Magnitude of astigmatism – A comparison between eyes
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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