Amblyopia can be caused by deprivation of vision early in life by vision-obstructing disorders such as congenital cataracts, by strabismus (misaligned eyes), or by anisometropia (different degrees of myopia or hypermetropia in each eye).
Strabismic amblyopia
Strabismus, sometimes erroneously also called ''lazy eye'', is a condition in which the eyes are misaligned. Strabismus usually results in normal vision in the preferred sighting (or "fellow") eye, but may cause abnormal vision in the deviating or strabismic eye due to the discrepancy between the images projecting to the brain from the two eyes.
Adult-onset strabismus usually causes double vision (diplopia), since the two eyes are not fixated on the same object.
Children's brains, however, are more neuroplastic, and therefore can more easily adapt by suppressing images from one of the eyes, eliminating the double vision.
This plastic response of the brain, however, interrupts the brain's normal development, resulting in the amblyopia.
Strabismic amblyopia is treated by clarifying the visual image with glasses, and/or encouraging use of the amblyopic eye with an eyepatch over the dominant eye or pharmacologic penalization of it.
Penalization usually consists of applying atropine drops to temporarily dilate the pupil, which leads to blurring of vision in the good eye.
This helps to prevent the bullying and teasing associated with wearing a patch, although application of the eyedrops is more challenging. The ocular alignment itself may be treated with surgical or non-surgical methods, depending on the type and severity of the strabismus.
Refractive or anisometropic amblyopia
Refractive amblyopia may result from anisometropia (unequal refractive error between the two eyes). Anisometropia exists when there is a difference in the refraction between the two eyes.
The eye which provides the brain with a clearer image (closer to 20/20) typically becomes the dominant eye.
The image in the other eye is blurred, which results in abnormal development of one half of the visual system.
Refractive amblyopia is usually less severe than strabismic amblyopia and is commonly missed by primary care physicians because of its less dramatic appearance and lack of obvious physical manifestation, such as with strabismus.
Frequently, amblyopia is associated with a combination of anisometropia and strabismus.
Amblyopia in those that maintain binocular functions can be treated successfully up to a later age than those with strabismic amblyopia.
Pure refractive amblyopia is treated by correcting the refractive error early with prescription lenses and patching or penalizing the good eye.
Meridional amblyopia is a mild condition in which lines are seen less clearly at some orientations than others after full refractive correction.
An individual who had an astigmatism at a young age that was not corrected by glasses will later have astigmatism that cannot be optically corrected.
Form-deprivation and occlusion amblyopia
Form-deprivation amblyopia (''Amblyopia ex anopsia'') results when the ocular media become opaque, such as is the case with cataracts or corneal scarring from forceps injuries during birth.
These opacities prevent adequate visual input from reaching the eye, and therefore disrupt development. If not treated in a timely fashion, amblyopia may persist even after the cause of the opacity is removed.
Sometimes, drooping of the eyelid (ptosis) or some other problem causes the upper eyelid to physically occlude a child's vision, which may cause amblyopia quickly.
Occlusion amblyopia may be a complication of a hemangioma that blocks some or all of the eye.
Further Reading
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