Clearing up angle kappa

 

Clearing up angle kappa

Refractive
September 2022/EYEWORLD

by Liz Hillman
Editorial Co-Director

Angle kappa—“People don’t fully understand it, but most don’t even realize they don’t,” Daniel Chang, MD, told EyeWorld.

Dr. Chang is the first to admit that he didn’t. The subject of centration is 1) poorly defined in the literature, 2) inconsistent in its application, and 3) probably not as important for IOL surgery as most people suspect.

Dr. Chang and George Waring IV, MD, wrote a perspective in 2014 describing what they found to be inconsistencies with definitions, applications, and use of various ocular reference axes and angles.1 They also proposed a new, practical, and reproducible coordinate system for centration of refractive treatments.

“When someone asks me about angle kappa, I ask how much effort they want to make to fully understand it.” Dr. Chang said. “The concepts are actually quite elegant, but the tough part is to unlearn what was previously partially understood.”

Angle kappa background

Angle kappa, Dr. Chang said, is based on 100-year-old concepts and terminology that were originally created for the management of strabismus, which he said requires a different type of function and precision than intraocular surgery.

“Angle kappa is defined as the angle that subtends the visual axis and the pupillary axis. Angle lambda (previously angle kappa) subtends the line of sight and the pupillary axis. If this sounds confusing, add the fact that the visual axis has at least three different definitions involving nodal points; line of sight and pupillary axis depend on pupil location and thus change with lighting and accommodation; and throw in the question of whether these lines actually intersect to form an angle,” Dr. Chang said.

With these concepts being used for lens centration, Dr. Chang said they fall short of what physicians need them to do. “When you try to apply these to intraocular surgery, they don’t work. We’ve been stuck applying old concepts and terminology to a new problem of a very different nature,” he said.

When and how did physicians start using angle kappa with lens surgery? Dr. Chang said it started with presbyopia-correcting IOLs. The rings on these IOLs made any decentration relative to the pupil margin an obvious discrepancy.

“There was an assumption that the ‘misalignment’ was causing some visual dissatisfaction,” he said. “I think that’s where it started. When we observe a discrepancy, we associate that with an outcome, and we start doing something about it. Terminology is simply the way we communicate our observations.” A 2011 study suggested that “there may be a role of misalignment between the visual and pupillary axis (angle kappa).”2

Dr. Chang said “people took preoperative ‘angle kappa’ measurements and suggested that higher angle kappas would result in problematic outcomes. Without clear evidence, centration became a convenient scapegoat for undesirable surgical outcomes. Unfortunately, other important factors, like IOL material and design, were not given as much consideration.”

Nomenclature changes

Dr. Chang and Dr. Waring attempted to address the ambiguity with the current nomenclature and proposed a new, reproducible definition and technique for centering IOLs on what they called the subject-fixated coaxially sighted corneal light reflex (SF-CSCLR). They also called the chord distance between the SF-CSCLR and the pupil center “chord mu.” Since putting forth this concept, scientists at Carl Zeiss Meditec suggested calling it the “Chang-Waring reflex” and “Chang-Waring chord.” Dr. Chang noted that “angle kappa” is actually a chord distance, not an angle. The naming has since moved toward chord kappa, and Dr. Chang hopes that the new terminology would help to clarify.

Dr. Chang said that the pupil center defines the pupillary axis. 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.

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“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.”

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 industry 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 decentration 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 outcomes, and I’ve yet to see visual problems from putting a presbyopia-correcting lens in these patients.”

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About the physician

Daniel Chang, MD
Cataract and Refractive Surgeon
Empire Eye and Laser Center
Bakersfield, California

Douglas Koch, MD
Department of Ophthalmology
Cullen Eye Institute
Baylor College of Medicine
Houston, Texas

References

1. Chang DH, Waring GO. The subject-fixated coaxially sighted corneal light reflex: a clinical marker for centration of refractive treatments and devices. Am J Ophthalmol. 2014;158:863–874.
2. Prakash G, et al. Predictive factor 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 Ophthalmol. 2022;16:1321–1329.
4. Fu Y, et al. Influence of angle kappa and angle alpha on visual quality after implantation of multifocal intraocular lenses. J Cataract Refract Surg. 2019;45:1258–1264.