Magnitude of astigmatism – A comparison between eyes

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Abstract

Purpose

Astigmatism is a highly prevalent refractive error and while studies typically focus to describe the axis symmetry between eyes, little is known about the refractive symmetry. Therefore, this study determined the astigmatic power symmetry between eyes in a large clinic population.

Methods

A clinical chart review was conducted at three optometric practices in the United States, the United Kingdom and Canada and subjective refraction data from 88,891 patients 14–70 years of age who presented with at least −0.25DC refractive astigmatism in at least one eye were included in the analysis. Data were obtained at these practices between January 2014 and March 2017. The overall distribution (%) and magnitude (DC) of astigmatism was determined and refractive differences between eyes were identified.

Results

The mean age of the patients was 42.1 ± 15.9 years and included 51,685 (58%) female and 37,206 (42%) male patients. In this data pool of 177,782 eyes, 10.9% required zero astigmatic correction, while 56.2% had astigmatism of −0.25 to −0.75DC. In total 23.9% of patients presented with astigmatism of at least −0.75DC in only one eye, while the other eye had 0 to −0.50DC. Overall, the difference in astigmatism between eyes was less than −0.75DC for 82.1% of astigmatic patients. For patients who presented with astigmatism of −1.00DC in the right eye, 80.8% of them had an astigmatic prescription of −1.00 ± 0.50DC in the left eye. For an astigmatic prescription of −4.00DC in the right eye, only 40.6% of patients exhibited astigmatism of −4.00DC ± 0.50DC in the left eye.

Conclusions

The majority of patients exhibited a difference in astigmatism between eyes of less than −0.75DC, however the refractive cylinder power symmetry was significantly lower in patients with higher refractive astigmatism.

Introduction

Astigmatism is reported to be the most common refractive error but is rivaled by myopia in certain ethnicities [1]. Astigmatism prevalence is dependent on age, ethnicity, race, and has been associated with spherical ametropia [1], [2], [3], [4], [5], [6]. The prevalence has been studied extensively [1], [2], [3], [4], [5], [6] and none more comprehensively than Hashemi et al. whose meta-analysis included 135 articles on astigmatism [1]. Hashemi et al. concluded that the prevalence of astigmatism in children was 14.9% and 40.4% in adults.

There is evidence that inter-ocular symmetry exists for astigmatism [6], [7], [8] and other ocular parameters. However, studies comparing the symmetry of astigmatism, focus primarily on the axis distribution and less on the refractive cylinder power [9], [10], [11]. In a pool of 5,505 patients who exhibited astigmatism of a least 0.75DC in at least one eye, Young et al. found that 49% of these patients had astigmatism of at least 0.75DC in one eye only [3] and Satterfield also found a surprisingly high percentage (26%) of subjects with unilateral astigmatism amongst the astigmatic patient cohort [12]. This seemed like an inordinately high prevalence and has clinical implications when prescribing toric soft lenses where the choice of lens design could incorporate prism ballast in the toric lens, thus creating potential binocular imbalance. A previous report by Luensmann et al. [2] described the spectacle prescription data of 101,973 patients which included ametropic and emmetropic conditions and the prevalence of astigmatism but did not report on the symmetry between eyes. This presented an opportunity to determine the level of symmetry of refractive astigmatism between the right and left eyes in this patient cohort and to calculate the proportion of the patients who have astigmatism in one eye (unilateral astigmatism) only as a secondary objective.

Section snippets

Methods

In a retrospective chart review, de-identified spectacle prescription data were collected from three clinical eye care institutions including, the Eye Care Center with multiple locations in Alabama (United States), a large Optometry office in Hereford (United Kingdom), and the School of Optometry & Vision Science in Waterloo (Canada). Details on the method of subjective refraction to determine the spectacle prescription were not recorded and may have varied between practitioners. Ethics

Patient cohort

The astigmatic group consisted of 88,891 patients (87.2% of the entire sample) who presented with astigmatism of at least −0.25DC in at least one eye and included 51,685 (58%) female and 37,206 (42%) male patients. The average age of the patients was 42.1 ± 15.9 years. Based on the eye with the larger absolute value of the spherical equivalent refraction this study included 63,837 (71.8%; CI: 71.5–72.1) myopes, 23,752 (26.7%; CI: 26.4–27.0) hyperopes and 1,302 (1.5%; CI: 1.4–1.5) emmetropes.

Magnitude of refractive astigmatism per eye

The

Discussion

The threshold is critical when comparing data on ametropia from different publications. In a meta-analysis by Hashemi they found that the most commonly reported threshold for astigmatism was −0.50DC or −0.75DC [1]. Even though a low prevalence of 0.3% [14] and 0.7% [15] was found for children in Vietnam and Thailand, other countries such as China reported between 25% [16] and 41% [17] for similar age groups for astigmatism of at least −0.75DC. Data from the US showed that 23% of young adults

Conclusion

The analysis of this large population of astigmatic clinic patients has revealed that there is high symmetry of astigmatic power between right and left eyes indicating that more than 4 in 5 patients exhibit a difference in astigmatism between eyes of no more than −0.50DC. Astigmatic power symmetry was high in lower astigmatic powers; however, it gradually decreased with increasing levels of astigmatism. Approximately 1 in 4 patients exhibited astigmatism of at least −0.75DC in one eye only,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This project was funded by the Centre for Ocular Research & Education (CORE), University of Waterloo, Canada.

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