Correlation of major components of ocular astigmatism in myopic patients

doi:10.1016/j.clae.2015.06.005

Contact Lens and Anterior Eye

Volume 39, Issue 1, February 2016, Pages 20-25

https://doi.org/10.1016/j.clae.2015.06.005 Get rights and content

Highlights

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The strongest correlation was observed between refractive and corneal astigmatism.
•
A significant correlation was found between refractive astigmatism and ORA.
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Less significant correlation was observed between corneal astigmatism and ORA and possible compensation of corneal astigmatism by ORA.
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Identifying the type of astigmatism and preoperative measurement of ocular residual astigmatism is recommended prior to any refractive procedure.

Abstract:

Purpose

To investigate the correlation of major components of ocular astigmatism in myopic patients in an academic hospital.

Methods

This cross-sectional study was conducted on 376 eyes of 188 patients who were referred to Farabi Eye Hospital for refractive surgery. Preoperative examinations including refraction and corneal topography were performed for all candidates to measure refractive and corneal astigmatism. Ocular residual astigmatism was calculated using vector analysis. Pearson’s correlation and ANOVA analysis were used to evaluate the strength of the association between different types of astigmatism. Both eyes were defined as cluster and the Generalized Estimating Equations (GEE) analysis were performed.

Results

Mean age of 119 women (63.3%) and 69 men (36.7%) was 27.8 ± 5.7 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 amount of ORA was 0.65 ± 0.36 D.There was a significant correlation between ORA and refractive astigmatism(r = 0.23, p < 0.001), corneal and refractive astigmatism (r = 0.91, p < 0.001) and a weak correlation between ORA and corneal astigmatism (r = 0.13, p = 0.014). There was a significant correlation between J0 and J45 values of ORA and corneal astigmatism (p < 0.001).

Conclusion

There is a significant correlation between ORA and refractive astigmatism, refractive and corneal astigmatism and a weak correlation between ORA and corneal astigmatism in refractive surgery candidates. Identifying the type of astigmatism and preoperative measurement of ocular residual astigmatism is highly recommended prior to any refractive surgery, especially in cases with significant astigmatism.

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.

Section snippets

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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View full text

Correlation of major components of ocular astigmatism in myopic patients

Highlights

Abstract:

Purpose

Methods

Results

Conclusion

Introduction

Section snippets

Patients and methods

Pre-operative examinations

Statistical analysis

Results

Discussion

J. Cataract Refract. Surg.

Astigmatism in Chinese preschool children: prevalence, change, and effect on refractive development

Br. J. Ophthalmol.

Characteristics of astigmatism in a population of schoolchildren, Dezful, Iran

Optom. Vis. Sci.

High prevalence of astigmatism in the 40 to 64 year old population of Shahroud, Iran

Clin. Exp. Ophthalmol.

Refractive errors in a rural Chinese adult population: the Handan eye study

Ophthalmology

Prevalence of refractive errors among school children in Northeastern Iran

Ophthalmic. Physiol. Opt.

Refractive errors in an elderly Japanese population: the Tajimi study

Ophthalmology

The prevalence of refractive errors and its determinants in the elderly population of Mashhad, Iran

Ophthal. Epidemiol.

Prevalence of refractive error in rural Myanmar: the Meiktila eye study

Ophthalmology

High prevalence of refractive errors in a rural population: ‘Nooravaran Salamat’ mobile eye clinic experience

Clin. Exp. Ophthalmol.

Refractive error in urban and rural adult Chinese in Beijing

Ophthalmology

Genes and environment in refractive error: the twin eye study

Invest. Ophthalmol. Vis. Sci.

The visual and functional impacts of astigmatism and its clinical management

Ophthal. Physiol. Opt.

New method of targeting vectors to treat astigmatism

J. Cataract Refract. Surg.

The association between residual astigmatism and refractive errors in a population-based study

J. Refract. Surg.

Astigmatism and its components in 6-year-old children

Invest. Ophthalmol. Vis. Sci.

Corneal and refractive astigmatism in adults: a power vectors analysis

Optom. Vis. Sci.