ENDOTHELIAL KERATOPLASTY

  Researchers are working hard to understand the complex interaction of factors behind the high graft detachment and re-bubbling rates associated with Descemet’s membrane endothelial keratoplasty (DMEK), according to a presentation here. “We are all well aware of the advantages of DMEK in terms of the visual results, rapid recovery and low rejection rates, but primary graft detachment is still a problem that remains to be solved in order for DMEK to reach its full potential,” Friedrich Kruse MD told delegates attending the 2013 Congress of the European Society of Ophthalmology (SOE). Dr Kruse, University of Erlangen- Nürnberg, in Erlangen, Germany, outlined some of the key factors believed to play a role in graft detachment and high re-bubbling rates in DMEK patients. Compared to Descemet’s stripping automated endothelial keratoplasty (DSAEK), the donor preparation for DMEK is more difficult given the need to consistently separate Descemet’s membrane without tearing, which may result in loss of that donor. "Older donors are also preferred for DMEK since their Descemet’s membrane tends to be less adherent than in younger corneas and therefore easier to separate.” Graft thickness is also likely to play a role in graft adhesion and donor age is again a factor here, said Dr Kruse. “Younger donors are only to be used by very experienced surgeons because the Descemet’s membrane roll is usually very tight. It relates to the age of the patient - in general the older the patient the thicker the membrane. So while we can use young donors for DSAEK, it is probably not a good idea for DMEK, at least for less experienced surgeons, because unfolding the tight graft tends to result in more trauma for the endothelial cells and a higher risk of detachment,” he said. Another factor to watch for in selecting donor tissue is the presence of pathologies such as cataract and pseudoexfoliation syndrome, said Dr Kruse. “A donor who has undergone cataract surgery means a corneal incision and also side port incisions and this is a very big problem in preparing the tissue. Pseudoexfoliation syndrome also needs to be watched for because it renders an abnormal structure of Descemet’s membrane and increases the likelihood of eventual graft detachment,” he said. Current research is also seeking to shed light on the complex role played by adhesion proteins in Descemet’s membrane in increasing the risk of detachment, said Dr Kruse. “The fact is that we do not really know why a graft sticks in certain cases and not in others. There are significant interindividual variations in the concentration of proteins such as fibronectin, vitronectin, amyloid P and the various fibulin glycoproteins, among others, in the interfacial matrix layer of Descemet’s membrane. There are more of these proteins concentrated in the central corneal zone, so that might also go some way towards explaining why the detachment always starts in the periphery,” he said. Host characteristics are also not to be overlooked in the search for reduced detachment rates, said Dr Kruse. “In our experience, we get much better results from Fuchs’ endothelial dystrophy patients than those with pseudophakic bullous keratopathy. Additional pathologies are also a problem, as a shallow anterior chamber can make DMEK surgery even trickier,” he said. Graft overlap also needs to be borne in mind. A recent study carried out by Dr Kruse’s group found that complete removal of Descemet’s membrane in the area of the graft allows better adhesion following DMEK surgery. Looking at 100 eyes that underwent DMEK by the same surgeon, the study examined the zone between donor and graft Descemet’s membrane. Based on the size of descemetorhexis, patients were divided into three groups: group one of 15 patients in which the graft and the membrane overlapped completely (100 per cent), group two of 21 patients in which no overlap was seen and in which the graft was separated from Descemet’s by a small gap and group three of 69 patients with variable overlap. Significant graft detachment appeared in 10 eyes of group one and three eyes of group two. The area of peripheral detachment was significantly greater in group one than in group two. Re-bubbling was performed in three eyes of group one due to persistent major detachment but was not necessary in group two, said Dr Kruse. “Our results suggest that complete removal of Descemet’s membrane in the area of the graft allows better adhesion of the graft and greater overlap is a risk factor leading to graft detachment. Thus patients with greater overlap have to be followed up more carefully,” he said. Insertion of the graft also poses a significant problem for DMEK procedures at the moment, said Dr Kruse. “This is a big problem because there is no specially adapted insertion technique. Our group is currently using an intraocular lens shooter, but this is not available in the United States, and so they have to improvise to put the graft into the anterior chamber. We are currently working with a manufacturer to design a shooter so hopefully this will be resolved in the near future,” he said.