EyeWorld China September 2019 Issue

Effect of Astigmatism in Topo-guided Treatments Dr. Rohit Shetty 7 Good understanding of cylinders and dynamic relationship between lens and cornea is essential to ensure successful astigmatic correctionwith Contoura TM . In future, use of the 3 Zsmodel and ray tracing technology is expected to provide further insights in topo- guided astigmatism treatment. Figure 3: Galilei pie charts is a valuable tool to understand cylinders Cylinder with predominant lower order aberration CASE 1 CASE 2 Cylinder with predominant higher order aberration custom ablation treatment (T-CAT) protocol modified by topographical neutralization technique (TNT) and refraction with WaveLight ® EX500, along with a standard cross-linking protocol. A significant gain in UCVA and BCVA was seen with good stability at 12-month follow-up, highlighting the ongoing improvement in patient’s vision beyond the initial laser treatment. In addition, manifest astigmatism; steepest keratometry, refractive, keratometricand topographic data reached statistical significance after the triple procedure, thus highlighting the beneficial effect of cross-linking in keratoconus patients. One of the important considerations while treating astigmatism is understanding cylinders. Cylinders are made of higher and lower order aberrations (HOA and LOA) and GALILEI pie charts are a great tool to understand cylinders (Figure 3). “If there are two patients, both with high cylinders, but one has predominant astigmatism and other one predominant coma; the axis changes according to the dominance,’ explained Dr. Shetty. In case 1, when the patient has predominant astigmatism with LOA, the patient’s axis and topolyzer axis match. In this situation a WFO- ablation would result in a superior quality visual outcome. However, in case 2, where the measured axis and patient axis are different, the GALILEI map shows both astigmatism and coma. A WFO-ablation in this patient will have a residual cylinder of 90% coma and 5% astigmatism and hence poor quality of vision with glass correction. “This is where Contoura TM plays an important role, where it corrects the HOA, and the resulting cylinder is predominantly 2nd order astigmatism explaining the superior quality vision”, highlighted Dr. Shetty. Another common confusion that occurs while treating astigmatism is when the patient accepts higher cylinder than that measured on topography. The reason for this confusion is explained by real- time aberrometry of lenticular astigmatism, which shows corneal astigmatism being balanced by the lens astigmatism. Prof. Shetty estimated that at least 10-12% of his ContouraTM patients have issues with higher cylinders than what is seen on topography. A model of 3 Z’s consisting of Zernike pyramid, zero rule and lens compensation by zonules, might allow better understanding of interplay of lens and cornea. Ray tracing, a new computer- based algorithm that uses data from several different measurements, is expected to provide individualized ablation profile for treatment of astigmatism in the near future.