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Posted on 10-31-2016

Amblyopia, also referred to as "lazy eye", is a unilateral or infrequently bilateral condition in which the best corrected visual acuity is poorer than 20/20 in the absence of any obvious structural anomalies or ocular disease. Amblyopia represents a syndrome of compromising deficits, rather than simply reduced visual acuity, including:
• Increased sensitivity to contour interaction effects(difficulty processing closely spaced print)
• Abnormal spatial distortions and uncertainty
• Unsteady and inaccurate monocular fixation
• Poor eye tracking ability
• Reduced contrast sensitivity
• Inaccurate accommodative(focusing) response

Form Deprivation Amblyopia:
When a physical obstruction along the line of sight prevents the formation of a well-focused, high-contrast image on the retina, the result is form deprivation amblyopia. This obstruction can occur in one or both eyes and must take place before the age of 6-8 years for amblyopia to develop. The degree to which amblyopia develops depends on the time of onset and the extent of the form deprivation. Congenital cataract is the most frequent cause of form deprivation amblyopia.

Refractive Amblyopia:
Refractive amblyopia results from either high but equal amounts (isoametropic) or significant unequal amounts (anisometropic) of uncorrected nearsightedness or farsightedness.  Over time, this subtle type of visual form deprivation blur delays normal neurophysiological development of the visual pathway and visual areas of the brain.  The difference in uncorrected prescription between eyes causes a blurred image in the eye with the greater nearsightedness or farsightedness, disrupting the normal neurophysiological development of the visual pathway and visual areas of the brain.  Generally, the greater the difference, the more severe the amblyopia.

Strabismic Amblyopia:
Strabismic amblyopia is most commonly associated with an early onset (<6-8 years of age) of constant right or constant left eye turn. The two eyes then receive different visual images, causing confusion and double vision. To eliminate these problems, the visual system actively inhibits or suppresses the image from the turned eye. This active inhibition over time causes an alteration in the brains perception of space that results in a loss of visual acuity.

Development of Amblyopia:
The visual pathways develop from birth to approximately 6-8 years of age, with the most rapid development occurring in infancy. During this time, the visual system is susceptible to known amblyogenic factors: form deprivation, blur, and misalignment of the eyes. If left untreated, the two amblyogenic mechanisms, form deprivation and abnormal binocular inhibition, cause a progressive reduction of visual acuity until approximately 6-8 years of age, at which time visual acuity stabilizes. The child's age when exposed to an amblyopia-inducing condition appears to be the most important determinant for the development of amblyopia. Amblyopia of one eye usually produces little handicap and few symptoms because the patient typically has good visual acuity in the normal eye. The most significant problems usually result from a decrease in stereopsis, which may result in avoiding certain activities and less efficient vision performance in driving and near eye-hand coordination activities. In addition, amblyopia may contribute to later onset of strabismus(eye turn). Difficulty processing visual information is approximately three times greater for children whose refractive errors are corrected after 4 years of age than for those corrected earlier which is associated with early learning deficits.

Risk factors:
Prematurity
Low birth weight
Cerebral palsy
Cognitive impairment
Family history of eye turn, significant unequal or high equal amounts of near or farsightedness,         congenital cataract
Smoking or drug use during pregnancy 

Prevention:
Amblyopia is a preventable and a treatable condition especially if detected early. Screening for causes of form deprivation amblyopia should be conducted by the infant's primary care physician within the first 4-6 weeks after birth, and children at risk should be monitored yearly throughout the sensitive developmental period (birth to 6-8 years of age).

Treatment:
Treatment should be directed toward the two primary etiologies of amblyopia: form deprivation and binocular inhibition. Amblyopia therapy effectively restores normal or near-normal visual function by developing more extensive brain wiring to the visual cortex. It improves monocular deficits of visual acuity, monocular fixation, accommodation, and ocular motility. The final step in amblyopia therapy, if possible, is to develop normal binocular vision. The establishment of binocular vision eliminates or significantly reduces the underlying binocular inhibition, which increases the probability of maintaining visual acuity improvements.
    Optical correction: The rationale for correcting the refractive error with spectacles or contact lenses is to ensure that the retina of each eye receives a clear optical image.
    Occlusion:  Covering the better seeing eye to force activation of the poorer seeing eye is the basis of occlusion.  This method has a high rate of non-compliance due to the negative impact on the patients quality of life and rendering them incapable to learn effectively in the classroom.  Occlusion treatment is aimed at improving visual acuity of the poorer eye however this method does not address the other mechanism of amblyopia which is binocular inhibition.  Treating the condition monocularly does not guarantee binocularity.
    Active vision therapy: Vision therapy refers to the total treatment program, which may include passive therapy options (e.g., spectacles, occlusion, pharmacologic agents) and active therapy. With such passive treatment options as optical correction and occlusion, the patient experiences a change in visual stimulation without any conscious effort. Active therapy is designed to improve visual performance by the patient's conscious involvement in a sequence of specific, controlled visual tasks or procedures that provide feedback about the patient’s performance. Active vision therapy for amblyopia is designed to remediate deficiencies in four specific areas: eye movements and fixation, spatial perception, accommodative efficiency, and binocular function. The goal of vision therapy is remediation of these deficiencies, with subsequent equalization of monocular skills and, finally, integration of the amblyopic eye into binocular functioning.   

Care of Patient with Amblyopia, Optometric Clinical Practice Guideline, American Optometric Association.  Updated 2004.

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