Anisometropia is a condition in which a person's eyes have substantially differing refractive power.
[2][3] Patients may have up to 3 diopters of anisometropia before the condition becomes clinically significant due to headache, eye strain, double vision or photophobia.
The name of the condition comes from its four Greek components: an- "not", iso- "same", metr- "measure", ops "eye".
[5] Anisometropia is caused by common refractive errors, such as astigmatism, far-sightedness, and myopia, in one eye.
[7] 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.
However, since most people do not show any clear symptoms, the condition usually is found during a routine eye exam.
[6] For those with large degrees of anisometropia, the wearing of standard spectacles may cause the person to experience a difference in image magnification between the two eyes (aniseikonia) which could also prevent the development of good binocular vision.
It is possible for spectacle lenses to be made which can adjust the image sizes presented to the eye to be approximately equal.
The formula for iseikonic lenses (without cylinder) is: where: t = center thickness (in metres); n = refractive index; P = front base curve (in 1/metres); h = vertex distance (in metres); F = back vertex power (in 1/metres), (essentially, the prescription for the lens, quoted in diopters).
At a difference of 3 diopters the lenses would however be very visibly different—one lens would need to be at least 3 mm thicker and have a base curve increased by 7.5 spheres.
As alluded to above, one method of producing more iseikonic lenses would be to adjust the thickness and base curve of the second lens.
The usual recommendation for those needing iseikonic correction is to wear contact lenses.
Refractive surgery causes only minimal size differences, similar to contact lenses.
In a study performed on 53 children who had amblyopia due to anisometropia, surgical correction of the anisometropia followed by strabismus surgery if required led to improved visual acuity and even to stereopsis in many of the children[9] (see: Refractive surgery).
[11] Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood.
[13] However, there are indications that anisometropia influences the long-term outcome of a surgical correction of an inward squint, and vice versa.
More specifically, for patients with esotropia who undergo strabismus surgery, anisometropia may be one of the risk factors for developing consecutive exotropia[14] and poor binocular function may be a risk factor for anisometropia to develop or increase.