|Classification and external resources|
Anisometropia (// US dict: an·ī′·sə·mə·trō′·pē·ə) is the condition in which the two eyes have unequal refractive power; that is, are in different states of myopia (nearsightedness), hyperopia (farsightedness) or in the extreme, antimetropia (wherein one eye is myopic and the other is hyperopic), the unequal refractive states cause unequal rotations thus leading to diplopia and asthenopia. Gross anisometropia is the difference of more than 2 diopters between the eyes.
Anisometropia can adversely affect the development of binocular vision in infants and children if there is a large difference in clarity between the two eyes. The brain will often suppress the vision of the blurrier eye in a condition called amblyopia, or lazy eye.
The name is from four Greek components: an- "not," iso- "same," metr- "measure," ops "eye."
One study estimated that 6% of those between the ages of 6 and 18 have anisometropia.
For those with large degrees of anisometropia, spectacle correction may cause the person to experience a difference in image magnification between the two eyes which could also prevent the development of good binocular vision.
The solution for spectacle wearers to the problem that spectacle correction may cause the person to experience a difference in image magnification between the two eyes is spectacles incorporating iseikonic lenses. Iseikonic lenses present adjusted image sizes to the eye compared to standard lenses. The formula for iseikonic lenses (without cylinder) is:
where: t = center thickness (in meters) n = refractive index P = front base curve h = vertex distance (in meters) F = back vertex power (essentially, the prescription for the lens)
(see Practical Optical Dispensing by David Wilson)
If the difference between the eyes is up to 3 diopter spheres iseikonic lenses can compensate. At a difference of 3 diopter spheres the lenses would however be very visibly different - one lens would need to be at least 3mm thicker and have a base curve increased by 7.5 spheres.
Contact lens correction
The usual recommendation for those needing iseikonic correction is to wear contact lenses in their normal prescription since the effect of vertex distance is removed and the effect of center thickness is also almost removed, meaning there is minimal and likely unnoticeable image size difference when wearing contacts. This is a good solution for those who can tolerate contact lenses and for whom the contact lenses create sufficient acuity.
- Czepita D, Goslawski W, Mojsa A. "[Occurrence of anisometropia among students ranging from 6 to 18 years of age.]" Klin Oczna. 2005;107(4-6):297-9. Polish. PMID 16118943.