Eyeworld

SEPTEMBER 2022 | EYEWORLD | 57 R Contact Chang: dchang@empireeyeandlaser. com; www.empireeyeandlaser.com Koch: dkoch@bcm.edu References 1. Chang DH, Waring GO. The subject-fixated coaxially sighted corneal light reflex: a clinical marker for centration of refrac- tive treatments and devices. Am J Ophthalmol. 2014;158:863–874. 2. Prakash G, et al. Predictive fac- tor and kappa angle analysis for visual satisfactions in patients with multifocal IOL implantation. Eye (Lond). 2011;25:1187–1193. 3. Sandoval HP, et al. The effects of angle kappa on clinical results and patient-reported outcomes after implantation of a trifocal intraocular lens. Clin Ophthal- mol. 2022;16:1321–1329. 4. Fu Y, et al. Influence of angle kappa and angle alpha on visual quality after implanta- tion of multifocal intraocular lenses. J Cataract Refract Surg. 2019;45:1258–1264. 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 visu- al 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 out- comes? 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 mythi- cal 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 re- producible 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 sub- jective (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 pre- operative angle kappa in patients who received a multifocal IOL. Despite these and other find- ings, 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 "an- gle kappa" across the board. Rather, as he and Dr. Waring concluded in their paper, a "sub- ject-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." Overall, Dr. Chang said that aligning to the SF-CSCLR or considering preoperative chord kappa may be "splitting hairs," which is why he doesn't typically place too much emphasis it. "We defined it, and my goal is to help in- dustry put it in their devices, so surgeons don't have to think through the intricacies of the topic," he said. "I personally still center on the SF-CSCLR as best as I can, but I know that de- centration is only one of many possible causes of residual astigmatism." Douglas Koch, MD, who defined angle kappa as the distance between the corneal light reflex when the patient is coaxially fixating on a light source and the center of the entrance of the pupil, said that it has "yet to be proven" whether angle kappa is an important issue. "That said, Dr. Chang's and Dr. Waring's article was hugely helpful in defining a measurement that we could use to study this topic," he said. "However, I'm not sure that angle kappa is as important as we once thought that it was," Dr. Koch said. "I think it is important for people to continue to look at it, and if I see a large angle kappa, I discuss it with patients and tell them that we don't know, and it might be an issue. But I am not aware of any studies that convincingly correlate angle kappa and out- comes, and I've yet to see visual problems from putting a presbyopia-correcting lens in these patients." continued from page 54

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