Research Article: Distribution of Posterior Corneal Astigmatism According to Axis Orientation of Anterior Corneal Astigmatism

Date Published: January 27, 2015

Publisher: Public Library of Science

Author(s): Toshiyuki Miyake, Kimiya Shimizu, Kazutaka Kamiya, Chen-Wei Pan.

http://doi.org/10.1371/journal.pone.0117194

Abstract

To investigate the distribution of posterior corneal astigmatism in eyes with with-the-rule (WTR) and against-the-rule (ATR) anterior corneal astigmatism.

We retrospectively examined six hundred eight eyes of 608 healthy subjects (275 men and 333 women; mean age ± standard deviation, 55.3 ± 20.2 years). The magnitude and axis orientation of anterior and posterior corneal astigmatism were determined with a rotating Scheimpflug system (Pentacam HR, Oculus) when we divided the subjects into WTR and ATR anterior corneal astigmatism groups.

The mean magnitudes of anterior and posterior corneal astigmatism were 1.14 ± 0.76 diopters (D), and 0.37 ± 0.19 D, respectively. We found a significant correlation between the magnitudes of anterior and posterior corneal astigmatism (Pearson correlation coefficient r = 0.4739, P<0.001). In the WTR anterior astigmatism group, we found ATR astigmatism of the posterior corneal surface in 402 eyes (96.6%). In the ATR anterior astigmatism group, we found ATR posterior corneal astigmatism in 82 eyes (73.9%). Especially in eyes with ATR anterior corneal astigmatism of 1 D or more and 1.5 D or more, ATR posterior corneal astigmatism was found in 28 eyes (59.6%) and 9 eyes (42.9%), respectively. WTR anterior astigmatism and ATR posterior astigmatism were found in approximately 68% and 91% of eyes, respectively. The magnitude and the axis orientation of posterior corneal astigmatism were not constant, especially in eyes having high ATR anterior corneal astigmatism, as is often the case in patients who have undergone toric IOL implantation.

Partial Text

In the surgical correction of astigmatism, it is important to accurately determine the magnitude and the axis orientation of the corneal astigmatism. Since the amount of anterior corneal astigmatism is far larger than that of posterior corneal astigmatism, the anterior corneal surface plays a more vital role in astigmatic correction than the posterior corneal surface. [1] Recently, the development of new technologies, such as slit-scanning devices, Scheimpflug devices, and optical coherence tomography, has made possible the quantitative measurement of the posterior corneal curvature in a clinical setting. [2–15] Moreover, a new toric intraocular lens (IOL) nomogram has been recently proposed because of the presence of posterior corneal astigmatism. [16] However, there have so far been no quantitative studies on the distribution of posterior corneal astigmatism that take the axis orientation of anterior corneal astigmatism into consideration. Since the magnitude and the axis orientation of anterior corneal astigmatism are easier to determine with the daily use of an autokeratometer or a manual keratometer, this evaluation of posterior corneal astigmatism according to the individual axis orientation of anterior corneal astigmatism may give us deep insights into astigmatic correction, especially when toric IOL implantation is performed. The purpose of this study is to retrospectively assess the distribution of posterior corneal astigmatism according to the axis orientation of anterior corneal astigmatism in a large cohort of healthy subjects.

The demographics of the study population are summarized in Table 1. The numbers of eyes in the age groups were as follows: 18 to 29 year-old group, 84, 30 to 39 year-old group, 83; 40 to 49 year-old group, 83; 50 to 59 year-old group, 84; 60 to 69 year-old group, 94; 70 to 79 year-old group, 97; and group of 80—year-olds and over, 83 eyes. The mean magnitudes of anterior and posterior corneal astigmatism were 1.14 ± 0.76 diopters (D) (range: 0 to 4.90 D), and 0.37 ± 0.19 D (range: 0 to 1.20 D), respectively. With-the-rule (WTR) astigmatism of the anterior corneal surface was found in 68.4% (416 eyes), while against-the-rule (ATR) astigmatism of the posterior corneal surface was found in 91.3% (555 eyes) of the entire study population. We found a significant correlation between the magnitudes of anterior and posterior corneal astigmatism (Pearson correlation coefficient r = 0.4739, P<0.001). Figs. 1 and 2 show the respective distributions of anterior and posterior corneal astigmatism in each age group. We found a tendency for a high prevalence of ATR anterior corneal astigmatism with aging, whereas most eyes in all age groups showed ATR posterior corneal astigmatism. In the present study, our results demonstrated that approximately 68% of the eyes of the entire study population had WTR astigmatism of the anterior corneal surface, and that approximately 91% of eyes had ATR astigmatism of the posterior corneal surface. These ATR findings were in line with previous findings of posterior corneal astigmatism, as shown in Table 2. [3–9, 13] Moreover, our results also showed a tendency for a high prevalence of ATR anterior corneal astigmatism with aging, whereas most eyes showed ATR posterior corneal astigmatism in all age groups, which was in agreement with previous reports on age-related changes in corneal astigmatism. [8–10] The mean change in ATR astigmatism of the anterior corneal surface (0.18 D in 5 years) was far greater than that in WTR astigmatism of the posterior corneal surface (0.02 D in 5 years), [10] suggesting that the shape of the posterior corneal surface was relatively unchanged by aging.   Source: http://doi.org/10.1371/journal.pone.0117194