Amblyopia is a condition in which one eye cannot see better than 20/30 even with glasses or contacts and the reduced vision is not due to another disease within the eye. This condition is commonly called “lazy eye”.
Amblyopia primarily affects children under the age of ten because the visual system is actively developing.
The mechanism of amblyopia can be due to occlusion, refractive error, or an eye turn.
No matter which mechanism is the cause of amblyopia, the condition affects the way the brain interprets vision rather than how the eye is able to see.
Since the development of the portion of the brain responsible for developing and interpreting vision is mature by the age of ten, amblyopia will not occur even if there is a decrease in vision after that age.
Occlusion Amblyopia
Occlusion amblyopia, also called form deprivation amblyopia, is caused by an obstruction of vision in one eye. This obstruction must be constant and only affect one eye or affect one eye more severely than the other eye.
Several conditions that are present at birth can result in occlusion amblyopia including a constant droopy eyelid, cataracts of the lens, or a cloudy cornea.
In addition to conditions that are present at birth, some causes of occlusion amblyopia may be acquired throughout life including a scar on the central cornea or a tumor on the eyelid that causes obstruction.
Form deprivation amblyopia is the least common cause of amblyopia but is often the most severe and most difficult to treat.
Refractive Amblyopia
There are three types of refractive error amblyopia.
Anisometropic amblyopia occurs when one of the eyes is much stronger or weaker than the other, isometropic amblyopia occurs when neither eye is able to have a clear image, and meridional amblyopia occurs when one eye has much more astigmatism than the other eye.
Anisometropic amblyopia is often symptomless compared to isometropic amblyopia because in anisometropic amblyopia the non-amblyopic eye can still see clearly.
Since one eye has a clear image and the other has a blurry image, the brain automatically turns off the blurry eye and the connections between the brain and that eye are weakened resulting in anisometropic amblyopia.
Isometropic amblyopia occurs when both eyes have blurry images, and the brain forms weak connections with both eyes.
Meridional amblyopia occurs due to a difference in the astigmatism between the two eyes. Astigmatism can cause the eye to focus in two points instead of one and cause blurred vision.
Like anisometropic amblyopia, meridional amblyopia occurs when one eye is much blurrier than the other and the brain weakens the connections with that eye.
Strabismic or Eye Turn Amblyopia
If one of the eyes is turned inward or outward, this is called strabismus. This condition can be present at birth or develop throughout childhood.
Since the eyes are not aligned, they cannot focus on the same object at the same time. This forces the brain to either have double vision or suppress one eye.
If one eye is suppressed the brain turns off the perception from that eye for some time and only uses the other eye for sight.
In causes of constant eye turns, the turned eye will be suppressed constantly and result in a weak connection between the eye and the brain and cause strabismic amblyopia.
Treatment for Amblyopia
To treat amblyopia, the cause of the amblyopia must be determined and if possible corrected.
For occlusion amblyopia, surgery to remove the obstruction may result in normal vision development.
In refractive error amblyopia, glasses or contact lenses may improve clarity and allow normal development.
Strabismic amblyopia will often require vision therapy to improve the control of the eye turn and break the suppression of the eye before the eye can develop correctly.
In many cases, vision therapy can improve the speed and outcome of the amblyopia regardless of the cause by training the brain to use both eyes.
