Binocular States of the Eyes
The spherical equivalent of a refractive state is defined as the algebraic sum of the spherical component and half of the astigmatic component. Anisometropia refers to any difference in the spherical equivalents between the 2 eyes. Uncorrected anisometropia in children may lead to amblyopia, especially if 1 eye is hyperopic. Although adults may be annoyed by uncorrected anisometropia, they may be intolerant of initial spectacle correction. Unequal image size, or aniseikonia, may occur, and the prismatic effect of the glasses will vary in different directions of gaze, inducing anisophoria (disparate ocular alignment in different directions of gaze). Anisophoria may be more bothersome than aniseikonia for patients with spectacle-corrected anisometropias.
Even though aniseikonia is difficult to measure, anisometropic spectacle correction can be prescribed in such a manner as to reduce aniseikonia. Making the front surface power of a lens less positive can reduce magnification. Decreasing center thickness also reduces magnification. Decreasing vertex distance diminishes the magnifying effect of plus lenses as well as the minifying effect of minus lenses. These effects become increasingly noticeable as lens power increases. Contact lenses may provide a better solution than spectacles for some patients with anisometropia, particularly children, in whom fusion may be possible.
Unilateral aphakia is an extreme example of hyperopic anisometropia arising from refractive ametropia. In the adult patient, spectacle correction produces an intolerable aniseikonia of about 25%; contact lens correction produces aniseikonia of about 7%, which is usually tolerated. If necessary, the clinician may reduce aniseikonia still further by adjusting the powers of contact lenses and simultaneously worn spectacle lenses to provide the appropriate minifying or magnifying effect via the Galilean telescope principle. With the near-universal adoption of intraocular lenses for the correction of aphakia, this problem is now rarely encountered. For further information on the correction of aphakia, see Chapter 6.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.