J Korean Ophthalmol Soc.  1982 Sep;23(3):613-619.

A Study on the Axis of the Astigmatism

• Lee SC
• Choi O

Abstract

Regular astigmatism is the condition in which the refractive power changes gradually from one meridian to the next by uniform increments, and the two principal meridians are placed at right angles to each other. Astigmatism is one of the most important cause that is concerned with visual disturbance. When there are no serious deterioration of the visual acuity and no symptoms of asthenopia astigmatic errors do not require correction. But if either of those two conditions is present, the errors should be corrected by lenses. A false position of the cylinder axis in a corrected lens creates a new astigmatism. The larger the error in the position of the correcting cylinder, the larger is the amount of the resultant astigmatic error. The axis of new astigmatism does not coincide with the axis of the cylinder lens nor the original astigmatism of the eye. When the amount of astigmatism is large, the axial error may impair the visual acuity. The two kinds of errors in position of axis that caused respectively a blur of letters or visual acuity of 20/30 on Snellen chart by rotating the trial cylinder in the trial frame, and the reduced amount of cylinder power that caused blur of letters on Snellen chart were studied in 210 eyes with astigmatism of less than 2 diopters. The results were as follow: 1. When the patients recognized blurring by rotating the trial cylinder, the average error in position of axis was respectively 18 degrees +/- 0.42 in cylinder power of 0.25D - 0.50D, 11 degrees +/- 0.29 in 0.75D - 1.00D, 9 degrees +/- 0.29 in 1.25D - 1.50D and 4 degrees +/- 0.24 in 1.75D - 2.00D. 2. At the visual acuity of 20/30 by rotating the trial cylinder, the average error in position of axis was respectively 70 degrees +/- 0.22 in cylinder power of 0.25D - 0.50D, 29D +/- 0.79 in 0.75D - 1.0DD, 17 degrees +/- 0.37 in 1.25D - 1.50D and go +/- 0.36 in 1.75D - 2.00D. 3. In astigmatism of less than 1.50D, the error in position of axis in against-the-rule astigmatism was significantly smaller than that in with-the-rule or that in oblique when the patients, recognized blurring by rotating the trial cylinder. 4. In astigmatism of less than 1.50D, the error in position of axis in against-the rule astigmatism was significantly smaller than that in with-the-rule or that in oblique at the visual acuity of 20/30 by rotating the trial cylinder. 5. In astigmatism of less than 1.50D, the larger the amount of spherical lens, the larger was the error in position of axis when the patients recognized blurring by rotating the trial cylinder. 6. In astigmatism of less than 1.50D, the larger the amount of spherical lens, the larger was the error in position of axis at the visual acuity of 20/30 by rotating the trial cylinder. 7. When the patients recognized blurring by reducing cylinder power, the reduced cylinder power was respectively 0.19D +/- 0.01 in cylinder power of 0.25D - 0.50D, 0.29D +/- 0.02 in 0.75D - 1.00D, 0.35D +/- 0. 02 in 1.25D - 1.50D and 0.38D +/- 0.01 in 1.75 - 2.00D. 8. In cylinder power of 1.25D - 1.50D, the reduced amount of cylinder power that caused blurring was significantly smaller in against-the-rule astigmatism than in with-the-rule.

MeSH Terms

Asthenopia
Astigmatism*
Axis, Cervical Vertebra*
Humans
Meridians
Visual Acuity