Date Published: September 7, 2016
Publisher: Public Library of Science
Author(s): Qinghe Jing, Yating Tang, Dongjin Qian, Yi Lu, Yongxiang Jiang, Wei Li.
To evaluate the characteristics of the posterior corneal surface in patients with high myopia before cataract surgery.
We performed a cross-sectional study at the Eye and ENT Hospital of Fudan University, Shanghai, China. Corneal astigmatism and axial length were measured with a rotating Scheimpflug camera (Pentacam) and partial coherence interferometry (IOLMaster) in a high-myopia study group of 167 eyes (axial length ≥ 26 mm) and a control group of 150 eyes (axial length > 20 mm and < 25 mm). Total corneal astigmatism and anterior corneal astigmatism values were higher in the high-myopia group than in the control group. There was no significant difference in posterior corneal astigmatism between the high-myopia study group and the control group. In the study group, the mean posterior corneal astigmatism (range 0 – −0.9 diopters) was –0.29 diopters (D) ± 0.17 standard deviations (SD). The steep corneal meridian was aligned vertically (60°–120°) in 87.43% of eyes for the posterior corneal surface, and did not change with increasing age. There was a significant correlation (r = 0.235, p = 0.002) between posterior corneal astigmatism and anterior corneal astigmatism, especially when the anterior corneal surface showed with-the-rule (WTR) astigmatism (r = 0.452, p = 0.000). There was a weak negative correlation between posterior corneal astigmatism and age (r = –0.15, p = 0.053) in the high-myopia group. Compared with total corneal astigmatism values, the anterior corneal measurements alone overestimated WTR astigmatism by a mean of 0.27 ± 0.18 D in 68.75% of eyes, underestimated against-the-rule (ATR) astigmatism by a mean of 0.41 ± 0.28 D in 88.89% of eyes, and underestimated oblique astigmatism by a mean of 0.24 ± 0.13 D in 63.64% of eyes. Posterior corneal astigmatism decreased with age and remained as ATR astigmatism in most cases of high myopia. There was a significant correlation between posterior corneal astigmatism and anterior corneal astigmatism when anterior corneal astigmatism was WTR. If posterior corneal astigmatism is not accounted for when selecting toric intraocular lenses for high-myopia patients, the use of anterior corneal astigmatism measurements alone will lead to overestimation of WTR astigmatism and underestimation of ATR and oblique astigmatism.
Myopia is the most common human eye disorder worldwide, affecting 85%–90% of young adults in Asian countries such as Singapore and Taiwan . Of rural Chinese, 26.7% of those ≥ 30 years old and 22.9% of those ≥ 40 years old are affected [2,3]. In China, the prevalence of high myopia, defined as eyes with spherical equivalent refractive error of at least −6.00 D and axial length of at least 26.0 mm , is ~1.3%–26% [2,3]. In 2004, Tuft and Bunce  found an association between increased axial length and lower mean age of cataract surgery. As there is an increasing prevalence of cataract surgery in younger age groups, it is important to understand the relationship between high myopia and cataracts. Pan et al.  confirmed an association between myopia and nuclear and posterior subcapsular (PSC) cataracts. Many cataract patients with high myopia also have astigmatism, and treatment with a toric intraocular lens (IOL) has been effective in patients with high myopic astigmatism [7,8]. However, previous methods of calculating toric IOL power that are based on anterior corneal astigmatism measurements have led to residual astigmatism and refractive errors in a small percentage of patients . As total corneal astigmatism includes both anterior and posterior corneal astigmatism, the total corneal astigmatism cannot be optimally calculated with anterior corneal astigmatism measurements and the keratometric index alone. Previous studies have reported a mean magnitude of posterior corneal astigmatism of −0.30 D . Furthermore, the anterior corneal surface shifted from with-the-rule (WTR) to against-the-rule (ATR) astigmatism with age, whereas the posterior corneal surface continued to show ATR astigmatism in most cases [10–16]. Therefore, patient-specific posterior corneal astigmatism measurements are required to estimate total corneal astigmatism. As the characteristics of corneal astigmatism in high myopia remain unknown, we performed a cross-sectional study of anterior and posterior corneal astigmatism measurements in high-myopia patients and compared these values with those obtained in controls.
In this cross-sectional study, we recruited patients scheduled for cataract surgery at the Eye and ENT Hospital of Fudan University, Shanghai, China, between September 1 and December 31, 2014. Patients were classified into two groups, cataract patients with high myopia and cataract patients without myopia. Axial length and corneal biometric data were collected from all patients. Patients with a history of previous ocular surgery, corneal disease, uveitis, glaucoma, wearing contact lenses within the previous 2 weeks, or age younger than 40 years were excluded. The study was approved by the Human Research Ethics Committee of the Eye and ENT Hospital of Fudan University and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from each patient.
The study included 317 eyes of 197 patients. There were 167 eyes of 98 patients in the high-myopia group and 150 eyes of 99 patients in the control group. The patient demographics are listed in Table 1. The Km values of the anterior and posterior corneal surfaces were lower in the high-myopia group than in the control group (Table 2). The magnitude of astigmatism of the total cornea and the anterior corneal surface was higher in the high-myopia study group than in the control group, whereas the posterior corneal astigmatism showed no statistically significant difference between the two groups. The anterior corneal surface astigmatism was mainly WTR in both groups (47.9% of eyes in the high-myopia group and 58% of eyes in the control group), whereas the posterior corneal surface astigmatism was mainly ATR in both groups (87.43% of eyes in the high-myopia group and 88% of eyes in the control group). In the high-myopia study group, there was no statistically significant difference for any corneal parameter between the axial-length subgroups (Table 3).
The study of corneal astigmatism is greatly enhanced by improvements in the accuracy of the measuring instruments. The mean magnitude of posterior corneal astigmatism is approximately –0.3 D and the steep posterior surface corneal meridian is aligned vertically in more than 85% of eyes [10,13–15]. However, the characteristics of posterior corneal astigmatism in high myopia are less well known. Our study found that in high myopia, the mean posterior corneal astigmatism was −0.29 ± 0.17 D. Furthermore, the posterior corneal steep meridian was aligned vertically in 87.43% of eyes and did not change with advancing age. There was a positive correlation between posterior corneal astigmatism and anterior corneal astigmatism, especially when the anterior corneal surface showed WTR astigmatism. Finally, anterior corneal measurements alone were inaccurate in estimating total corneal astigmatism.