EyeWorld India March 2017 Issue

20 EWAP FEATURE March 2017 Views from Asia-Pacific Tips - from page 18 magnitude and axis of astigmatism when measured with a diagnostic device,” Dr. Tipperman said. “In cases of marked differences, the surgeon should carefully analyze the Placido ring images and look for distortion of the rings, which could indicate ABM dystrophy or keratitis sicca.” When biometry measurements seem unreliable, Dr. Tipperman has found it is often helpful to treat the patient aggressively with artificial tears for a week or two and then repeat all the measurements. Calculators’ role Stephen Klyce, PhD, adjunct professor, Department Ophthalmology, Icahn School of Medicine of Mount Sinai, New York, said that although not all devices yield identical keratometry values, most should provide accurate measurements for normal healthy corneas. However, for eyes that have undergone surgery— keratorefractive or intraocular— standard keratometry values can provide measurements that don’t reflect the true central corneal average curvature, Dr. Klyce said. To address this challenge, several corneal topographers have developed indices that form an average curvature of the central cornea over the pupil, and these can improve the prediction accuracy of IOL calculations. Dr. Klyce co-authored one of the first papers to demonstrate such use of indices, and various investigators have contributed their methods to the ASCRS IOL calculator (iolcalc.ascrs.org). 2 Dr. Klyce noted that many of the IOL equations have been refined with data from specific devices, so it is important to follow the recommendations provided for the different approaches. Dr. Rubenstein agreed that it is important to use methods such as the ASCRS IOL calculator to FAM Han Bor, MD Senior Consultant, NHG Eye Institute @ Tan Tock Seng Hospital 11 Jalan Tan Tock Seng, Singapore Tel. no. +65-6357-7726 Fax no. +65-6357-7735 han_bor_fam@ttsh.com.sg I n approaching patients for any IOL, contact lens users should be off contact lenses for specified periods, depending on the type of contact lens. Keratometry should be done before any other investigations, certainly before any contact examination studies including applanation tonometry or any eyedrops. This is to avoid distorting the cornea and corrupting the measurements. If there is any doubt, the measurement should be repeated on separate occasions. If the corneas are too dry, wetting agents are used to lubricate the eyes. 50-year-old myopic female Initial 1 weeks later Keratometry 7.57mm @ 1750 7.18mm @ 850 Mean: 45.79D 7.27mm @ 1590 7.15mm @ 690 Mean: 46.81D Astigmatism 2.42D @ 85 0 0.78D @ 69 0 The 1 st biometry was done less than 12 hours after stopping contact lens wear. In determining the cornea power for toric IOLs, I use a few devices including the IOLMaster 700, LenStar, Galilei and OPD III. The ocular registration of the IOLMaster 700 is a nice feature. It measures the corneal astigmatism superimposed against captured corneal landmarks. This registration is transferred to Callisto on the microscope to assist in aligning the toric IOL during surgery. This helps avoid the error of head or postural tilt. I routinely scrutinize the measurement analysis of all my devices. I look at the standard deviations of the measurements; the lower the better. I look for concurrence of the measured axes. The measurements should make sense and the difference between the two eyes should not be too great. Use the established checklist to ensure consistently good readings. The different devices may measure differently and generally most of them are good. The more important thing is knowing each device well. Checklist for keratometry: Repeat keratometry if: • 40D > Average Corneal Power > 48D • Astigmatism for either eye is > 3.00D • Difference of average corneal power > 1.0D between eyes • Difference of cornea astigmatic power > 1.0D between eyes • Poor fixation e.g. mature cataract, etc. • Uncooperative or non-communicative patients. If I am dissatisfied with the topographic maps, I will look at the mires on the raw image for corneal irregularities which may affect the quality of the maps. The use of topography is important in excluding irregular cornea and keratoconus. I use Galilei to look at the posterior corneal astigmatism as well. For calculating the toric power I use Barrett’s toric formula. Barrett’s toric formula incorporates an accurate nomogram for posterior corneal astigmatism. When we analyzed our postoperative outcomes of our initial toric IOL implantations, the prediction errors agreed with Barrett’s toric formula. This shows the importance and significance of posterior cornea toricity. In microincision phacoemulsification, the influence of surgically induced astigmatism may appear minimal but nonetheless it does impact the final statistical outcome. If this is personalized, this should be included in the final calculation. Editors’ note: Dr. Fam is a consultant for Abbott Medical Optics (Abbott Park, Illinois) and Zeiss. A good reading typified by low SD for repeated measurements. The axes concurred.