In anisometropia, there is a difference in refractive power between the two eyes.
Epidemiology: Anisometropia of at least 4 diopters is present in less than 1 % of the population.
Etiology: The reason for the varying development of the two eyes is not clear. This primarily congenital disease is known to exhibit a familial pattern of increased incidence.
Pathophysiology: In anisometropia, there is a difference in refractive power between the two eyes. This refractive difference can be corrected separately for each eye with different lenses as long as it lies below 4 diopters. Where the difference in refraction is greater than or equal to 4 diopters, the size difference of the two retinal images becomes too great for the brain to fuse the two images into one. Known as aniseikonia, this condition jeopardizes binocular vision because it can lead to development of amblyopia (anisometropia amblyopia). The aniseikonia, or differing size of the retinal images, depends not only on the degree of refractive anomaly but also depends significantly on the type of correction. The closer to the site of the refraction deficit the correction is made, the less the retinal image changes in size. Correction with intraocular lenses results in almost no difference in image size. Contact lenses produce a slight and usually irrelevant difference in image size. However, eyeglass correction resulting in a difference of more than 4 diopters leads to intolerable aniseikonia (see Table 7.4).
Symptoms: Anisometropia is usually congenital and often asymptomatic. Children are not aware that their vision is abnormal. However, there is a tendency toward strabismus as binocular functions may remain underdeveloped. Where the correction of the anisometropia results in unacceptable aniseikonia, patients will report unpleasant visual sensations of double vision.
Diagnostic considerations: Anisometropia is usually diagnosed during routine examinations. The diagnosis is made on the basis of refraction testing.
Treatment: The refractive error should be corrected. Anisometropia exceeding 4 diopters cannot be corrected with eyeglasses because of the clinically relevant aniseikonia. Contact lenses and, in rare cases, surgical treatment are indicated. Patients with unilateral aphakia or who do not tolerate contact lenses will require implantation of an intraocular lens.
H Correction of unilateral aphakia with unilateral glasses is usually con-traindicated because it result in aniseikonia of approximately 25%.
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