Anisometropia refers to a condition when two eyes have unequal refractive power. Generally, a difference in power of one diopter (1D) or more is the accepted threshold to label the condition anisometropia. Patients can tolerate 3 D of anisometropia before it becomes clinically symptomatic with headaches, asthenopia, double vision and photophobia.In certain types of anisometropia, the visual cortex of the brain will not process images from both eyes together (binocular summation), 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 persons vision in the affected eye is still not correctable to 20/20. The name is from four Greek components: an- "not," iso- "same," metr- "measure," ops "eye." Antimetropia is a rare sub-type of anisometropia, in which one eye is myopic (nearsighted) and the other eye is hyperopic (farsighted). Around 0.1% of the population may be antimetropic. Causes
Anisometropia is caused by common refractive errors, such as astigmatism, far-sightedness, and myopia, in one eye.Anisometropia is likely the result of both genetic and environmental influences.Some studies suggest, in older adults, developing asymmetric cataracts may cause worsen anisometropia. However, anisometropia is associated with age regardless of cataract development: a rapid decrease in anisometropia during the first years of life, an increase during the transition to adulthood, relatively unchanging levels during adulthood but significant increases in older age.
Thus, in the first eye the size of the image formed on the retina will be 1.17% smaller than without spectacles (although it will be sharp, rather than blurry), whilst in the second eye the image formed on the retina will be 5.36% smaller. As alluded to above, one method of producing more iseikonic lenses would be to adjust the thickness and base curve of the second lens. For instance, theoretically it could be set to t = 5 mm = 0.005 m and P = 14.5 m−1, with all other parameters unchanged.