Non-Orthogonal Corneal Astigmatism among Normal and Keratoconic Brazilian and Chinese populations

ABSTRACT

Purpose: To investigate the prevalence of non-orthogonal astigmatism among normal and keratoconic Brazilian and Chinese populations.

Methods: Topography data were obtained using the Pentacam High Resolution (HR) system ^® from 458 Brazilian (aged 35.6 ± 15.8 years) and 505 Chinese (aged 31.6 ± 10.8 years) eyes with no history of keratoconus or refractive surgery, and 314 Brazilian (aged 24.2 ± 5.7 years) and 74 Chinese (aged 22.0 ± 5.5 years) keratoconic eyes. Orthogonal values of optical flat and steep powers were determined by finding the angular positions of two perpendicular meridians that gave the maximum difference in power. Additionally, the angular positions of the meridians with the minimum and maximum optical powers were located while being unrestricted by the usual orthogonality assumption. Eyes were determined to have non-orthogonal astigmatism if the angle between the two meridians with maximum and minimum optical power deviated by more than 5° from 90°.

Results: Evidence of non-orthogonal astigmatism was found in 39% of the Brazilian keratoconic eyes, 26% of the Chinese keratoconic eyes, 29% of the Brazilian normal eyes and 20% of the Chinese normal eyes.

Conclusions: The large percentage of participants with non-orthogonal astigmatism in both normal and keratoconic eyes illustrates the need for the common orthogonality assumption to be reviewed when correcting for astigmatism. The prevalence of non-orthogonality should be considered by expanding the prescription system to consider the two power meridians and their independent positions.

KEYWORDS:

• Cornea
• astigmatism
• non-orthogonal astigmatism
• optical power

Acknowledgments

We thank Dr. Steve Jones, from the Ocular Biomechanics Group at the University of Liverpool, and Mrs. Lynn White, the clinical director of UltraVision CLPL, for their comments, which greatly improved the revised version of this manuscript.

Declaration of interest

All authors of this article declare that they have no conflict of interest.

Additional information

Funding

This work was funded by an Innovate UK Knowledge Transfer Partnership program grant 009521/UVP016.