There are a number of different eye disorders that can cause a lazy eye (amblyopia):
 
Strabismic amblyopia
Strabismic amblyopia is the most common cause of a lazy eye. The condition involves a squint (strabismus) in one eye, which is noticeable because the eyes look in different directions. 
 
For example, one eye may look straight ahead and the other may look inwards, outwards, up, or down. To avoid double vision, the brain ignores the signals from the eye with a squint and only ‘sees’ images from the normal eye. As the affected eye is not being used, over time the squinting eye will become lazy.
 
Strabismus can be inherited, or it may be caused by long- or short-sightedness, an injury, or a viral illness.
 
Anisometropic amblyopia
Anisometropic amblyopia is where a refractive error (poor focusing of light) occurs due to short-sightedness (myopia), long-sightedness (hypermetropia), or astigmatism (where the surface of the lens is uneven, causing blurred vision).
 
A refractive error means that there is a difference of refraction in each eye. Refraction is where light rays change direction when they pass between two mediums of different densities, such from the air into the eye. Both eyes are usually affected to the same degree.
 
In anisometropic amblyopia,however, a child who is long-sighted, for example, will be more long-sighted in one eye compared with the other. As a result, their brain will ignore the signals from the eye that has the biggest refractive error (is most long-sighted). As a result, this eye may become lazy.
 
Ametropic amblyopia
Ametropic amblyopia is a reduction in the vision of both eyes. It usually occurs when a child has large, uncorrected refractive errors. The image that is produced on the retina, at the back of the eye, is constantly blurred as a result of undetected long- sightedness, or astigmatism.
 
Stimulus deprivation amblyopia
Stimulus deprivation amblyopia is the rarest form of amblyopia, but it is often the most severe. It is caused when one eye or both eyes are prevented from seeing and become lazy. This may be due to:
  • Eye disease; such as a corneal ulcer (a sore on the transparent layer at the front of the eye) or scar,
  • Congenital cataract (clouding of the lens of the eye that is present from birth),
  • Droopy eye lid (ptosis, or blepharoptosis), or
  • Glaucoma (a group of eye conditions that affect vision)
Stimulus deprivation amblyopia can also be caused by an eye injury, or surgery to the eye.
 
Learn more about diagnosing lazy eye.

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