Eyeworld

EW CORNEA 70 March 2011 A lthough Descemet's strip- ping endothelial kerato- plasty (DSEK) offers distinct advantages over penetrating keratoplasty (PK), hyperopic shift remains a prob- lem after the endothelial procedure. A new mathematical model, presented online in November 2010 in Investigative Ophthalmology & Vi- sual Science (IOVS), may help to pre- dict such refractive changes after DSEK surgery. "This model was applied to four patients, and the average predicted hyperopic shift in the overall power of the eye was calculated to be 0.83 D," wrote lead study author Richard Y. Hwang, M.D., Ph.D., Duke Eye Center, Durham, N.C. That change accounted for, on average, 93% of patients' measured refractive shifts. Discussing the model "Initially, DSEK was thought to be a relatively neutral procedure regard- ing changes in refractive error," Dr. Hwang noted. Not so. Evidence currently sug- gests endothelial grafts cause hyper- opic shift in the cornea and that this needs to be taken into account dur- ing pre-op assessments. For instance, the hyperopic shift would affect IOL choice in patients undergoing pha- coemulsification and corneal trans- plantation as a combination procedure. "We developed a mathematical model based on donor graft corneal central thickness and donor CP [cen- tral to peripheral graft] ratio to pre- dict changes in the posterior curvature of the cornea and thus the overall refractive changes of the re- cipient eye after DSEK surgery," Dr. Hwang noted. "Perhaps this model can be the first step in developing a tool to preoperatively estimate the hyperopic shift after DSEK surgery." The mathematical model is a very detailed one, consisting of many assumptions and guidelines for accurate prediction. Readers in- terested in using the model should refer to the study, "Refractive Changes After Descemet Stripping Endothelial Keratoplasty: A simpli- fied mathematical model," directly. It should be understood that further studies are needed to "refine the va- lidity of this model," according to Dr. Hwang. Assuming that the model is cor- rect, it creates a vision for the "ideal" corneal graft. "Our mathe- matical model suggests that, in the- ory, an ideal corneal graft would be one cell layer thick without any vari- ation in thickness because such a graft would have negligible effects on refraction," Dr. Hwang reported. "Conversely, thicker tissue grafts with smaller CP ratios result in larger hyperopic shifts. With regard to pre-operative recipient corneal measurements, a larger recipient corneal thickness and recipient pre- surgical posterior radius of curvature result in relatively smaller hyperopic shifts." Model variables include pre-op corneal thickness, graft thickness, graft CP ratio, and recipient poste- rior corneal curvature, and some of these variables affect refractive changes more than others. "Of the four variables of our mathematical model, the two vari- ables that can be controlled with graft cutting (graft tissue thickness and CP ratios) can have a significant effect on refractive changes, where the smaller the CP ratio and the larger the graft thickness, the larger the refractive changes," Dr. Hwang noted. "Variations in the recipient posterior radius of curvature can also affect the refractive change, but vari- ations in the recipient corneal thick- ness have the least effect on the overall refractive changes." Further, not all refractive changes after DSEK are hyperopic in nature. "Grafts with CP ratios ap- proximately greater than 1 can re- sult in greater refractive power (myopic shift) of the cornea and eye (although other variables can also affect the ratio at which the shift be- comes myopic)," Dr. Hwang re- ported. "Perhaps with improved dissection of donor tissue and more precise control of the corneal graft CP ratio and thickness, a specific donor corneal shape can be cut to target a refractive goal for the pa- tient." This study confirms previous re- ports of hyperopic shift, and it goes a step further as "the first model to predict refractive changes based on by Matt Young EyeWorld Contributing Editor Mathematical model helps predict hyperopic shift after DSEK A patient's eyes 3 days after undergoing DSEK with peripheral scraping Source: Mark A. Terry, M.D. continued on page 71 Study continued from page 68 Nonetheless, the use of DSAEK led to "an excellent outcome thus far," Dr. Gorovoy reported. Mark Packer, M.D., clinical as- sociate professor of ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, said epithelial downgrowth overall is a scary thought among ophthalmolo- gists. "Any time the eye is open for a while, there is an avenue for epithe- lial cells to get into the anterior chamber," Dr. Packer said. Beyond corneal transplants, Dr. Packer said epithelial ingrowth is worrisome for him in the area of glaucoma. "It has been described with shunts and tubes as well," Dr. Packer said. As for this study, Dr. Packer said it is interesting that the immune re- sponse turned out to be beneficial. "[The investigators] feel that the rejection somehow slowed down the growth of epithelium," Dr. Packer said. "It's an unusual situation where rejection is a good thing." EW Editors' note: Dr. Gorovoy has no financial interests related to this study. Dr. Packer has no financial interests related to his comments. Contact information Gorovoy: mgorovoy@gorovoyeye.com Packer: 541-687-2110, mpacker@finemd.com

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