Correlation of major components of ocular astigmatism in myopic patients
Introduction
Astigmatism is a common optical disorder and exists in most human eyes in subtle amounts. Numerous studies have reported the prevalence of astigmatism in different ages [1], [2], [3], [4], [5], [6], [7], rural and urban populations [8], [9], [10] and ethnic groups [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Previous studies have reported the prevalence of astigmatism from 11.3% up to 70% in related studies [5], [7], [8]. Different factors have been suggested in the development of astigmatism including age, gender, ethnicity, genetic predisposition, eye lid pressure and unequal tension of extraocular muscles on the cornea [11], [12].
Refractive or total astigmatism is comprised of anterior corneal and ocular residual astigmatism (ORA). Anterior corneal curvature is the main component of ocular astigmatism and causes the greatest optical effect; however, ORA is considered as the non-corneal component of total refractive astigmatism and is often caused by internal optics of the eye, especially the crystalline lens [12]. ORA cannot be measured independently and is calculated by the vectorial difference between the corneal and refractive astigmatism [13].
Since, ORA is not calculated directly, it seems to have an special importance in patients undergoing refractive surgery. However, the relationship between ORA and other types of astigmatism is not clear [14]. Many studies have evaluated the correlation between refractive and corneal astigmatism and found that refractive astigmatism is mainly corneal in origin [15], [16], [17]. However, results regarding the contribution of ORA have been conflicting, with some studies finding a significant correlation between ORA and total astigmatism in normal astigmats [15] and others stating no significant relation between the two entities [18]. Astigmatism affects visual acuity and contrast sensitivity and compared to other refractive errors, is more difficult to treat. Additionally, with the advent of excimer laser techniques and the unpredictability of visual outcome after laser refractive surgeries in eyes with astigmatism [19] and lower efficiency of these surgeries in correcting astigmatism that is mainly due to internal optics of the eye [20], it is crucial to determine the pattern of astigmatism and its components preoperatively. The purpose of this study is to evaluate the correlation between refractive, corneal and ocular residual astigmatism in patients undergoing refractive surgery.
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Patients and methods
This cross-sectional study was conducted on 376 eyes of 188 patients who referred to Farabi Eye Hospital for refractive surgery. The tenets of the Declaration of Helsinki were followed for all study procedures. Patients were included in this study if they were aged 20 years or older, had regular astigmatism of ≤6 D, and best-corrected visual acuity (BCVA) of 6/6 or better.
The exclusion criteria included any previous eye surgeries, trauma to the eye, corneal scar, corneal or lens opacity
Pre-operative examinations
Refractive astigmatism was determined by manifest refraction using Zywave aberrometry predicted phoropter refraction (PPR) (Bausch & Lomb, Rochester, NY, USA). Zywave’s estimate of refractive error was then compared to subjective manifest refraction and confirmed with HEINE BETA 200 retinoscope (Herrsching, Germany) and Topcon RM8800 auto-refractometer (Tokyo, Japan). Repeatability and validity of Zywave’s refractive error measurements have been confirmed [21].
Each patient’s best visual acuity
Statistical analysis
In this study, Pearson correlation analysis and scatter plots were used for investigating the correlation between refractive, corneal and ocular residual astigmatism. The analysis of variance (ANOVA) was used to compare the mean power of astigmatism in different myopic groups.
Both eyes were defined as cluster and the Generalized Estimating Equations (GEE) analysis were performed. The data analysis was performed using SPSS software version 20. A p value of less than 0.05 was considered
Results
Of 188 patients (376 eyes), 119 (63.3%) were female. Mean age of patients was 27.8 ± 5.7 years (range, 20–52 years). Mean refractive error based on spherical equivalent was −3.59 ± 1.95 D (range, −0.54 to −10.22 D). Mean refractive and corneal astigmatism was 1.97 ± 1.3 D and 1.85 ± 1.01 D, respectively. Mean ocular residual astigmatism was 0.65 ± 0.36 D (Table 1).
A Pearson correlation coefficient was computed to assess the relationship between the amounts of ORA, refractive and corneal astigmatism. As
Discussion
In this study, we found a significant correlation between ORA, refractive and corneal astigmatism and the strongest correlation existed between corneal and refractive astigmatism.
Many studies have emphasized on the correlation between corneal and refractive astigmatism. In 1996 Mckendrick reported a significant correlation between corneal and total refractive astigmatism using vector analysis [26]. Our results regarding the correlation between corneal and total refractive astigmatism were also
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