EyeWorld India December 2018 Issue

Preoperative diagnostics for cataract surgery by Liz Hillman EyeWorld Senior Staff Writer An ASCRS Young Eye Surgeons (YES) Clinical Committee-sponsored webinar featured an in- depth discussion about preoperative diagnostics for cataract surgery M itchell Weikert , MD, associate professor and residency program director, Cullen Eye Institute, Baylor College of Medi- cine, Houston, and Kevin M. Miller , MD , Kolokotrones Chair in Ophthalmology, chief of cataract and refractive surgery, Stein Eye Institute, David Geffen School of Medicine at UCLA, Los Angeles, provided expert insights for “Know Your Tools/Toy Box: Preoperative Diagnostics.” The webinar was hosted by YES Clinical Committee members Zaina Al-Mohtaseb, MD , Houston, and Samuel Lee, MD , Sacramento, California. It included specific case examples in which Dr. Weikert and Dr. Miller shared their thoughts on topog- raphy, tomography, biometry and specific differences among preop- erative diagnostic devices; their ex- perience with different calculators and formulas; and their perspective on handling special circumstances like how long to wait before taking measurements after a patient stops rigid gas permeable lens use or has a Salzmann’s nodule removed to ensure reliability. EyeWorld caught up with Dr. Weikert and Dr. Miller after the webinar so they could answer a few questions from attendees of the webinar, as well as follow-up ques- tions that expound upon the topics that were discussed during the hour-long educational event. Attendee: What is the differ- ence between axial curvature and instantaneous curvature? Dr. Miller: On some ma- chines, axial is called sagittal and instantaneous is called tangential. With axial maps, the software as- sumes that light rays, as they travel through the cornea and entrance pupil, all intersect somewhere along the optical axis of the eye. Instantaneous maps don’t make that assumption; they allow rays to intersect wherever they actually do, and often times that’s off the optical axis. What does that mean when you display those maps? You get a greater smoothing function with axial maps. The cornea will look smoother than perhaps it actually is. If you want to see the hills and valleys, the little bumps, the little dry spots, then you look at the instantaneous or tangential map. For astigmatism planning, I use axial maps, and if I’m trying to trouble shoot an eye that’s not seeing all that well and doesn’t refract very well, I will often use a tangential map to highlight subtle pathology. Dr. Weikert: It’s the same data, just analyzed two different ways. Axial curvature looks at a point on the corneal surface and references the radius of curvature at that point to the visual axis. By tying the surface curvature at every point to the visual axis, the devices end up averaging the curvature. You may lose a little detail, especially in the peripheral cornea, but this method can reduce a lot of noise. Instantaneous maps determine curvature by looking at small areas adjacent to the point of interest and don’t tie their analysis to the visual axis. By doing it this way you get more detail, but you can also get more noise. We look at both maps, since they each can provide useful information. I like to use axial maps to assess astig- matism, but I think instantane- ous maps will often give you the cornea’s true shape and highlight irregularities and subtle sources of vision loss. Attendee: With the Pentacam (Oculus, Wetzlar, Germany), which Ks are used for astigmatism man- agement in cataract surgery (toric or LRI)? Dr. Miller: I have a Galilei G6 (Ziemer, Port, Switzerland) and a Pentacam HR, but I tend to use the Pentacam more. The Pentacam produces simulated keratometry values, so it will give you the Sim Ks of the anterior cornea that a cor- neal topographer would produce. But the Pentacam also measures the posterior surface, calculat- ing total corneal power and total corneal astigmatism. So that’s what I use now; I use the total corneal astigmatism and put those Sim Ks into the appropriate formulas. Dr. Weikert: I use the Gali- lei, which combines Placido and Scheimpflug imaging, and I don’t have access to a Pentacam. Since the Galilei is a tomographer, it can also provide the total corneal power by ray tracing light through the anterior and posterior cor- neal surfaces. When we plan our astigmatism correction, we look at the total corneal power, but when selecting the magnitude of correction, we still rely on popu- lation norms to assess the poste- rior contribution to the patient’s astigmatism. Current devices still have a lot of variability in their assessment of posterior corneal astigmatism. Attendee: Do you use manual Ks ever? Dr. Miller: I do not, but I do think there’s a value. There is something to be gained expe- rience-wise in learning how to obtain manual Ks. For instance, one of the things you find when you use a manual keratometer is that it’s hard to nail the axis when the amount of astigmatism is low. You also learn that centration is important. I wouldn’t use manual Ks anymore because there’s just continued on page 48 EWAP CATARACT/IOL 47 December 2018