What is Divergent Strabismus (Exotropia)?
Divergent strabismus, also known as exotropia, is an outward misalignment of one or both eyes where the visual axes diverge away from each other, most commonly manifesting when viewing at distance. 1, 2
Core Definition and Mechanism
Exotropia results from either passive mechanisms related to orbital and globe anatomy, or active innervational mechanisms that cause divergence of the visual axes, which is normally compensated by fusional convergence. 3
Intermittent exotropia (IXT) is the most common form of divergent strabismus, characterized by an outward deviation that is typically intermittent rather than constant, with the eye deviating outward particularly during distance viewing. 1, 2
The intermittent nature occurs when fusional convergence mechanisms temporarily fail to compensate for the underlying divergent tendency. 3
Clinical Classification (Burian System)
The classification is based on the difference between distance and near deviation angles: 3
- True divergence excess: Deviation significantly larger at distance than near (typically ≥10 prism diopters difference)
- Pseudo-divergence excess: Initially appears as divergence excess but reveals equal angles after monocular occlusion test
- Basic exotropia: Distance and near angles are approximately equal (within 10 prism diopters)
- Convergence insufficiency: Near angle greater than distance angle
Common Presentations and Symptoms
Patients frequently present with closure of one eye (especially in bright sunlight), asthenopia (eye strain), and diplopia (double vision), though symptoms are often absent despite visible misalignment. 2, 3
Additional symptoms include eyes feeling tired, excessive blinking, and reduced quality of life. 2
The deviation is typically comitant (similar in all gaze directions) and increases with fatigue, illness, or inattention. 1
Epidemiology
The estimated prevalence of intermittent exotropia in children ranges from 0.1% to 3.7%, making it the most common form of childhood divergent strabismus. 2, 4
Female sex and hypoxia at birth are identified as potential risk factors. 2
The condition typically presents in early childhood but may not be diagnosed until later years. 1
Important Clinical Distinction
This condition differs fundamentally from divergence insufficiency (discussed in the provided guidelines), which is an esotropia (inward deviation) that worsens at distance—essentially the opposite problem. 5 Divergence insufficiency represents a failure of the eyes to diverge adequately for distance viewing, while exotropia represents excessive divergence.
Natural History
Using validated control measures, multicenter prospective studies demonstrate that the rate of conversion from intermittent exotropia to constant exotropia is relatively low, though the condition is generally progressive without treatment. 2
The angle of deviation and control of the deviation are considered the main parameters of disease severity and are among the four core outcomes for surgical management studies. 2
Treatment Implications
Treatment is indicated when there is an increasing tropia phase (constant deviation) to preserve or restore binocular function and achieve normal ocular alignment. 1
Surgical therapy (typically bilateral lateral rectus recession) is the preferred definitive treatment by most pediatric ophthalmologists, with success rates of approximately 75-92%. 1, 4
Medical treatment including optical correction and binocular visual training may provide temporary relief but is generally not definitive. 1, 3
Preoperative deviation angle is the strongest predictor of surgical success, with smaller angles (around 31 prism diopters) correlating with better outcomes compared to larger angles (42 prism diopters or greater). 4