What causes unilateral exotropia?

Causes of Unilateral Exotropia

Unilateral exotropia can be caused by sensory vision loss, mechanical/orbital factors, neurological conditions, or as a consequence of previous strabismus surgery. 1

Primary Causes

• Sensory exotropia: Associated with unilateral vision loss on a structural basis, where poor vision in one eye leads to outward deviation of that eye 1
• High unilateral myopia: Particularly in cases of high axial myopia, which can cause a combined exotropia and hypotropia pattern in the affected eye 2
• Mechanical restrictions: Including thickening of extraocular muscles, particularly the inferior rectus muscle, which can cause combined hypotropia and esotropia 3
• Orbital factors: Such as inferior orbital fat hernia or irregular soft tissue masses in the orbit that can affect eye alignment 3

Secondary Causes

• Consecutive exotropia: Occurs after surgical correction of esotropia, either immediately post-surgery or developing over time 1
• Neurological conditions: Including Duane syndrome, congenital fibrosis syndrome, and ocular myasthenia gravis 1
• Craniofacial abnormalities: Can lead to altered orbital anatomy affecting eye alignment 1

Risk Factors

• Female sex: Some studies show intermittent exotropia is twice as frequent in girls than in boys 1
• Prematurity and perinatal morbidity: Associated with higher risk of developing exotropia 1
• Genetic disorders: Can predispose to exotropia development 1
• Refractive errors: Particularly astigmatism, myopia, and anisometropia 1
• Maternal factors: Including substance abuse and smoking during pregnancy 1
• Family history of strabismus: Indicates possible genetic component 1

Clinical Presentation

• Heavy eye syndrome: In cases of unilateral high axial myopia, patients may present with combined exotropia and hypotropia, with limitation of elevation in abduction and a V-pattern deviation 2
• Restrictive pattern: In cases with mechanical restrictions, patients may show limited eye movements in specific directions 3
• Amblyopia: Common in patients with unilateral exotropia, particularly when present from early childhood 3, 2

Diagnostic Considerations

• Pseudoexotropia: Not true strabismus but caused by a disparity between the visual and anatomic axes of the eyes (positive angle kappa) 1
• Dissociated horizontal deviation: A divergent misalignment typically occurring in patients with a history of infantile esotropia 1

Clinical Pitfalls

• Misdiagnosis of pseudoexotropia: Important to distinguish true exotropia from pseudoexotropia by careful examination of corneal light reflexes and cover testing 1
• Overlooking underlying causes: Unilateral exotropia, especially when sudden in onset, may signal underlying neurological or orbital pathology requiring imaging studies 1, 3
• Underestimating amblyopia risk: While severe amblyopia is uncommon in intermittent exotropia, it can occur in constant unilateral exotropia and requires prompt treatment 1
• Inadequate imaging: MRI or CT imaging of orbits and brain may be necessary to identify structural causes of unilateral exotropia 3, 2

Understanding the specific cause of unilateral exotropia is crucial for determining the appropriate treatment approach, which may include surgery (unilateral or bilateral procedures), prism therapy, or management of underlying conditions 4, 5, 6.