Intermittent Exotropia

Key Points at a Glance

1. What Is Intermittent Exotropia?

Intermittent exotropia is a type of strabismus in which one eye deviates outward while the other eye is fixating on a target, combined with periods when both eyes are aligned and no outward deviation occurs. The eyes are usually straight, but the exotropia becomes apparent when tired, distracted, upon waking, when looking at distant objects, or in bright sunlight outdoors.

It accounts for about 75–90% of exotropia cases and is found in about 1% of the general population. It is the most common type of strabismus, occurring in about 0.14% of elementary school children. Onset is typically between infancy and around 8 years of age, most commonly around 3–4 years. It usually begins before age 5.

Long-Term Course

The outcomes of untreated cases are reported as follows:

Spontaneous resolution: about 10%
No change: about 40%
Progression to constant exotropia: about 50% (becomes manifest even at near)

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%. It may also transition to constant exotropia due to aging and other factors.

Q Does intermittent exotropia resolve on its own without treatment?

About 10% resolve spontaneously, but about 50% progress to constant exotropia. The remaining 40% show no change. Although spontaneous resolution is possible, the risk of progression is high, so regular follow-up is necessary.

2. Main symptoms and clinical findings

Subjective symptoms

In the early stage, the eyes can maintain alignment with fusional convergence, but exotropia tends to occur during fatigue, poor physical condition, or immediately after waking. Children rarely complain of subjective symptoms, so caution is needed.

Diplopia: Noticed when one eye deviates outward. If the deviation is small, it may be perceived as blurred vision. If the deviating eye is suppressed, diplopia is not noticed.
Asthenopia: Convergence effort to maintain alignment induces excessive accommodation. It may also cause headache, vomiting, or nausea.
Positional myopia: Accommodation is stimulated by convergence effort, causing myopia. This occurs during binocular vision, so unaided visual acuity decreases when both eyes are open. During monocular vision, the refractive state remains unchanged.
Closing one eye: In bright outdoor settings, exotropia occurs and the patient closes one eye. The mechanism is unknown, but strong light outdoors makes fusion difficult and triggers it. Children may complain of glare.

Clinical findings (findings confirmed by the doctor during examination)

Eye alignment during orthophoria is good, and binocular vision develops almost normally. The following findings are observed.

Extended near point of convergence: Accompanied by reduced fusional amplitude.
Characteristics of infantile onset: Eccentric fixation may lead to anomalous retinal correspondence and monofixation syndrome. Mild amblyopia is also observed in about 5% of cases.
Narrowing of binocular visual field: Even in the phoric state, it is 20–30 degrees (narrower than the normal 40 degrees).

Control Evaluation Scale

The control status of exotropia is evaluated using the following scale.

Score	Status
5	Constant exotropia
4	Exotropia >50% of examination time
3	Exotropia <50% of examination time
2	No exotropia unless occluded, recovery >5 seconds
1	Recovery 1–5 seconds
0	Recovery <1 second (exophoria)

Distance stereoacuity is an objective measure of deviation control and deterioration of fusion. Near stereoacuity can be used as an indicator of disease progression.

Q Is squinting one eye in bright outdoor light a symptom of intermittent exotropia?

Squinting one eye in bright outdoor light is a characteristic symptom of intermittent exotropia. Strong light makes it difficult to maintain fusion, triggering the manifestation of exotropia. If this occurs repeatedly, an eye examination is recommended.

3. Causes and Risk Factors

The cause of intermittent exotropia is multifactorial and cannot be explained by a single factor. The following factors are thought to be involved.

Innervational factors: Imbalance in the innervation of the divergence center in the brainstem tegmentum or divergence burst cells in the midbrain reticular formation leads to outward deviation of the eyes.
Abnormal position of extraocular muscle pulleys: Causes mechanical deviation of eye movements.
Fusion insufficiency: There is a classical theory that it is a congenital defect of fusion function.
Abnormal AC/A ratio: A high AC/A ratio (accommodative convergence to accommodation ratio) has been reported.
Refractive error: Uncorrected myopia reduces accommodative convergence, leading to exophoria (Donders’ theory).
Genetics: Frequent family history.
Craniofacial abnormalities and neurological defects: Predispose to exotropia.
Maternal smoking during pregnancy and low birth weight: Significant and independent risk factors for the development of horizontal strabismus.

