Transplanting Whole Cornea Not Always Necessary
Dr Donald Tan from the Singapore National Eye Centre and Professor John Dart, from the Moorfields Eye Hospital NHS Foundation Trust and the University College in London, and their team describe the latest advances in corneal transplantation in The Lancet's third paper in the ophthalmology series.
In 2010, over 42,000 cornea transplants were carried out in the USA, in comparison with 12,600 solid-organ transplants in 2008 that include kidney, liver, lung, pancreas, heart, and intestine transplants, whilst in the UK, doctors performed 3,565 corneal, 2,671 kidney, and 689 liver transplants in 2010 and 2011.Eye banks are not able to fulfill the worldwide demand on corneas, which means long waiting lists for corneal transplantation in most developing countries.
A fundamental change that occurred within recent years is for specialist surgeons to adopt a newer form of lamellar transplant surgery, which selectively replaces only those layers of the cornea that are diseased.
Deep anterior lamellar keratoplasty (ALK) is replacing PK for disorders, which affect the corneal stromal layers, whilst eliminating the risk of endothelial rejection. Endothelial keratoplasty (EK), a procedure in which the surgeons selectively replace endothelial patient's corneal endothelium, has led to much faster and predictable visual outcomes.
PKs advantage is that it has potentially the best optical result, given that no lamellar-corneal interface problems exist due to transplanting the entire cornea. The procedure is also relatively easy to perform and can be used for any indication. However, the disadvantage is the risk of complications. These include the risk of rejection and graft failure. Rates vary depending on the disease being treated.
Given that ALK is performed in that part of eye that is not served by immune function, the risk of many complications remains low. However, the procedure is generally technically more demanding and can potentially fail if the host endothelium is not healthy. EK generally produces an early visual recovery and better visual outcomes and lower rates of complications as compared with PK. However, there is a potential that it can lead to suboptimal vision unless the corneal stroma is relatively free of opacity.
According to Tan, in cases of long term endothelial dysfunction the corneal stroma becomes chronically swollen and scarred, and even though the endothelial layer is replaced in EK, the remaining host stroma may still be scarred even though it may be much less swollen and therefore clearer, yet this can also reduce vision.
During the EK procedure, surgeons add an extra layer of donor tissue onto the recipient stroma. This can have a similar effect to double-glazing because of the 2 layers. Because this can sometimes affect the quality of vision, the percentage of EK patients who regain perfect (6/6) vision is lower than that in PK. However in PK surgeons need to make a lot of corneal sutures that distort the cornea, causing astigmatism. PK can often lead to wide ranges of refractive errors, given that the donor cornea refractive power can be unpredictable.
This affects PKs quality of vision in a different way and is generally far worse than the "double glazing" effect in EK. Given that the sutures need to be taken out after PK means that visual rehabilitation may not be until months later, in some cases it may take up to 6 months to stabilize into a decent vision. In comparison, EK has no sutures, no distortion of the original cornea, and no dramatic or unpredictable changes in the refractive error of the eye, which means vision can be rehabilitated in as fast as a couple of weeks. According to all published EK studies, overall, vision is far superior with EK than PK.
Other new therapies consist of ocular surface reconstruction and artificial corneal (keratoprosthesis) surgery. Both have become more widely available due to rapid advances in techniques. Both of these advances together have better outcomes and expanded the number of cases of corneal blindness that can be treated successfully.
Femtosecond-laser-assisted-surgery, i.e. improving surgical accuracy, bioengineered corneas as well as medical treatment for endothelial disease may also play a part in future therapies. Other potential therapies that are currently investigated are cultured human corneal endothelial cells for endothelial repair as well as gene therapy for corneal endothelial regeneration. In a concluding statement the researchers say:
"In the field of corneal transplantation, conventional penetrating keratoplasty procedures are being rapidly replaced by newer forms of selective lamellar keratoplasty, which provide improved visual outcomes, high graft survival rates, and less postoperative complications. Further developments in artificial cornea technology, stem-cell transplants, and corneal endothelial therapies are on the horizon in this rapidly evolving ophthalmic subspecialty."