What is amblyopia?

What is Amblyopia?

Amblyopia is a developmental disorder of the central nervous system that results in reduced visual acuity, typically in one eye, that cannot be attributed solely to structural abnormalities of the eye and occurs due to abnormal processing of visual images during early development. 1

Definition and Characteristics

Amblyopia is characterized by:

• Unilateral (most common) or bilateral reduction in best-corrected visual acuity
• Occurs in an otherwise structurally normal eye
• May include deficits in contrast sensitivity and accommodation
• Often the fellow "good" eye also has subtle functional deficits 1

Classification by Cause

Amblyopia is classified into several types based on underlying cause:

1. Refractive Amblyopia

• Anisometropic amblyopia: Develops when unequal refractive error causes the image on one retina to be more poorly focused than in the fellow eye

  • Risk increases with greater degrees of anisometropia or astigmatism
  • May occur with or without strabismus 1
  • Significant anisometropia is considered >1.50D in hypermetropia or >2.50D in myopia 2

• Bilateral refractive (isoametropic) amblyopia: Less common form with bilateral reduction in visual acuity due to bilateral blurred retinal images 1

2. Strabismic Amblyopia

• Results from constant tropias that are not alternating or unequally alternating (typically esodeviations)
• Caused by competitive inhibitory interaction between neurons processing nonfusible inputs from the two eyes
• Leads to dominance of cortical vision centers of the fixating eye and reduced responsiveness to input from the nonfixating eye 1

3. Visual Deprivation Amblyopia

• Caused by complete or partial obstruction of the visual axis
• Common causes include:
  • Congenital or early-onset cataracts
  • Corneal opacities
  • Intraocular inflammation
  • Vitreous hemorrhage
  • Ptosis 1
• Often the most severe and difficult to treat form of amblyopia
• Visual acuity is frequently 20/200 or worse
• Unilateral obstruction typically causes more severe amblyopia than bilateral obstruction of similar degree 1

4. Occlusion (Reverse) Amblyopia

• Can occur as a complication of patching therapy for amblyopia treatment
• Develops in the previously normal eye due to prolonged occlusion 1

Clinical Significance and Impact

Amblyopia is:

• The leading cause of monocular vision loss in children 3
• Associated with approximately 5% prevalence in Germany (though data varies between countries) 4
• A preventable cause of visual loss if detected and treated early 5
• Associated with symptoms including blurry vision, visual fatigue, headaches, diplopia in vertical gaze, and difficulties with depth perception if left untreated 2

Diagnosis and Screening

Early detection is crucial:

• The U.S. Preventive Services Task Force recommends vision screening for all children at least once between three and five years of age 3
• The American Association for Pediatric Ophthalmology and Strabismus and the American Academy of Pediatrics recommend:
  • Age-appropriate red reflex testing
  • Examination for signs of strabismus
  • Vision chart testing 3
• Complete diagnosis includes:
  • Cycloplegic refraction (using cyclopentolate 0.2% and phenylephrine 1%) 2
  • Visual acuity measurement
  • Orthoptic assessment 6

Treatment Approaches

Treatment is most effective when initiated early in life:

1. Correction of Refractive Error:

   • Complete correction of refractive error using glasses or contact lenses as initial treatment 2
   • Contact lenses may be preferred for refractive origin anisometropia 2

2. Occlusion Therapy:

   • Patching of the non-amblyopic eye for 2-6 hours daily 2, 3
   • For moderate amblyopia, 2 hours of patching daily is as effective as 6 hours 3

3. Pharmacological Penalization:

   • Atropine eye drops (1%) can be used as an alternative to patching 7, 3
   • Daily atropine is as effective as daily patching for moderate amblyopia 3

4. Optical Penalization:

   • Using specially designed lenses to correct differences in image size for symptomatic aniseiconia 2

5. Treatment of Associated Conditions:

   • Amblyopia should be treated before strabismus surgery 2
   • Strabismus should be treated by correcting significant refractive errors initially 2

Treatment Timing and Prognosis

• Children younger than seven years receive the most benefit from treatment 3
• Older children may still benefit, though to a lesser extent 3
• Recent evidence suggests some potential for improvement even in adulthood through perceptual learning techniques 8
• Amblyopia recurs in approximately 25% of children, making continued surveillance important 3

Important Clinical Considerations

• Early diagnosis and treatment are critical to prevent permanent vision loss 2, 5
• Immediate treatment is recommended for significant anisometropia to prevent permanent vision loss 2
• Adults can generally tolerate up to 3.00 D of difference in refractive correction between both eyes 2
• Photophobia and blurred vision may last up to 2 weeks with atropine treatment 7
• Systemic absorption of atropine may cause side effects including dryness, tachycardia, and central nervous system effects 7