Anisometropia

Anisometropia
Pronunciation	/ænˌaɪsəmɪˈtroʊpiə/ ann-EYE-sə-mi-TROH-pee-ə
Specialty	Ophthalmology, optometry
Symptoms	One eye is myopia other eye is hyperopia
Complications	Amblyopia

Anisometropia is when two eyes have unequal refractive power.^[1] 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.

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.

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.^[2]

Causes[]

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Diagnosis[]

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Treatment[]

Spectacle correction[]

For those with large degrees of anisometropia, the wearing of standard spectacle 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. This can make it very difficult to wear glasses without symptoms such as headaches and eyestrain. However, the earlier the condition is treated, the easier it is to adjust to glasses.

It is possible for spectacle lenses to be made which can adjust the image sizes presented to the eye to be approximately equal. These are called iseikonic lenses. In practice though, this is rarely ever done.

The formula for iseikonic lenses (without cylinder) is:

${\displaystyle {\textrm {Magnification}}={\frac {1}{(1-({\frac {t}{n}})P)}}\cdot {\frac {1}{(1-hF)}}}$

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).

If the difference between the eyes is up to 3 diopters, iseikonic lenses can compensate. 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.

Example[]

Consider a pair of spectacles to correct for myopia with a prescription of −1.00 m⁻¹ in one eye and −4.00 m⁻¹ in the other. Suppose that for both eyes the other parameters are identical, namely t = 1 mm = 0.001 m, n = 1.6, P = 5 m⁻¹, and h = 15 mm = 0.015 m.
Then for the first eye ${\displaystyle {\textrm {Magnification}}={\frac {1}{(1-(0.001/1.6)\times 5)}}\cdot {\frac {1}{(1-0.015\times -1)}}=1.0031\times 0.9852=0.9883=98.83\,\%}$,
while for the second eye ${\displaystyle {\textrm {Magnification}}={\frac {1}{(1-(0.001/1.6)\times 5)}}\cdot {\frac {1}{(1-0.015\times -4)}}=1.0031\times 0.9434=0.9464=94.64\,\%}$.

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. Then for the second eye the magnification would become ${\displaystyle {\textrm {Magnification}}={\frac {1}{(1-(0.005/1.6)\times 14.5)}}\cdot {\frac {1}{(1-0.015\times -4)}}=1.0475\times 0.9434=0.9882=98.82\,\%}$,
which is much closer to that of the first eye.

In this example the first eye, with a −1.00 diopter prescription, is the stronger eye, needing only slight correction to sharpen the image formed, and hence a thin spectacle lens. The second eye, with a −4.00 diopter prescription, is the weaker eye, needing moderate correction to sharpen the image formed, and hence a moderately thick spectacle lens — if the aniseikonia is ignored. In order to avoid the aniseikonia (so that both magnifications will be practically the same, while retaining image sharpness in both eyes), the spectacle lens used for the second eye will have to be made even thicker.

Contact lenses[]

The usual recommendation for those needing iseikonic correction is to wear contact lenses. 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. This is a good solution for those who can tolerate contact lenses.

Refractive surgery[]

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^[3] (see: Refractive surgery).

Epidemiology[]

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.^[4]

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 teenagers and young adults, and more prevalent after presbyopia sets in, progressively increasing into old age.^[4]

One study estimated that 6% of those between the ages of 6 and 18 have anisometropia.^[5]

Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood.^[6]

Anisometropic persons who have strabismus are mostly far-sighted, and almost all of these have (or have had) esotropia.^[7] 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^[8] and poor binocular function may be a risk factor for anisometropia to develop or increase.^[9]

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