lecture 6

• Amblyopia


  Eye condition where vision is reduced in one (or both) eyes and is not correctable by glasses, contacts or surgery, and is not due to eye disease
• Amblyopia is a cerebral visual impairment considered to derive from abnormal visual experience (e.g., strabismus, anisometropia)

  
• Amblyopia was originally defined as poor vision, or blunt sight

  
• The most commonly assessed aspect of the amblyopia deficit is visual acuity, usually done with a Snellen chart

  
• Amblyopia most commonly affects one eye but may occur in both eyes
• Amblyopia incidence is 3% of children under six years
• In children under 3, amblyopia affects about 50% of those with strabismus and 18% of those with anisometropia, but this ratio reverses in adults
• Amblyopia develops


  Due to disruption in visual processing during visual development period
• Amblyopia is caused by


  Abnormal retinal input during childhood, so the brain does not fully recognize the images received by the amblyopic eye

  
• Neural impairment of the visual pathway can occur


  When normal visual development is disrupted, leading to loss, distortion or rearrangement of connections within the visual cortex
• Forms of amblyopia


  • Amblyopia from anisometropia
  • Stimulus deprivation amblyopia
  • Amblyopia from strabismus
  • Combination of anisometropia and strabismus amblyopia
• Amblyopia from anisometropia


  • No strabismus, but one eye's image is out of focus, so connections between retina and cortex do not form a precise topographic map

  
• Vision changes in amblyopic eye from anisometropia


  • Acuity reduced
  • Contrast sensitivity reduced, especially at high spatial frequencies
  • Vernier acuity reduced
  • Spatial localization degraded

  
• Spatial frequency


  Measured in cycles per degree, with 30 cycles/degree for a Snellen 6/6 letter
• Anisometropic amblyopes show different amounts of acuity and contrast sensitivity loss depending on their degree of anisometropia
• Stimulus deprivation amblyopia


  Caused by absence of form vision stimulus to visual receptors during critical visual development period, e.g. from cataracts, corneal opacity, etc.

  
• Deprivation amblyopia is associated with losses of contrast sensitivity, mainly at high spatial frequencies
• Amblyopia from unilateral cataract


  Much of the input from the deprived eye to the cortex is lost, leading to very poor acuity (less than 20/200)

  
• Therapy for cataract-induced amblyopia


  Remove cataract surgically, provide optical correction, patch good eye 40-50% of day to force use of poor eye
• Children with congenital bilateral cataract have better vision than unilateral, but still substantially below normal
• Spatial vision


  Ability to see and discriminate features and patterns in the visual field, related to changes in luminance across space
• Amblyopia from strabismus


  • Deficits are more complicated, with possible creation of a new fixation point away from the fovea, and various rearrangements of retina-cortex connections
• Two possible theories for neural changes in strabismic amblyopia are reduction in thalamic-cortical afferents/V1 neurons, and neural topographical disarray
• Visual conditions to investigate in amblyopia


  • Crowding
  • Vernier acuity
  • Spatial uncertainty
  • Shape discrimination
  • Deficits in movement and direction
  • Visual perception (spatial vision)
  • Suppression
• Crowding


  Small letters on Snellen chart tend to run together, as nearby objects affect visibility of each other
• Magnitude of crowding effect is proportional to acuity degradation in anisometropic amblyopes, but can be much greater than acuity deficit in strabismic amblyopes
• Vernier acuity


  Ability to detect misalignment or positional offset between visual stimuli
• Vernier acuity is reduced in both anisometropic and strabismic amblyopes, but much more so in strabismic
• Vernier acuity can improve with practice in amblyopic and normal adults
• Vernier acuity


  A form of visual hyperacuity, dependent on cortical processing and slightly affected by optical factors, considered a valuable indicator of cortical visual function

  
• Spatial uncertainty


  Amblyopes have deficits in fine-level spatial localization (vernier acuity) and coarse-level spatial localization (bisection, dot placement tasks)
• Shape discrimination


  Amblyopes have deficits in distinguishing irregular from regular shapes, not dependent on contrast
• Shape discrimination performance is generally poorer in the amblyopic eye compared to the normal eye
• Diagnosing amblyopia


  1. Determine if visual acuity is decreased and cannot be improved with corrective lenses

  2. Perform cover test to rule out manifest heterotropia

  3. Conduct refraction to check for anisometropia

  4. Examine fundus to rule out organic causes

  5. Check fixation behavior

  6. Perform 4 diopter base-out prism test for anisometropic amblyopia

  
• Anisometropic amblyopia


  Amblyopia caused by a refractive difference of more than 1.5 diopters between the eyes

  
• Strabismic amblyopia


  Amblyopia associated with a history of strabismus
• Deprivation amblyopia


  Amblyopia caused by unilateral visual deprivation during infancy and early childhood
• Idiopathic amblyopia


  Amblyopia with no known cause
• Bilateral visual deprivation amblyopia


  Amblyopia caused by uncorrected high bilateral hypermetropia or congenital nystagmus

  
• Careful examination of the fundus is indicated in all cases of amblyopia associated with strabismus