What is amblyopia (lazy eye)?

What is Amblyopia (Lazy Eye)?

Amblyopia is a developmental disorder of the central nervous system characterized by reduced best-corrected visual acuity in one or, less commonly, both eyes that occurs in an otherwise structurally normal eye due to abnormal processing of visual images during early childhood. 1

Core Definition and Pathophysiology

Amblyopia represents a reduction in visual acuity that cannot be attributed solely to structural abnormalities of the eye or visual pathway. 1 The condition results from abnormal visual experience during the critical period of visual development, typically before age 7-8 years. 2

Beyond reduced visual acuity, affected eyes demonstrate:

• Deficits in contrast sensitivity 1
• Impaired accommodation 1
• Subtle functional deficits that may also affect the fellow "normal" eye 1

Classification by Underlying Cause

Amblyopia is categorized into distinct types based on etiology: 1

Refractive Amblyopia

• Anisometropic amblyopia: Develops when unequal refractive error between eyes causes one retinal image to be more poorly focused, resulting from both direct image blur effects and interocular competition 1
• Bilateral refractive (isoametropic) amblyopia: Less common form caused by bilateral high refractive errors producing blurred retinal images in both eyes 1
• Greater degrees of anisometropia or astigmatism increase both risk and severity 1

Strabismic Amblyopia

• Occurs with constant, non-alternating tropias (typically esodeviations) 1
• Results from competitive or inhibitory neuronal interactions between nonfusible inputs from the two eyes, leading to cortical dominance of the fixating eye and chronically reduced responsiveness to the nonfixating eye 1

Visual Deprivation Amblyopia

• Caused by complete or partial obstruction of the visual axis degrading the retinal image 1
• Common causes include congenital cataracts, corneal opacities, vitreous hemorrhage, and ptosis 1
• This is the least common but most severe and difficult-to-treat form 1
• Unilateral deprivation produces worse outcomes than bilateral due to added interocular competition, often resulting in visual acuity of 20/200 or worse 1
• For visually threatening unilateral cataracts, removal and optical correction by 2 months of age provides better prognosis 1

Epidemiology and Clinical Significance

Amblyopia affects approximately 2-5% of the population, with prevalence varying by race and ethnicity. 1, 3 In the United States:

• Latinx children show higher prevalence (2.6%) compared to African American children (1.5%) 1
• Amblyopia affects over 6 million people and is the leading cause of monocular vision loss in adults under age 45 1
• The projected lifetime risk of visual loss from amblyopia is at least 1.2% 3

Globally, an estimated 99.2 million people had amblyopia in 2019, with projections reaching 221 million by 2040. 1

Critical Clinical Pitfalls

The presence of anisometropia increases odds of amblyopia by 6.5 to 26 times, while strabismus increases odds by 2.7 to 18 times. 1 However, amblyopia is unusual in children with intermittent exotropia. 1

Children with developmental delay have a sixfold greater prevalence of amblyopia compared to full-term healthy children. 1

Most studies confirm that 53-64% of anisometropic amblyopia cases affect the left eye more than the right, though strabismic amblyopia shows similar distribution between eyes. 1

Treatment Context

Amblyopia is amenable to therapy and cost-effective to treat, with earlier intervention providing better outcomes, though treatment has proven effective even in older children and teenagers. 1, 4 Treatment modalities include optical correction, patching, pharmacological penalization (such as atropine), Bangerter filters, and digital therapeutics, in addition to managing the underlying amblyogenic cause. 1, 5