Anisometropia is the condition in which the two eyes have unequal refractive power. Each eye can be nearsighted (myopia), farsighted (hyperopia) or a combination of both, which is called antimetropia. Generally a difference in power of two diopters or more is the accepted threshold to label the condition anisometropia.
In certain types of anisometropia, the visual cortex of the brain will not use both eyes together (binocular vision), and will instead suppress the central vision of one of the eyes. If this occurs often enough during the first 10 years of life while the visual cortex is developing, it can result in amblyopia, a condition where even when correcting the refractive error properly, the person's vision in the affected eye is still not correctable to 20/20.
Anisometropia is one of the causes of Amblyopia (lazy-eye).
Simple anisometropia occurs when only one eye has a refractive error. The eye can be either hyperopic (farsighted) or myopic (nearsighted). This form of anisometropia can be treated using eyeglasses. Simple anisometropia causes one eye to see a blurry image while the other eye sees a clear image.
Compound anisometropia occurs when both eyes are either hyperopic (farsighted) or myopic (nearsighted). However, there is still a significant difference in the refractive errors of the two eyes, so they will need dramatically different prescriptions. This form of anisometropia causes both eyes to see blurry images, but one eye’s vision will be significantly blurrier.
Mixed anisometropia occurs when both eyes have refractive errors but one is myopic (nearsighted), and the other is hyperopic (farsighted).
Though anisometropia can be present from birth, it is not typically diagnosed until childhood. Children who have anisometropia may not show any symptoms. For the very young, parents should note any preferential looking, eye turn, headaches or failure to reach developmental milestones especially with mobility. Ideally, all children should have a comprehensive vision exam by an optometrist during the first year of life to help identify this condition and others.
Generally speaking, with Anisometropia, one sees a smaller image in one eye and a larger image in the other eye. The result is that their overall vision is often blurred. Anisometropia can also result in diplopia (double-vision) which can produce the following symptoms:
Treatment for Anisometropia also depends on the severity. For some, the difference between the two eyes is manageable, especially with a minimal difference. Other patients may require corrective lenses, contact lenses or corrective surgery. If anisometropia has caused amblyopia, then treatment may include vision therapy, patching, or drops.
The name is from four Greek components: an- "not," iso- "same," metr- "measure," ops "eye."
An estimated 6% of subjects aged 6 to 18 have anisometropia.