4. Diagnosis and Examination Methods

Diagnosis of intermittent exotropia involves a combination of multiple ocular alignment tests and refraction tests. Because the fusion range is wide, the strabismic angle may vary with each measurement; the maximum angle is detected to determine surgical indication.

Ocular Alignment Tests

Hirschberg test: A simple test that estimates ocular deviation from the position of the corneal light reflex.
Alternate cover test: Each eye is covered for 2 seconds alternately. This test is particularly important for intermittent exotropia.
Alternate prism cover test (APCT): Quantitatively measures the deviation using prisms.
Cover-uncover test: Cover one eye and observe the position of the covered eye from the side.

Eye Movement and Refraction Tests

Eye movement test: Important for differentiating A-V pattern strabismus and convergence insufficiency. Nine-direction ocular alignment is measured with a large synoptophore.
+3.0D near addition test: Add +3.00D lenses to distinguish pseudo-divergence excess due to high AC/A ratio.
Patch test: Cover one eye for 30 minutes to control tonic fusional convergence, thereby differentiating pseudo-divergence excess from true divergence excess.

Classification

Basic type

Difference between distance and near: within 10 PD

AC/A ratio: normal

Fusional vergence amplitude: normal

Features: most common type

Divergence excess type

Distance deviation: 10 PD or more greater than near

Subclassification: apparent divergence excess and true divergence excess

Differentiation: determined by patch test and +3.0D addition

Convergence insufficiency type

Near deviation: 10 PD or more greater than distance

AC/A ratio: normal or low

Features: symptoms tend to appear during near work

Pseudo-divergence excess type

Characteristic: Near deviation increases after 30–60 minutes of monocular occlusion.

Differential: The difference from distance deviation is within 10 PD.

Essence: A condition similar to basic type.

Q What types of intermittent exotropia are there?

It is classified into four types: basic, divergence excess, convergence insufficiency, and pseudo-divergence excess. The type is determined based on the difference between distance and near deviation and the AC/A ratio, and the treatment plan is decided. For details, see the “Diagnosis and Examination Methods” section.

5. Standard Treatment

Treatment for intermittent exotropia involves non-surgical and surgical approaches depending on the severity and control of the condition. The most effective treatment is surgery.

Non-surgical Treatment

Observation: Indicated for patients with good control and no symptoms.
Correction of refractive error: In myopic patients, corrective lenses may improve control.
Overcorrecting minus lenses: Stimulate accommodative convergence to reduce exodeviation. Useful in cases with high AC/A ratio.
Optical treatment: Only indicated when the deviation angle is small.
Vision therapy: Indicated for deviation <25 PD, no microtropia, and presence of near stereopsis. Patient motivation is essential, and the best age is 8–12 years. If suppression is present, perform anti-suppression training, then fusion training, and concurrently convergence training for convergence insufficiency.
Convergence training: Useful for convergence insufficiency type. Office-based training is more effective than home-based in children ²⁾. Results are inconsistent in adults ²⁾.
Part-time occlusion: Used in very young children.
Base-in prism: Promotes fusion but reduces fusional convergence reserve, so it is rarely used for long-term management.

Surgical Treatment

Surgical Indications

Surgery is considered when any of the following criteria are met:

Increase in the exotropic phase or frequency of the manifest phase
Increase in the angle of deviation
Decrease in stereoacuity
Progression from intermittent to constant exotropia
Persistent asthenopia or diplopia
Cosmetic or psychological concerns

Timing of Surgery

In principle, surgical candidacy is determined after age 4. To preserve binocular vision, surgery is ideally performed between ages 5 and 10. A study comparing 45 cases of intermittent exotropia and 31 cases of constant exotropia reported that achieving normal stereoacuity (≤60 arcseconds) required surgery before age 7 and within 5 years of onset; after progression to constant exotropia, only 39% achieved normal stereoacuity.

Surgical Techniques

The surgical procedure is selected based on the distance angle of deviation.

Bilateral Lateral Rectus Recession

Indications: Basic type, divergence excess type

Features: Most common surgical procedure

Recession-Resection Procedure

Procedure: Lateral rectus recession + medial rectus resection in one eye

Indications: Chosen when amblyopia is present

Bilateral Medial Rectus Resection

Indications: Useful for convergence insufficiency type

Features: When near deviation is large

For cases with large-angle deviation exceeding 50 PD, bilateral lateral rectus recession combined with resection of one or more medial rectus muscles is performed.

