Corneas are the most commonly transplanted tissue worldwide, and rapid advancements mean the long-developed technique of complete transplantation (penetrating keratoplasty/PK) is no longer necessary in many cases. This third paper in The Lancet Series on ophthalmology discusses the latest advances in corneal transplantation, and is written by Dr Donald Tan, Singapore National Eye Centre, Singapore, and Professor John Dart, Moorfields Eye Hospital NHS Foundation Trust, London, and University College London, UK, and colleagues.
In the USA, more than 42,000 cornea transplants were completed in 2010, compared with 12,600 solid-organ transplants (2008), covering kidney, liver, lung, pancreas, heart, and intestine. In the UK in 2010 and 2011, there were 3565 corneal, 2671 kidney, and 689 liver transplantations. Eye banks cannot, however, match demand worldwide, and there are long waiting lists for corneal transplantation in most developing countries.
The adoption by specialist surgeons of newer forms of lamellar transplantation surgery, which selectively replace only diseased layers of the cornea, has been a fundamental change in recent years. Deep anterior lamellar keratoplasty (ALK) is replacing PK for disorders affecting the corneal stromal layers, while eliminating the risk of endothelial rejection. Endothelial keratoplasty (EK) which selectively replaces the corneal endothelium in patients with endothelial disease, has resulted in more rapid and predictable visual outcomes.
Advantages for PK are that it has potentially the best optical result since no lamellar-corneal interface problems exist as the whole cornea is being transplanted. It is also relatively easy to perform and can be used for any indication. However, the risk of complications including rejection and graft failure are a disadvantage (with rates varying depending on the disease being treated). ALK is performed in the part of eye that is not served by immune function, resulting in a low risk of many complications. However it is usually more technically demanding, and can fail if the host endothelium is not healthy. EK usually results in early visual recovery and better visual outcomes than PK, with lower complications. However, it can result in suboptimal vision unless the corneal stroma is relatively free of opacity.
Tan explains that in cases of long term endothelial dysfunction, the corneal stroma becomes not only chronically swollen but also gets scarred, so that even though the endothelial layer is replaced in EK, the remaining host stroma, although much less swollen (and therefore clearer) may still be scarred. This can reduce vision.
In EK, an extra layer of donor tissue is added onto the recipient stroma, so it can have a similar effect to double glazing—because there are 2 layers, sometimes this can affect visual quality, so the percentage of EK patients which regain perfect (6/6) vision are less than in PK. However, because PK requires a lot of corneal sutures which distort the cornea and cause astigmatism. Often wide ranges of refractive errors can occur with PK (the donor cornea refractive power can be unpredictable); the visual quality of PK is also affected in a different way, usually far worse than the "double glazing" effect in EK. Because sutures need to be removed, and only months later, visual rehabilitation after PK can be very prolonged (it may take 6 months for decent vision to stabilize). In EK, because there are no sutures, no distortion of the original cornea, and no dramatic or unpredictable changes in the refractive error of the eye, visual rehabilitation can be as fast as a couple of weeks. In all published EK studies, vision is far superior overall to PK.
Other emerging therapies are ocular surface reconstruction and artificial corneal (keratoprosthesis) surgery, which have become more widely available because of rapid advances in these techniques. Collectively, these advances have improved outcomes, and expanded the number of cases of corneal blindness that can be treated successfully. Femtosecond-laser-assisted-surgery (improving surgical accuracy), bioengineered corneas, and medical treatment for endothelial disease are also likely to play a part in future treatment. Other possibilities being examined are cultured human corneal endothelial cells for endothelial repair, and gene therapy for corneal endothelial regeneration.
The authors conclude: "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."
Dr Donald Tan, Singapore National Eye Centre, Singapore. T) +65 98181962 E) snecdt@pacific.net.sg
Professor John Dart, Moorfields Eye Hospital NHS Foundation Trust, London, and University College London, UK. E) j.dart@ucl.ac.uk
Journal
The Lancet