|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 (aniseikonia) which could also prevent the development of good binocular vision.
The solution for spectacle wearers to the problem that spectacle correction may cause aniseikonia is to use 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.
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.
Refractive surgery causes only minimal size differences, similarly as 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 (see: Refractive surgery#Children).
A determination of the prevalence of anisometropia has several difficulties. First of all, the measurement of refractive error may vary from one measurement to the next. Secondly, different criteria have been employed to define anisometropia, and the boundary between anisometropia and isometropia depend on their definition.
Several studies have found that anisometropia occurs more frequently and tends to be more severe for persons with high ametropia, and that this is particularly true for myopes. Anisometropia follows a U-shape distribution according to age: it is frequent in infants aged only a few weeks, is more rare in young children, comparatively more frequent in teen-agers and young adults, and more prevalent after presbyopia sets in, progressively increasing into old age.
Aneisotropic persons who have strabismus are mostly far-sighted, and almost all have (or have had) esotropia. However, there are indications that for patients with esotropia who undergo strabismus surgery, anisometropia one of the risk factors for developing consecutive exotropia.
- 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.
- William F. Astle; Jamalia Rahmat; April D. Ingram; Peter T. Huang (December 2007). "Laser-assisted subepithelial keratectomy for anisometropic amblyopia in children: Outcomes at 1 year". Journal of Cataract & Refractive Surgery 33 (12): 2028–2034. doi:10.1016/j.jcrs.2007.07.024.
- Barrett BT, Bradley A, Candy TR (September 2013). "The relationship between anisometropia and amblyopia". Progress in Retinal and Eye Research 36: 120–58. doi:10.1016/j.preteyeres.2013.05.001. PMC 3773531. PMID 23773832.
- Quote: "When strabismus is present in an anisometropic individual, it is almost always of the convergent type and is generally found in anisohyperopes but not anisomyopes." Barrett BT, Bradley A, Candy TR (September 2013). "The relationship between anisometropia and amblyopia". Progress in Retinal and Eye Research 36: 120–58. doi:10.1016/j.preteyeres.2013.05.001. PMC 3773531. PMID 23773832.
- Yurdakul NS, Ugurlu S (2013). "Analysis of risk factors for consecutive exotropia and review of the literature". Journal of Pediatric Ophthalmology and Strabismus 50 (5): 268–73. doi:10.3928/01913913-20130430-01. PMID 23641958.