Physicians making sense of the cornea transplant alphabet soup.
Reviewed by Sadeer Hannush, MD
Corneal transplants have evolved over time from full thickness grafts to layer-specific procedures, according to Sadeer Hannush, MD, an attending surgeon for the cornea service at Wills Eye Hospital, a professor of ophthalmology at Thomas Jefferson University, and medical director of Lions Eye Bank of Delaware Valley in Philadelphia. Pennsylvania.
Delivering a lecture on the evolution of corneal transplantation surgery at the 2022 University of Toronto Walter Wright Symposium, Hannush described various techniques that have been used over time to treat corneas that are no longer clear or regular in shape.
“The indications and techniques of corneal transplantation have evolved over the past 3 decades with the introduction of layer-specific procedures, creating an alphabet soup of nomenclature,” Hannush said.
Although partial-thickness corneal transplantation has overtaken full-thickness
corneal transplantation in popularity, a role for full-thickness corneal grafts remains, according to Hannush.
“Some corneal surgeons may argue that there are no more indications for a full-thickness transplant, but there are some, and severe corneal edema with endothelial dysfunction and stromal scarring is one
such indication,” said Hannush, adding that corneal macular dystrophy is an instance in which patients can benefit from a full-thickness procedure with the presence of stromal opacities and an abnormal endothelium with guttate changes.
Descemet stripping endothelial keratoplasty (DSEK) took the world by storm when it arrived on the scene in the early part of the millennium, said Hannush, crediting its introduction to Gerrit Melles, MD, PhD, from the Netherlands. Many ophthalmologists began to adopt the procedure in the mid-2000s, with it becoming the procedure of choice for patients with corneal edema caused by endothelial dysfunction, according to Hannush.
“At that time, the eye banks were not preparing tissue for us,” Hannush pointed out. “I was cutting the tissue myself, and I continue to do that at the Wills Eye Surgery Center today.”
The advantage of cutting the tissue yourself is that the surgeon can influence the thickness of the graft, according to Hannush. “This results in very thin lenticules that has been referred to as ultrathin descemet stripping automated endothelial keratoplasty (DSAEK),” Hannush said.
Surgeons and eye bank technicians can vary the thickness of the posterior lamellar graft in several ways, according to Hannush. Those ways include removing the epithelium, selecting a microkeratome head of 300 microns to 350 microns, adjusting the speed of the microkeratome pass (slow speed results in a deeper cut, a thicker cap, and a thinner graft), and modifying the pressure in the artificial chamber (the more you pressurize the artificial chamber, the more tissue prolapse there is, the thicker the cap and the thinner the graft).
For Descemet membrane endothelial keratoplasty (DMEK), in which there is true anatomic replacement of Descemet membrane and endothelium, tissue is now provided to corneal surgeons prestripped, prestained, prepunched, premarked, and preloaded, Hannush said. Some clinicians also see value in pretreating the tissue with amphotericin B to prevent, at least hypothetically, fungal infection post keratoplasty, noted Hannush.
“We don’t think that there is significant evidence that pretreatment with amphotericin makes a difference, but still surgeons feel better when they ask for it,” he said.
Citing statistics, Hannush pointed out that in the US, two-thirds of all grafts are endothelial grafts; of those, half are DMEKs and half are DSEKs, with a yearly increase in the number of DMEK procedures. By contrast, in Germany, 98% of all endothelial grafts are DMEK, said Hannush, citing data from Friedrich Kruse.
“Some surgeons feel that there are no indications for DSEK any longer,” Hannush said. “In the setting of a damaged iris and sutured posterior chamber intraocular lens implant, there is direct communication between the anterior segment and the vitreous cavity. You could do a DMEK, but it would be fraught with complications. There would be a higher risk of graft dislocation, less so with DSEK. So there is still a role for DSEK in 2023.”
Another innovation in DMEK surgery is the use of sulfur hexafluoride (SF6) gas, noted Hannush. He pointed to the research of José Güell, MD, in Barcelona, Spain, which has suggested that the use of SF6 gas for prolonged tissue support may decrease the rebubble rate with DMEK.1
“Most of us are using SF6, but some prefer to use air,” Hannush, said, stressing that it is critical to use a nonexpansile concentration of SF6 gas because of the possibility of expansion of the gas in the anterior chamber, resulting in severely high pressure.
DMEK success can be improved in several ways, according to Hannush.
“There are ways to increase the odds that the graft is inserted correct side up,” he said, noting that grafts placed upside down can result in failure and adding, “If you have an anterior segment OCT (optical coherence tomography) on the operating room microscope, that can be helpful to you.”
Although DSEK and DMEK replace the posterior 4% of the cornea in the setting of endothelial dysfunction, such as Fuchs corneal endothelial dystrophy and bullous keratopathy, deep anterior lamellar keratoplasty (DALK) may be the procedure of choice for corneal stromal dystrophies, scars, and ectatic disorders not involving the endothelium and Descemet membrane, according to Hannush. He noted that with DALK, the anterior 96% of the cornea is replaced and that big-bubble DALK is the most popular version of this procedure.
Permanent keratoprosthesis surgery involves the placement of an artificial device in the cornea when the setting of corneal conditions may not be amenable to allograft transplantation, as in the case in the setting of multiple graft failures, Hannush explained.
The latest advance in corneal transplantation is eliminating the transplant altogether and allowing the patient’s own endothelial cells to redistribute—a procedure called Descemet stripping only, Hannush noted.
“The idea is to remove the central part of [the Descemet] membrane (approximately 4 mm), letting the healthy peripheral endothelium migrate into that area,” he said, pointing out that the procedure would initially result in edema but that a crystal-clear cornea may follow a few weeks later.
For the immediate future, Hannush maintained that DMEK will probably be the procedure of choice for most instances of endothelial dysfunction with or without comorbidities.