REFRACTIVE EWAP DECEMBER 2022 19 That point in space is a simple concept and probably more important than defining the whole axis. Positioning an IOL at the pupil center can be tricky because the pupil is dilated intraoperatively. The visual axis, actually defined by nodal points, is even trickier. “How do I know where the nodal point is? Many assume the corneal light reflex (CLR) defines the visual axis. We show that the CLR can be a good marker, but it’s not actually the visual axis. Why keep thinking of axes when these points in space are more consistent, applicable, and relevant? We defined the specific coaxially sighted corneal light reflex (CSCLR) when the patient is actually looking at it as a patient (or subject) fixated coaxially sighted light reflex (SF-CSCLR). “It’s a leap to call the SF-CSCLR the visual axis, thus it would be an additional leap to define the distance between the SF-CSCLR and the pupil center as angle kappa. Nevertheless, the spatial relationship between the SF-CSCLR and the pupil center is important, so we called it chord mu. This has subsequently been dubbed the ‘Chang-Waring chord’ and more recently chord kappa,” he said. Impact on outcomes Does considering chord kappa impact outcomes? Not really, Dr. Chang said. “Preoperative chord kappa does not appear to correlate with postoperative lens position or outcomes. Where the lens sits relative to the SF-CSCLR is what matters.” Dr. Chang said the visual axis is “that mythical place where everyone wants alignment, but no one knows really knows how to find it. We can find where the SF-CSCLR lies, and most studies that have touted alignment to the ‘visual axis’ are actually aligning to the SF-CSCLR.” How you identify the precise light reflex with your microscope can be tricky as well, he continued. A microscope would have to offer a light source coaxial with the surgeon’s view; the Lumera (Carl Zeiss Meditec) offers two of them. Then a patient would just have to fixate on one of the coaxial lights. “If viewed correctly, the SF-CSCLR is reproducible preoperatively, intraoperatively, and postoperatively. This definitive point in space can center the eye’s coordinate system, so you could start aligning things and studying them consistently,” he said. What happens with decentration? You induce astigmatism, Dr. Chang said, which is correctable. Several studies have looked at angle kappa, which typically refers to the physical quantity of chord kappa and have not supported its impact on outcomes of presbyopia-correcting IOLs. One recent study concluded “the magnitude of preoperative angle kappa had no apparent effect on the refractive, visual acuity or subjective (visual disturbances, quality of vision, satisfaction) clinical outcomes with this trifocal IOL.”3 Another study included a larger dataset and found no difference with outcomes of preoperative angle kappa in patients who received a multifocal IOL. Despite these and other findings, there are still studies being published that have concluded that angle kappa can affect the visual quality after multifocal IOL implantation.4 Dr. Chang went back to his point that with different definitions and usages, it’s difficult to draw conclusions about the influence of “angle kappa” across the board. Rather, as he and Dr. Waring concluded in their paper, a “subject-fixated coaxially sighted corneal light reflex avoids the shortcomings of current ocular axes for clinical application and may contribute to better consensus in the literature and improved patient outcomes.” Myoung Joon Kim, MD Renew Seoul Eye Centre 528 Teheran-ro, Gangnam-gu, Seoul mjmjkim@gmail.com ASIA-PACIFIC PERSPECTIVES R ecently, when I take questions about angle kappa from my colleagues, I find most of them are about the outcomes of multifocal IOLs. The questions are like if large angle kappa can be a cause of visual dissatisfaction after multifocal IOL implantation. I think it is more likely that other factors rather than angle kappa are causative in most real-world cases. I usually include terminology changes in my answer. Instead of angle, I use displacement length or distance in millimeters or micrometers. Chord mu means two-dimensional displacement of the entrance of the pupil center from the subject- fixated coaxially sighted corneal light reflex (SF - CSCLR). New terms are chord mu, chord length, chord distance, CW (Chang- Waring) chord, and so on. SF - CSCLR is a good, reliable reference point when aligning multifocal IOLs. Angle kappa seems a little overemphasized with respect to its impact on the visual outcome of cataract/refractive surgery. However, I mention angle kappa to my patients when apparent chord length is over 0.5 mm, which is about 3.75˚ of angle kappa. Some devices measure apparent distance, while some measure actual distance. Mean apparent distance is 0.3 mm, which is bigger than mean actual distance (0.2 mm) due to corneal magnification. Editors’ note: Dr. Kim declared no relevant financial interests.
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