Amblyopia

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De​finition

Vision loss due to the disuse or misuse of one eye

 

Pathogenesis

  • The exact mechanism is not known, but is primarily mediated by changes in the visual cortex.
     
  • During the critical period when vision develops (from birth until 3-5 years) [3], the visual cortex must calibrate itself to the retinas.
     
  • As an oversimplification, this is achieved through trial and error with good associations being reinforced and incorrect ones suppressed.
     
  • If there is less information from one eye or the information is “less valuable”, the brain will suppress this eye and dedicate more “processing power” (i.e. cortical neurons) to the better functioning eye
    • This was directly demonstrated by Hubel and Wiesel in cats [1], for which they shared the 1981 Nobel Prize with Roger Sperry

 

Common Causes

  • Monocular Deprivation
      
    • Cataract, ptosis, eyelid hemangioma, trauma
  • Unequal refractive indices (anisometropia)
  • Strabismus – Misalignment of the foveas

 

Severity

The degree of decreased visual acuity is a function the patient’s age, the degree of impairment, and the duration of impairment

  • Worst during the critical period (birth until 3-5 years of age)
    • Therefore, early detection and intervention are key
  • Less severe if onset at a greater age, when the visual cortex is already more developed

 

Diagnosis

Preverbal children:

  • Assess fixation and ability to track with each eye occluded
    • Occlusion test – positive if child becomes fearful or agitated when the dominant eye is occluded but not the amblyopic eye
        
    • Vertical prism test – displace an image using a prism in one eye at a time.  Positive if child shifts their gaze when the prism is front of the dominant eye, but not with the amblyopic eye

Older children:

  • Formal vision testing with Snellen chart (different sized letters)
    • There will be less acuity in the amblyopic eye even with optimal correction
       
  • Crowding phenomenon – using both eyes, there is decreased acuity when attempting to focus on multiple objects at once

 

Treatment

  • Eliminate or reduce impairment
        
    • Alignment surgery or correction of refractive error
  • Occlusion therapy
  • Encourage use of the amblyopic eye by temporarily inhibiting the dominant eye [2] using:
    • A patch or glasses to cover the dominant eye
    • Atropine or other cycloplegic drops in the dominant eye
  • Different intensities (i.e. hours per day) based on age and degree of impairment
    • Occlusion therapy is not effective if the amblyopic eye cannot focus

 

Prognosis

  • Optimal improvement if intervention instituted by age 9-10 [3] although there can be significant improvement up to age 18 [4].
  • With treatment, most patients have significant improvement, with some achieving their maximum potential acuity [5]
  • However, many individuals experience lifelong difficulty with hand eye coordination and the perception of complex images [6]

References

1.  Hubel, D. H. & T. N. Wiesel, Receptive Fields Of Single Neurones In The Cat's Striate Cortex, Journal of Physiology, (1959) 148, 574-591.

2.  Arthur SM.  Are amblyopia treatments really all equal? Is that even the right question?  .  Am Orthopt J. 2012;62:1-3.

3. Epelbaum M, Milleret C, Dufier JL.  The sensitive period for strabismic amblyopia in humans.  Ophthalmology. 1993;100(3):323.

4.  Scheiman MM, Hertle RW, Pediatric Eye Disease Investigator Group, et. al.  Arch Ophthalmol. 2005;123(4):437.

5.  Beardsell R, Clarke S, Hill M.  Outcome of occlusion treatment for amblyopia.  J Pediatr Ophthalmol Strabismus. 1999;36(1):19.

6.  Mirabella G, Hay S, Wong AM.  Deficits in perception of images of real-world scenes in patients with a history of amblyopia.  Arch Ophthalmol. 2011;129(2):176.