicated when the deviation angle is small and the patient complains of asthenopia or diplopia.
Refractive correction: Myopia may improve control with corrective lenses.
Vision Therapy / Convergence Training
Indications for vision therapy: Corrected visual acuity equal in both eyes, deviation angle <25Δ, intermittent, near stereopsis present, and patient motivation essential. Optimal age is 8–12 years.
Sequence of training: Suppression elimination training → fusion training → convergence training. Combined therapy (surgery + vision therapy/occlusion) is more effective than either alone.
Convergence training: Particularly useful for convergence insufficiency type. Office-based training is more effective than home-based in children 5). Results are inconsistent in adults 5).
Base-in prisms: Promote fusion but reduce fusional convergence reserve, so they are rarely used for long-term management.
Main surgical procedures
Bilateral lateral rectus recession (BLR): Basic procedure for basic type and divergence excess type.
Unilateral recession and resection (R&R): Selected when amblyopia is present. Success rate comparable to BLR for divergence excess type IXT (83.3%)2).
Bilateral medial rectus resection: Useful for convergence insufficiency type. When near deviation is large.
Large-angle cases >50Δ: Combined bilateral lateral rectus recession and medial rectus resection.
Surgery for A-V pattern exotropia
With oblique muscle overaction (V pattern): Horizontal rectus surgery combined with inferior oblique weakening.
With oblique muscle overaction (A pattern): Horizontal rectus surgery combined with superior oblique weakening.
Without oblique muscle overaction: Perform Trick method (vertical transposition of horizontal rectus insertion).
| Deviation angle (Δ) | Bilateral lateral rectus recession (mm) | Lateral rectus recession + medial rectus resection (mm) |
|---|---|---|
| 15 | 4.0 | 4.0 / 3.0 |
| 20 | 5.0 | 5.0 / 4.0 |
| 25 | 6.0 | 6.0 / 4.5 |
| 30 | 7.0 | 6.5 / 5.0 |
| 35 | 7.5 | 7.0 / 5.5 |
| 40 | 8.0 | 7.5 / 6.0 |
| 50 | 9.0 | 8.5 / 6.5 |
Surgery is considered when any of the following apply:
Surgery is generally considered after age 4. To achieve normal stereoacuity (60” or better), surgery should be performed by age 7 and within 5 years of onset. Surgery during the intermittent exotropia stage results in 93% achieving normal stereoacuity, compared to only 39% after progression to constant exotropia.
In pediatric surgery, a drift of 10–25Δ is observed compared to the immediate postoperative alignment. It is ideal to achieve an immediate postoperative esotropia within 10 PD (intentional overcorrection). Drift in adults is less than in children.
Postoperative outcomes of BLR for divergence excess type IXT: distance 38.1±8.0PD → −1.5±7.6PD, near 26.3±9.1PD → −0.9±6.2PD, distance-near disparity (NDD) improved from 15.4 to 0.62).
In principle, after age 4. To achieve normal stereopsis (60” or better), surgery is recommended by age 7 and within 5 years of onset. Surgery during the intermittent stage allows 93% to achieve normal stereopsis, but only 39% after it becomes constant. Deterioration of control score, decreased stereopsis, asthenopia, and cosmetic concerns are criteria for surgical indication.
For divergence excess type intermittent exotropia, bilateral lateral rectus recession (BLR) and unilateral recession-resection (R&R) have been reported to have equal success rates of 83.3%2). The choice of procedure is determined by considering the amount of deviation, ocular alignment characteristics, and surgeon’s experience. For convergence insufficiency type, bilateral medial rectus resection may be effective.
The fusion maintenance mechanism in intermittent exotropia is primarily mediated by fusional convergence. Near fixation can relatively easily maintain orthophoria, but control breaks down at distance, leading to manifest exotropia.
Long-term effects of part-time occlusion
In the PEDIG randomized controlled trial by Hatt et al. (2023), the part-time occlusion group (3 hours/day) showed a significant improvement of 0.4 points in distance control score at 3 months compared to the observation group (95% CI 0.1 to 0.7) 1). At 6 months, a significant difference of 0.3 points persisted (95% CI 0.02 to 0.6), but verification of long-term effects remains a challenge.
Expanded indications for R&R procedure in divergence excess type IXT
Han et al. (2023) demonstrated the potential for expanded indications of the R&R procedure in divergence excess type IXT 2). It was confirmed that R&R has a comparable success rate (83.3%) even in divergence excess type, for which BLR was standard.
Surgery and neuroplasticity in adult infantile-onset exotropia
Littlewood et al. (2021) reported that an adult patient who developed exotropia in infancy and underwent four surgeries over 15 years achieved recovery of binocular single vision (BSV) and stereopsis (55 arc seconds) postoperatively 3). This suggests that surgery for adult infantile-onset exotropia may utilize residual neuroplasticity.
Case report of transient high myopia after surgery
Yoshimura et al. (2022) reported a case of transient high myopia in a 6-year-old girl after exotropia surgery (lateral rectus recession 6.0mm + medial rectus resection 6.5mm), where the operated eye changed from +0.25D to −9.00D 4). AS-OCT showed abnormal anterior segment findings (anterior chamber depth 1.955mm vs. contralateral 3.007mm, lens thickness 4.216mm vs. contralateral 3.528mm). Spontaneous recovery occurred within 8 weeks. The mechanism was speculated to be anterior segment ischemia → ciliary body detachment → zonular relaxation → lens deformation.
| Parameter | Operated eye | Contralateral eye |
|---|---|---|
| Refractive value (postop) | −9.00D | +0.25D |
| Anterior chamber depth | 1.955mm | 3.007mm |
| Lens thickness | 4.216mm | 3.528mm |
Use of adjustable sutures and botulinum toxin
Adjustable sutures are considered useful for reoperations and unpredictable cases in adults 5). Botulinum toxin injection can be used for new-onset cases, reinforcement of large-angle deviations, postoperative residuals, and small-angle cases 5), and is attracting attention as one of the options.
