Eyeworld

70 | EYEWORLD | SPRING 2026 R EFRACTIVE by Liz Hillman Editorial Co-Director About the sources Stephen D. Klyce, PhD, FARVO Adjunct Professor of Ophthalmology Icahn School of Medicine at Mount Sinai Port Washington, New York Marguerite B. McDonald, MD, FACS Clinical Professor of Ophthalmology NYU Langone Medical Center Clinical Professor of Ophthalmology Tulane University Health Sciences Center New Orleans, Louisiana OCLI Vision Oceanside, New York William Trattler, MD Director of Cornea Center for Excellence in Eye Care Miami, Florida D etecting irregular astigmatism is less about classification and more about recognition. As corneal imaging technologies continue to evolve, identifying subtle irregularities early has taken on greater importance—particularly in the context of cataract surgery planning and patient satisfaction. Understanding how these findings are detected, and why they matter clin- ically, is becoming an essential part of preopera- tive evaluation and occasionally postop when a patient is complaining of poor visual quality. Topography has been able to identify irregular astigmatism for decades, and even before that, said William Trattler, MD, manual keratometry, if it didn't line up properly, was a diagnostic tool. Stephen D. Klyce, PhD, FARVO, and Marguerite B. McDonald, MD, FACS, said in an email to EyeWorld that newer, "advanced" corneal imaging technologies can actually be less sensitive in detecting irregular astigmatism compared to Placido- based technologies. "When evaluating a patient with astig- matism, subtle irregularities are best detected with Placido-based devices, which are 20 times more sensitive than tomography-based devic- es," Dr. Klyce and Dr. McDonald said. "Stated differently, anterior corneal maps based on Placido technology are the most reliable and repeatable at detecting early pathology, such as early keratoconus." Dr. Klyce and Dr. McDonald provided a short history of the first correlations of corneal irregularities and vision, as demonstrated by corneal topography in 1996, and how it has evolved since. In that early work, irregularities in the mires were reflected in Placido images. "Small irregularities in the positions of the mires along hemi-meridians were averaged over the central cornea forming an index called the surface regularity index (SRI). 1 Correlation with clinical vision data provided the range of poten- tial visual acuity (PVA) that could be attained with a patient's cornea analyzed with corneal topography (Figure 1)," they said. "After that early work, characterization of irregular astig- matism is typically done by modeling the corne- al surface with the Zernike polynomial series of radial basis functions. Zernike polynomials were adopted by the ANSI and ISO Standards groups to describe the components of the distortions produced by irregular astigmatism. These are generally called the higher order aberrations (HOAs), while the lower order aberrations also Irregular astigmatism: what we're missing— and how to find it continued on page 72 Figure 1. These are the right and left corneas of a patient with keratoconus. Typically asymmetric in progression, the right eye has a surface regularity index (SRI) of 0.51, which correlates to an expected potential visual acuity (PVA) range of 20/10–20/30 (95% confidence interval). The more advanced left eye has an SRI of 1.95 corresponding to a PVA of 20/30–20/100. Source: Stephen D. Klyce, PhD, FARVO, and Marguerite B. McDonald, MD, FACS

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