Cylindrical Correcting Lenses and the Far Point Concept
The far point principles used in the correction of hyperopia and myopia are also employed in the correction of astigmatism with spectacle lenses. However, in astigmatism, the required lens power must be determined separately for each of the 2 principal meridians.
Cylinders in spectacle lenses produce both monocular and binocular distortion. The primary cause is meridional aniseikonia—that is, unequal magnification of retinal images in the various meridians. Although aniseikonia may be decreased by iseikonic spectacles, such corrections may be complicated and expensive, and most practitioners prefer to prescribe cylinders according to their clinical judgment. Clinical experience also suggests that adult patients vary in their ability to tolerate distortion, whereas young children always adapt to their cylindrical corrections.
The following guidelines may prove helpful in prescribing astigmatic spectacle corrections:
For children, prescribe the full astigmatic correction at the correct axis.
For adults, try the full correction initially. Give the patient a “walking-around” trial with trial frames before prescribing, if appropriate. (The experience of seeing the effects of motion is an important part of this process—just sitting in a chair with the proposed prescription in trial frames is not an adequate test.) Inform the patient about the need for adaptation. To reduce distortion, use minus cylinder lenses (now standard) and minimize vertex distance.
Because spatial distortion from astigmatic spectacles is a binocular phenomenon, occlude 1 eye to verify that spatial distortion is the cause of the patient’s difficulty.
If necessary, reduce distortion by rotating the axis of the cylinder toward 180° or 90° (or toward the old axis) and/or reduce the cylinder power. Adjust the sphere to maintain spherical equivalent, but rely on a final subjective check to obtain the most satisfactory visual result.
If distortion cannot be reduced sufficiently, consider contact lenses or iseikonic corrections.
For a more detailed discussion of the problem of, and solutions for, spectacle correction of astigmatism, see the reference.
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.