The early detection of keratoconus is essential

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1 REFRCTIVE SURGERY Identifying Keratoconus Suspects With Wavefront Preoperative wavefront analysis may help surgeons prevent erratic LSIK results and postoperative complications. Y EUGENIO M. CNDL, MD; LUR T. MULLER, MD; RICHRD F. DENNIS, MD; PRG. MJMUDR, MD; SHWN KLEIN, MD; ND RNDY J. EPSTEIN, MD The early detection of keratoconus is essential to the prevention of unpredictable LSIK results and postoperative iatrogenic keratectasia. lthough wavefront technology is still in its early stages, it may assist physicians in recognizing forme fruste keratoconus. THE ROLE OF TOPOGRPHY Corneal distortion induces myopia, regular and irregular astigmatism, and both lower- and higher-order aberrations. s a result, corneal topography has become one of the most widely accepted tools for diagnosing keratoconus and forme fruste keratoconus 1 (FFKC). Several software programs are available to aid in the detection and quantification of the severity of keratoconus. Rabinowitz and McDonnell 2 first reported a numerical method to differentiate keratoconic from normal corneas. This technique is based on central corneal power, the difference in central corneal power between fellow eyes, and the inferior-superior value. Maeda et al 3 later developed quantitative indices derived from videokeratoscopic data in order to create an automated keratoconus detection algorithm. Researchers recently proposed that Orbscan II slit scan topography (ausch & Lomb, Rochester, NY) is an effective means of identifying FFKC at an early stage. 4 They evaluated patients with suspicious preoperative videokeratography and observed a mean posterior elevation that was more than double that of the control group. The researchers found that patients with positive keratoconus screening tests had higher anterior and posterior elevations. They advised those patients with a posterior float of greater than 40 µm to avoid LSIK surgery. Few studies have characterized the higher-order aberrations encountered in patients with FFKC or keratoconus. Furthermore, because there have been reports that up to 8% of myopes presenting for refractive surgery exhibit suspicious features on topography, it is important to determine whether wavefront analysis Figure 1. Topography studies revealed inferior steepening in right eye () and left eye (). 52 I CTRCT & REFRCTIVE SURGERY TODY I JNURY 2004

2 REFRCTIVE SURGERY Figure 2. utomated screening tests revealed similarity to keratoconus in the right eye () and left eye (). may have a role in screening patients for FFKC before they undergo corneal refractive surgery. WVEFRONT NLYSIS The Zywave aberrometer (ausch & Lomb) is a wavefront-sensing device based on the Hartmann-Shack principle. It uses a wavelength of 780 nm and measures approximately 70 to 75 locations within the pupil. The Zywave system automatically measures the pupil size at the moment the wavefront image is captured, and it takes three consecutive measurements with a pupil size of at least 6 mm. To avoid instrument accommodation, the eye is fogged approximately 1.00 D during the measurement. The device measures lower- and higher-order aberrations. Wavefront errors, the difference between the measured and the ideal wavefront, are measured as the root mean square (RMS) and represent the magnitudes of the coefficients of the Zernike polynomials. Physicians may combine the Zywave wavefront data with Orbscan data in order to design a customized wavefront-based laser ablation profile for each patient. 5 Wavefront-guided laser ablation has been shown to more effectively treat pre-existing higher-order aberrations and the lower-order aberrations (sphere and cylinder) that have historically been treated with standard LSIK. 6 Wavefront and Keratoconus Corneal refractive surgery may induce iatrogenic keratectasia in patients with FFKC or keratoconus lthough videokeratography currently offers several software programs that may aid in the detection and quantification of the severity of keratoconus, we believe wavefront technology may prove helpful in screening patients at risk for ectasia after refractive surgery. Researchers recently compared the ocular wavefront Figure 3. Orbscan II maps demonstrated increased posterior float values for OD (0.056 mm) () and OS (0.052 mm) (), normal difference less than mm. JNURY 2004 I CTRCT & REFRCTIVE SURGERY TODY I 53

