Amblyopia

• Amblyopia


  Unilateral or less commonly, bilateral reduction of best corrected visual acuity that can not be attributed directly to the effect of any structural abnormality of the eye or the posterior visual pathway. Defect of central vision
• Causes of amblyopia


  • Strabismus
  • Anisometropia or high bilateral refractive error (Isoametropia)
  • Stimulus deprivation
• Prevalence: 2%-4% in the North American population
• Amblyopia is primarily a defect of central vision
• There is a critical period for sensitivity in developing amblyopia
• The time necessary for amblyopia to occur during critical period is shorter for stimulus deprivation than for strabismus or anisometropia
• Classification of amblyopia


  • Strabismic amblyopia
  • Anisometropic amblyopia
  • Amblyopia Due to bilateral high refractive error (isometropia)
  • Meridional amblyopia
  • Stimulus Deprivation Amblyopia
• Strabismic amblyopia


  • The most common form of amblyopia
  • Thought to result from competitive or inhibitory interaction between neurons carrying the nonfusible inputs from the two eyes, which leads to domination of cortical vision centers by the fixating eye and chronically reduced responsiveness to the nonfixating eye input
• Anisometropic amblyopia


  • Second in frequency
  • Develops when unequal refractive error in the two eyes causes the image on the one retina to be chronically defocused
  • Partly from the direct effect of image blur in the development of visual acuity
  • Partly from intraocular competition or inhibition
• Mild hyperopic or astigmatic anisometropia (1-2D) causes mild amblyopia
• Mild myopia anisometropia (less than -3D) usually doesn't cause amblyopia
• Unilateral high myopia (-6D) causes severe amblyopia visual loss
• The eyes of a child with anisometropic amblyopia look normal to the family and primary care physician
• Amblyopia Due to bilateral high refractive error (isometropia)


  • Results from large, approximately equal, uncorrected refractive error in both eyes of a young child
  • Hyperopia exceeding 5D & myopia excess of 10 D risk bilateral amblyopia
• Meridional amblyopia


  • Uncorrected bilateral astigmatism in early childhood may result in loss of resolving ability limited to chronically blurred meridians
• Stimulus Deprivation Amblyopia


  • Usually caused by congenital or early acquired media opacity
  • The least common but most damaging and difficult to treat
  • In bilateral cases acuity can be 20/200 or worse
• In children younger than 6 years with congenital cataract that occupy the central 3mm or more of the lens, must be considered capable of causing severe amblyopia
• Similar lens opacities acquired after 6 years are generally less harmful
• Small polar cataracts & lamellar cataracts may cause mild to moderate amblyopia or may have no effect on visual development
• Occlusion amblyopia is a form of deprivation caused by excessive therapeutic patching
• Diagnosis of amblyopia


  • Characteristics of vision alone cannot be used to reliably differentiated amblyopia from other form of visual loss
  • The crowding phenomenon is typical for amblyopia but not uniformly demonstrable
  • Afferent pupillary defect are Characteristic of optic nerve disease but occasionally appear to be present with amblyopia
• Multiple assessment using a variety of tests or performed on different occasions are sometimes required to make a final judgment concerning the presence and severity of amblyopia
• Binocular fixation pattern


  • A test for estimating the relative level of vision in the two eyes for children with strabismus who are under the age of about 3
  • Quite sensitive for detecting amblyopia but results can be falsely positive
  • Showing a strong preference when vision is equal or nearly equal in the two eyes, particularly with small angle strabismic deviations
• Modified Snellen technique


  • Directly measures acuity in children 3-6 years old
  • Often, however, only isolated letters can be used, which may lead to underestimated amblyopia visual loss
  • Crowding bar may help alleviate this problem
• Crowding bar


  Allows the examiner to test the crowding phenomenon with isolated optotype. Bar surrounding the optotype mimic the full optotype to the amblyopic child
• Treatment of amblyopia


  • Eliminating (if possible) any obstacle to vision such as a cataract
  • Correcting refractive error
  • Forcing use of the poorer eye by limiting use of the better eye
• Cataract removal


  1. Cataracts capable of producing amblyopia require surgery without unnecessary delay

  2. Removal of significant congenital lens opacities during the first 2-3 months of life is necessary for optimal recovery of vision

  3. In symmetrical bilateral cases, the interval between operations on the first and second eyes should be no more than 1-2 weeks

  4. Acutely developing severe traumatic cataracts in children younger than 6 years should be removed within a few weeks of injury, if possible
• Refractive correction

  In general, optical prescription for amblyopic eyes should correct the full refractive error as determined with cycloplegia
• Occlusion and optical degradation

  1. Full time occlusion of the sound eye

  2. Part-time occlusion

  3. Penalization
• Full time occlusion


  • Defined as occlusion for all or all but one waking hour
  • It is the most powerful means of treating of amblyopia by enforced use of the defective eye
  • The patch can either be left in place at night or removed at bedtime
  • Spectacle-mounted occluder or special opaque contact lenses can be used as an alternative to full-time patching if skin irritation or poor adhesion proves to be a significant problem
• Full time patching should generally be used only when constant strabismus eliminates any possibility of useful binocular vision because full time patching runs a small risk of perturbing binocularity
• Part-time occlusion


  • Defined as occlusion for 1-6 hours per day
  • The children undergoing part time occlusion should be kept as visually active as possible when the patch is in place
  • Compliance with occlusion therapy for amblyopia declines with increasing age
• Penalization


  • A cycloplegic agent (usually atropine 1% or homatropine 5%) once daily to the better eye
  • This form of treatment has recently been demonstrated to be as effective as patching for mild to moderate amblyopia
• Complications of therapy


  • Full time occlusion carries the greatest risk of this complication and requires close monitoring, especially in the younger child
  • Part time occlusion & optical degradation methods allow for less frequent observation but regular follow up is still critical
• The time required for completion of treatment depends on the degree of amblyopia, choice of therapeutic approach, compliance with the prescribed regimen, and age of the patient
• Unresponsiveness


  • Complete or partial Unresponsiveness to treatment occasionally affect younger children but often occurs in patients older than 5 years
  • Primary therapy should generally be terminated if there is a lack of demonstrable progress over 3-6 months with good compliance
  • Refraction should be carefully rechecked and the macula and optic nerve critically inspected for subtle evidence of hypoplasia or other malformation that might have been previously overlooked
• Recurrence


  • When amblyopia treatment is discontinued after fully or partially successful completion, approximately half of patients show some degree of recurrence
  • Maintenance therapy: Patching for 1-3 hours per day, Optical penalization with spectacles, Pharmacologic penalization with atropine 1 or 2 day per week
  • This may require periodic monitoring until age 8-10