EyeWorld Asia-Pacific June 2014 Issue

55 EWAP CORNEA June 2014 could understand the impact of aberration changes on visual acuity changes,” Dr. Hindman said. “We looked postoperatively at the aberrations at 1, 3, 6, and 12 months.” Investigators looked at how aberrations in the whole eye affected visual quality. When it came to postop higher order aberrations, they determined that they did not diminish by much. “We found there was a trend that the total average was decreasing, but it wasn’t significant,” Dr. Hindman observed. In addition, investigators noticed there was a decrease in corneal light scatter occurring with time. “We found that the interface reflectivity decreased more than the other sources of reflectivity,” Dr. Hindman said. It was the scatter that proved significant. “The improvement in vision in our series could be accounted for by the decrease in the scatter we were seeing in the cornea,” Dr. Hindman said. The change in scatter was not associated with changes in corneal edema as evidenced by stable corneal thicknesses. Corneal thickness, she pointed out, was another area of interest for a lot of people, who wanted to determine if making the donor button thinner would affect vision. “Ours was a relatively small study, and corneal thickness wasn’t our primary endpoint but was something we noticed didn’t affect the visual outcome,” Dr. Hindman said. “It didn’t matter if the host donor was thicker, if the total amount was thicker, or if the donor tissue was thicker.” However, between the 1- and 12-month postoperative marks, a decrease in scatter proved to be an important factor in visual improvement. “The variability induced by the scatter could account for the change in the vision we saw,” she said. Investigators found that this decrease was most pronounced along the interface. Focusing on scatter Just where is this scatter coming from? Dr. Hindman recently took a look at this in an animal model. Prior studies, she noted, had found one of the primary causes of light scatter was corneal wounding. The bulk of the cornea, she pointed out, is made up of the stroma. When the cornea is injured, there is a release of cytokines and growth factors. Keratocytes, the primary cell type in the stroma, become fibroblastic and can transform into wound- healing cells called myofibroblasts, she explained. Myofibroblasts make proteins that can assemble into a contractile apparatus that can close the wound. However, when in the cornea, these wound healing cells may contract the tissue and make the light bend in undesirable ways, or when these proteins are expressed, they can create scatter. Yet in her animal model of DSAEK, myofibroblast activity was not found at the interface. Why not? While there aren’t many other corneal wounds exactly like those in DSAEK surgery, where donor and host stromal tissue are put together in a lamellar fashion, the wound interface with the LASIK flap can be somewhat similar, she said. With LASIK, myofibroblasts were only found on the edges of the cut that come through the epithelium, she observed. This suggests that epithelial stromal interactions, which the DSAEK interface lacks, may be necessary for the myofibroblastic wound healing response. Dr. Hindman theorizes the scatter hindering vision in DSAEK may involve misalignment and abnormal spacing of collagen fibrils at the interface. Collagen fibrils in the corneal tissue orient into little sheets called lamellae. Normally, she explained, these are oriented to minimize the amount of scatter as the light passes through. With a hinged LASIK flap, when this flap is replaced the orientation of the lamellae remains the same. However, with donor tissue in DSAEK there is no way of telling the correct orientation. This, she thinks, may account for some of the interface haze that gets better with time. “My guess is there’s some remodeling that occurs with time that allows the lamellae to reorient and improve their approximation,” Dr. Hindman said. She believes it is the stromal- to-stromal interface created in DSAEK cases from which the scatter is arising. Dr. Hindman pointed out that deep anterior lamellar keratoplasty (DALK) and Descemet’s membrane endothelial keratoplasty (DMEK) patients, where there is no stromal-to- stromal interface, don’t have nearly as much scatter. While there can be technique drawbacks, vision with these other corneal transplant techniques tends to be better. Still, Dr. Hindman stressed that DSAEK is a good surgery. “I think overall patients do incredibly well, and this surgery has been a great advancement over PK,” she said. “We’re not talking about big vision limitations.” While DMEK may resolve many of the issues of higher order aberration and scatter induction, it can be more challenging for both the surgeon and the patient. Eventually, however, Dr. Hindman thinks the transition will be made. “I think we should continue to look toward DMEK as the next step in advancing endothelial keratoplasty,” she concluded. EWAP Editors’ note: Dr. Hindman has no financial interests related to this article. Contact information Hindman: Holly_Hindman@URMC.Rochester.edu