Corneal and external eye disease encompasses a very diverse range of conditions. At one end of the spectrum, it includes conditions which form the “bread and butter” of general or primary care ophthalmology, whilst at the opposite end of the spectrum it includes some conditions which may require very complex surgical or systemic treatment.
In the NHS, ophthalmologists with a sub-specialty interest in corneal and external eye disease are relatively thinly spread and a significant part of their work consists of tertiary referrals for the management of conditions that are rare or complex, or conditions that have proved refractory to first-line management. They are likely to maintain close links with other specialties such as dermatology and rheumatology for the joint management of systemic inflammatory conditions which compromise the ocular surface or cause corneal or scleral melting.
At its most severe, ocular surface disease can be painful, disfiguring and blinding. Patients with severe ocular surface disorders may require a considerable amount of support in coming to terms with, and managing their condition, sometimes over several decades. Chronic corneal or ocular surface disorders may also be punctuated unpredictably by acute episodes where more intensive treatment is required and it is important that the corneal and external eye disease service is configured so that patients can receive appropriate treatment in a timely fashion.
Corneal transplantation procedures form an important part of the surgical workload of the corneal specialist. The main reasons for corneal transplantation are to restore corneal transparency, to maintain or restore the integrity of the eye where corneal tissue has been lost to trauma, infection or melting and to restore a normal contour to a distorted cornea. Corneal transplants from deceased human donors are obtained from ocular tissue banks. Either the whole thickness of the cornea is transplanted (penetrating keratoplasty), the host endothelium and Descemet’s membrane may be retained (lamellar keratoplasty) or Descemet’s membrane and the endothelium may be transplanted alone (endothelial keratoplasty).
The techniques of penetrating and lamellar keratoplasty were developed several decades ago but a number of technological advances and refinements in technique have improved visual outcomes and speed of visual rehabilitation. Endothelial keratoplasty is a more recently developed procedure made possible by technological advances which allow the separation of the endothelium and Descemet’s membrane from the donor cornea without excessive damage to the endothelium. In situations where the corneal endothelium has failed but the other corneal layers are largely healthy, it potentially offers much faster visual rehabilitation than penetrating keratoplasty and does not weaken the eye structurally. However, it is a technically difficult procedure to perform and success rates (in terms of restoration and maintenance of corneal clarity) are lower than those achievable with penetrating keratoplasty, though increasing experience with the technique has been accompanied by a steady increase in success rates. NICE has undertaken a review of corneal endothelial transplantation (IPG304) which endorses the procedure but notes that it should only be carried out by surgeons with specific training in the technique.
NHS Blood and Transplant (NHSBT) is a Special Health Authority which coordinates the retrieval and supply of human tissue for transplantation in the UK. It maintains a corneal transplant database which tracks the use of each donated cornea, including the subsequent progress of the recipient eye. Periodic requests for information about each recipient are sent to the centre which performed the transplant.
Transplanted corneas are relatively protected from immunological rejection, largely because the cornea is normally avascular, and this means that it is generally possible to avoid the need for systemic immunosuppression and even for long term topical steroids. The risk of a rejection reaction is highest in the early stages following surgery and following other interventions such as corneal suture removal, but the risk never completely disappears even many years following transplantation.
The early stages of corneal graft rejection may be reversible with intensive topical steroids, but there is a point of no return beyond which the graft loses transparency irreversibly and often becomes vascularised. The risk of graft rejection is considerably increased where the recipient’s cornea is already vascularised or where there has been previous corneal inflammation or where the ocular surface is unhealthy. In these situations, long term topical or systemic immunosuppression may be needed to improve the likelihood of the graft remaining transparent.
Corneal grafts can sometimes fail without immunological rejection because of excessive loss of endothelial cells (which do not regenerate). This can occur if the donor endothelium was in poor condition (this is unlikely as the health of the endothelium is inspected before the donor cornea is released for transplantation), if there is excessive trauma during the transplantation process or there is accelerated loss of endothelial cells subsequently.
With penetrating and lamellar keratoplasty, the refractive status of the eye may take as much as a year to stabilise following surgery as the graft-host interface heals. Careful suturing technique and selective removal of sutures postoperatively can help to minimise astigmatism, but the net spherical refractive error (the overall degree of long- or short-sight) is unpredictable even with meticulous surgical technique, so spectacles or contact lenses are still likely to be required to achieve the optimum visual acuity.
For a small number of patients where the condition of the ocular surface and the degree of corneal vascularisation is such that a conventional corneal graft is almost certain to fail, an artificial cornea (keratoprosthesis) may be the only way to restore useful vision. A number of devices and techniques have been designed for this complex procedure which is carried out in a small number of specialist centres.
Where the corneal limbus has been extensively damaged (for instance by a chemical burn), there may be an insufficient number of remaining limbal stem cells to maintain the corneal epithelium. It is possible to transplant limbal tissue from the fellow eye if it is healthy (limbal stem cell autograft) to address this problem. Limbal stem cell allografting (where the graft is obtained from a living donor and augmented in tissue culture) may be possible where there is an insufficient supply of limbal stem cells in either eye), but this is a specialised procedure only available in a small number of centres. Long term immunosuppression may be required if the transplanted cells are to survive.