EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/1538634
FALL 2025 | EYEWORLD | 41 R BEYOND THE ROUTINE by Ellen Stodola Editorial Co-Director About the physicians Peter Hersh, MD Cornea and Laser Eye Institute Rutgers New Jersey Medical School Princeton University Teaneck, New Jersey Beeran Meghpara, MD Director of Refractive Surgery Co-Chief of the Cornea Service Wills Eye Hospital Philadelphia, Pennsylvania There are many great diagnostic devic- es available now to measure refraction, Dr. Meghpara said, adding that two popular devices are the iTrace (Tracey Technologies) and the OPD-Scan III (Nidek). These can provide a good starting point to use to refine the refraction in the phoropter. "Getting a good starting point is one of the keys to making this an efficient and accurate process. You could even go old school and use a retinoscopy to get a starting point," he said. An autorefractor is not as good in the circumstance of keratoconus, Dr. Hersh said, be- cause of the higher order aberrations, so there are a few instruments that are good adjuncts to help get started. The first is corneal topography because it can give you a map of the corneal surface and show you the optical irregularities K eratoconus is a unique disease in that its consequent vision loss, in most of its stages, involves corneal optics, according to Peter Hersh, MD. "The biomechanical weakness of the cornea in keratoconus causes a focal misshaping of the cornea, compromising its optical quality," he said. The cornea supplies two-thirds of the total refractive power of the eye, so having distortions in the corneal optical surface will diminish vision. Dr. Hersh and Beeran Meghpa- ra, MD, discussed finding the right refraction in patients with keratoconus, what to take into account, and considerations if patients are seek- ing surgery. In a perfect eye with a perfect cornea, re- fraction involves the normal Cartesian optics of sphere and cylinder, and those are called lower order aberrations, Dr. Hersh said, noting that these define normal refraction. The keratoconic cornea, because it is optically distorted, also has higher order aberrations. "When I'm discussing these with patients, I explain it's akin to visu- al static. It's like having static on a television. These are aberrations that are different from the standard sphere and cylinder that you have in a normal optical system. These impact patients' functional vision because they can't be corrected with spectacles," he said. "Spectacles can only correct sphere and cylinder, that is myopia, hyperopia, and astigmatism." It's important to consider these higher order aberrations, and it's important to determine a good starting point, Dr. Hersh said. Finding the right refraction can be tricky in keratoconus patients, Dr. Meghpara said, because they often have high amounts of astig- matism, and additionally, the astigmatism is irregular. "A lot of times the amount of astigma- tism in the cornea from the keratoconus doesn't necessarily match up to the amount they'll get in a refraction," he said. The amount of cylin- der we measure in glasses often isn't as high as what you're getting in the cornea, he said. The surgeon needs to know that you don't necessar- ily want to give all the astigmatism that you're measuring in a glasses prescription because it's so high, and it can be intolerable to wear. When the cornea is not routine: finding the right refraction in patients with keratoconus continued on page 42 Videokeratography over the pupil suggests refractive minus cylinder axis at approximately 50 degrees Source: Peter Hersh, MD