3 REFRCTIVE SURGERY Figure 4. Zywave images revealed increased higher-order RMS at 6 mm (normal less than 0.4) in the right eye () and left eye (). aberrations of 35 keratoconic eyes and 38 normal eyes. 12 Their results revealed statistically significant differences between normal and keratoconic eyes regarding higherorder aberrations, coma-like aberrations, and sphericallike aberrations under photopic as well as scotopic conditions. Of note was a dominance of coma-like aberrations over spherical-like aberrations in keratoconic eyes. Other investigators have studied the corneal wavefront aberrations in patients with keratoconus through elevation data obtained by corneal topographical analysis Their study suggested that keratoconic corneas could be differentiated from normal corneas with the Z3 term of Zernike polynomials in corneal topography. The use of the Zywave aberrometer to help confirm a diagnosis of FFKC has some limitations. Most of the quantitative wavefront measurements on patients with clinical keratoconus are not obtainable, due to the high degree of aberrations present in these eyes. lso, it is important that the surgeon perform the Zywave measurement when the pupil size is at or just above 6 mm. If full dilation/cycloplegia has occurred, the wavefront aberrations may be artificially increased. OUR STUDY We recently studied Zywave parameters in 33 eyes with probable FFKC (n = 10) and in 23 normal eyes. The mean refractive error in our FFKC patients was D versus D in the normal control group. We diagnosed FFKC using the criteria established by Randleman et al 15 : central K value greater than D and/or I-S value greater than 1.4. bnormal, localized steepening was observed on topography as well as positive Klyce/Maeda and Rabinowitz screening tests. These patients did not have any slit lamp or retinoscopic findings suggestive of keratoconus. If patients did not have a resting pupil size of greater than 6 mm in a dark room (which is necessary in order to obtain the RMS 6-mm value), we dilated their pupils with 2.5% phenylephrine and 1% tropicamide and obtained three Zywave measurements with the pupil at or above 6 mm. We obtained these measurements within 10 minutes of instilling the dilating drops. RESULTS In our study, the mean value for higher-order aberrations (RMS) in the FFKC versus the control groups was 1.27 µm versus 0.70 µm. This is in accordance with the previously reported findings of Maeda et al. The patients with FFKC also had greater vertical coma, with a mean value of 0.74 µm, versus 0.27 µm in the control group. Significantly elevated posterior floats on the Orbscan II maps were identified on patients with FFKC (mean = 0.06 vs 0.03 mm in normal controls). SCREENING LGORITHM In our practice, we have developed an effective screening algorithm for keratoconus and FFKC. If patients have a normal topography and fits the standard criteria, we counsel them to undergo refractive surgery. Every patient with suspicious topographic features undergoes Klyce/Maeda and Rabinowitz computerized screening tests. If these tests are positive for keratoconus, the patient undergoes Orbscan II topography. If patients have a posterior float of greater than 40 µm, we advise them against LSIK and suggest they consider surface ablation. We have recently expanded this process to include the use of Zywave wavefront analysis to our arsenal of screening tests for patients with possible FFKC. We have noted that patients with FFKC have an elevated RMS as well as a high degree of coma. Wavefront analysis has 54 I CTRCT & REFRCTIVE SURGERY TODY I JNURY 2004

4 aided us in identifying these patients, who may be at a higher risk for developing post-lsik keratectasia. Since adopting the screening algorithm outlined herein, we have not experienced any instances of keratectasia in any of our own patients following refractive corneal surgery. CSE EXMPLE 45-year-old white male came to our refractive practice and expressed an interest in undergoing refractive surgery. The patient stated that he had never been successfully fitted with contact lenses in the past. His CV, with a prescription of OD plano X 005 OD and plano X 005 OS was 20/25 OU. The slit lamp examination was negative for any clinical findings consistent with keratoconus. Upon performing topography (Figure 1), we observed significant inferior steepening in both eyes. Subsequently, according to our protocol, we performed automated keratoconus screening programs (Figure 2). These indicated a 46% and 56% similarity to keratoconus in the patient s right and left eyes, respectively. The patient s Orbscan II posterior float values were mm OD and mm OS (Figure 3). We then obtained Zywave images, which revealed elevated wavefront aberrations of 1.08 µm and 1.53 µm OD and OS, respectively (Figure 4). Significantly elevated third-degree coma was present as well (Figure 5). We diagnosed the patient with FFKC and informed him that he was not a candidate for refractive surgery. We referred him to our optometrist for contact lens fitting. CONCLUSION The adoption of wavefront analysis as a part of the routine workup of potential refractive surgery candidates in many refractive surgery practices is still in its early stages.

