Etiology
Amblyopia arises due to abnormal visual stimulation during the critical period for normal visual development (from birth to between 7 and 8 years).[10][11] Amblyopia is more commonly unilateral than bilateral.[12][13]
The etiology of amblyopia is varied. It may occur secondary to an underlying visual impairment (refractive error [a significant cause of pediatric amblyopia] or strabismus), as a result of visual stimulus deprivation, or due to combined mechanisms.[14] Strabismus and anisometropia are common causes of amblyopia, and may coexist.[12][15][16][17] See Classification.
Pathophysiology
Amblyopia is a developmental cortical disorder of the visual pathway caused by abnormal visual stimulus of the binocular cortical cells:[1][10][18][19]
Strabismus (misalignment of the eyes) is thought to result in amblyopia because of the competitive or inhibitory interaction between the cortical inputs from the two eyes. Cortical vision centers from the fixating eye dominate, and cortical vision centers from the nonfixating eye are chronically suppressed.
Anisometropia (unequal refractive error in the two eyes) causes the image on one retina to be chronically defocused. Amblyopia results from the direct effect of image blur on visual development. It also partly results from interocular competition (or inhibition) similar to that responsible for strabismic amblyopia.
Ametropia (both eyes are about equally defocused) causes no cortical competition or inhibition between the two eyes. Amblyopia results from image blur alone.
Corneal and/or lenticular astigmatism in a particular meridian may result in meridional amblyopia due to optical defocus. Amblyopia is partially due to loss of cells in the primary visual cortex with an orientation similar to the blurred meridian.
Occlusion of the visual axis or severe image blur caused by opacities in the visual axis may result in form-deprivation amblyopia. Unilateral occlusion is worse than bilateral occlusion of similar magnitude because interocular competition adds to the direct developmental impact of image degradation.
Classification
Etiologic classification
Strabismic amblyopia
Strabismus (misalignment of the eyes) results in amblyopia if the child prefers to fixate with one eye instead of freely alternating fixation between two eyes.
The nonfixating eye (or deviated eye) is suppressed as an adaptive mechanism to avoid double vision.
Prolonged suppression during early childhood leads to amblyopia.
Refractive amblyopia
Refractive errors defocus the retinal image and, if marked, can cause amblyopia.
May occur with strabismus.
Greater anisometropia or astigmatism increase the risk and severity.
Subtypes include:
Anisometropic amblyopia: caused by unequal refractive error between the two eyes. Most often develops in hyperopic (long-sighted) children who focus with the less hyperopic eye, leading to persistent blurring of the more hyperopic eye.
Ametropic (isoametropic) amblyopia: caused by high and similar refractive errors in the two eyes. Both high hyperopia and high myopia (nearsightedness) can cause sufficient retinal image blur to induce amblyopia, but amblyopia occurs more commonly with high hyperopia.
Meridional amblyopia: high astigmatism in one or both eyes causes blur of a retinal image in a particular meridian.
Bilateral refractive (isoametropic) amblyopia: uncommon and results from the effect of bilateral blurred retinal images.
Form deprivation amblyopia
This results from blurring of the retinal image caused by opacities in the cornea, anterior chamber, lens, vitreous, or retinal surface (e.g., macular hemorrhage).
Deprivation can also occur due to severe ptosis or prolonged occlusion of one or both eyes.
It is the least common cause, but is often the most severe and difficult to treat.
Occlusion (reverse) amblyopia
This is a form of deprivation amblyopia that develops after therapeutic patching or pharmacologic cycloplegia of the nonamblyopic eye.
Visual acuity usually returns to baseline without treatment.
Combined mechanism
Different subtypes of amblyopia can coexist in the same patient (e.g., strabismic and hyperopic anisometropic amblyopia frequently co-occur).
Some children with moderate to high hyperopia develop accommodative esotropia (inward turning of the eye). This is a type of strabismus resulting from the yoking of convergence and accommodation (which is necessary to focus the image in the hyperopic eye). If hyperopia differs between the two eyes, it is generally the more highly hyperopic eye that turns in and develops amblyopia.
International classification of diseases, 11th revision (ICD-11)[2]
Includes entities with the following classifications:
Amblyopia, type not specified
Strabismic amblyopia
Anisometropic amblyopia
Deprivation amblyopia
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