When A or V pattern strabismus coexists, the following measures are taken:

V pattern + inferior oblique overaction: Combined with inferior oblique weakening procedure
A pattern + superior oblique overaction: Combined with superior oblique weakening procedure
No oblique overaction: Trick method

The reported success rate after surgery is approximately 60–70%. However, in childhood surgery, a certain amount of “drift” of 10–25 PD compared to the immediate postoperative period is not uncommon, so intentional overcorrection to achieve an esotropia within 10 PD immediately after surgery is considered ideal. Drift in adults is less than in children.

Q When is the appropriate time to perform surgery?

In principle, the indication is determined after 4 years of age. Surgery is considered when there is an increase in the frequency of the exotropic phase, an increase in the angle of deviation, or a progression to constant deviation. For details, see the “Standard Treatment” section.

6. Pathophysiology and Detailed Mechanism of Onset

Both mechanical and innervational factors are involved in the onset of intermittent exotropia.

Innervational Factors

The divergence center in the brainstem tegmentum and divergence burst cells in the midbrain reticular formation control divergence movements. An imbalance in these innervations leads to outward deviation. Some theories suggest a congenital defect in fusion function, but since many patients maintain normal binocular vision, this is unlikely to be the primary cause.

Abnormal AC/A Ratio

The AC/A ratio (accommodative convergence to accommodation ratio) is measured using the heterophoria method and the gradient method. A high AC/A ratio is diagnosed when the difference in strabismus angle between distance and near is 10 PD or more. A high AC/A ratio is characteristic of the divergence excess type, and its involvement is assessed with a +3.0 D lens addition test.

Refractive Error and Accommodation

According to Donders’ theory, uncorrected myopia or hyperopia leads to insufficient accommodative convergence, resulting in exophoria.

Mechanism of Progression

Intermittent exotropia progresses through the following stages.

Exophoria → Intermittent exotropia → Constant exotropia

In early-onset cases, sensory adaptation leads to suppression, and patients often do not notice double vision. Therefore, the discovery is often triggered by the caregiver noticing an abnormal eye position.

Genetic predisposition is also involved, and the etiology is multifactorial. Abnormal positioning of the extraocular muscle pulleys causes deviation in eye movements as a mechanical factor.

7. Latest Research and Future Prospects (Research Stage Reports)

Transient Refractive Changes After Strabismus Surgery

Yoshimura et al. (2022) reported a case of a 6-year-old girl with intermittent exotropia who underwent lateral rectus recession of 6.0 mm and medial rectus resection of 6.5 mm, resulting in transient high myopia from +0.25 D to -9.00 D in the operated eye ¹⁾. AS-OCT confirmed ciliary body detachment, anterior chamber shallowing (1.955 mm vs. 3.007 mm in the fellow eye), and lens thickening (4.216 mm vs. 3.528 mm in the fellow eye). Spontaneous recovery occurred within 8 weeks.

The cause is presumed to be anterior segment ischemia induced by strabismus surgery, leading to uveitis, ciliary body detachment, zonular relaxation, and lens deformation ¹⁾.

Anterior segment ischemia is a complication that can occur when the anterior ciliary arteries are severed during rectus muscle surgery, and recovery takes 2 to 12 weeks in adults ¹⁾. Children have higher lens flexibility and may develop more severe myopia than adults ¹⁾.

Parameter	Operated Eye	Fellow Eye
Refractive value (postoperative)	-9.00 D	+0.25 D
Anterior chamber depth	1.955mm	3.007mm
Lens thickness	4.216mm	3.528mm

8. References

Yoshimura A, Miyata M, Muraoka Y, et al. Unilateral transient high myopization after pediatric strabismus surgery. Am J Ophthalmol Case Rep. 2022;25:101421.
American Academy of Ophthalmology. Adult Strabismus Preferred Practice Pattern. AAO. 2019.
Pang Y, Gnanaraj L, Gayleard J, Han G, Hatt SR. Interventions for intermittent exotropia. Cochrane Database Syst Rev. 2021;9(9):CD003737. PMID: 34516656.

Related keywords