5 REFRCTIVE SURGERY Figure 5. Zywave maps reveal increased third-order coma in the right eye () and left eye (). However, we would encourage clinicians to gain access to this technology and begin using it as a diagnostic aid. We feel that, in addition to surface topography and Orbscan studies of posterior corneal curvature, wavefront analysis can be a very useful aid in confirming which patients may have FFKC or subtle keratoconus. Furthermore, it may also support the case for performing surface ablation (which has not been associated with ectasia except under unusual circumstances) on patients with suspicious topographic features who lack any other clinical findings associated with keratoconus. Further prospective studies with larger numbers of patients are required in order to determine if this technology will be beneficial as a screening tool for all patients undergoing refractive surgery. Eugenio M. Candal, MD, is currently in private practice with Ophthalmic Consultants of oston. He holds no financial interest in any of the products mentioned herein. Dr. Candal may be reached at (800) ; emcandal@eyeboston.com. Laura T. Muller, MD, is currently in private practice with the Pasco Eye Institute in New Port Richey, Florida. She herein. Dr. Muller may be reached at (727) ; lauramuller@hotmail.com. Richard F. Dennis, MD, is an assistant professor at Rush Presbyterian-St. Luke s Medical Center in Chicago. He herein. Dr. Dennis may be reached at (847) ; rdennis@chicagocornea.com. Parag. Majmudar, MD, is an assistant professor at Rush-Presbyterian-St. Luke s Medical Center in Chicago. He herein. Dr. Majmudar may be reached at (312) ; pamajmudar@chicagocornea.com. Shawn Klein, MD, is currently a fellow at Rush Presbyterian-St. Luke s Medical Center in Chicago. He may be reached at (847) ; sklein@chicagocornea.com. Randy J. Epstein, MD, is an associate professor at Rush Presbyterian-St. Luke s Hospital in Chicago. Dr. Epstein is a paid consultant for ausch & Lomb. Dr. Epstein may be reached at (847) ; epstein@chicagocornea.com. 1. Wilson SE, Lin DT, Klyce SD. Corneal topography of keratoconus. Cornea. 1991;10:1: Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus. Refract Corneal Surg. 1989;5: Maeda N, Klyce SD, Smolek MK, Thompson HW. utomated keratoconus screening with corneal topography analysis. Invest Ophthalmol and Vis Sci. 1994;35:6: Rao SN, Raviv T, Majmudar P, Epstein RJ. Role of Orbscan II in screening keratoconus suspects before refractive corneal surgery. Ophthalmology. 2002;109: Hament WJ, Nabar V, Nuijts RM. Repeatability and validity of Zywave aberrometer measurements. J Cataract Refract Surg. 2002;28: Nuijts RM, Nabar VN, Hament WJ, Eggink F. Wavefront-guided versus standard laser in situ keratomileusis to correct low to moderate myopia. J Cataract Refract Surg. 2002;28: Seiler T, Quurke W. Iatrogenic keratectasia after LSIK in a case of forme fruste keratoconus. J Cataract Refract Surg. 1998;24: Mcleod SD, Kisla T, Caro NC, McMahon TT. Iatrogenic keratoconus: corneal ectasia following laser in situ keratomileusis for myopia. rch Ophthalmol. 2000;118: Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg. 1998;14; Joo C, Kim TG. Corneal ectasia detected after laser in situ keratomileusis for correction of less than -12 diopters of myopia. J Cataract Refract Surg. 2000;26: moils SP, Deist M, Gous P, moils PM. Iatrogenic keratectasia after laser in situ keratomileusis for less than -4.0 to -7.0 diopters of myopia. J Cataract Refract Surg. 2000;26: Maeda N, Fujikado T, Kuroda T, et al. Wavefront aberrations measured with Hartmann- Shack sensor in patients with keratoconus. Ophthalmology. 2003;109: Schwiegerling J, Greivenkamp JE. Keratoconus detection based on videokeratographic height data. Optom Vis Sci. 1996;73: Schwiegerling J, Greivenkamp JE, Miller JM. Representation of videokeratoscopic height data with Zernike polynomials. Opt Soc m. 1995;12: Randelman J, Russell, Ward M, et al. Risk Factors and prognosis for corneal ectasia after LSIK. Ophthalmology. 2003;110: I CTRCT & REFRCTIVE SURGERY TODY I JNURY 2004