Corneal Graft (Transplant) Surgery
What is the cornea?
The clear “window” of the eye approximately 0.5mm thick and 12mm across. It lies in front of the fluid-filled anterior chamber of the eye and the coloured iris. It is like the lens of a camera – any opacity or distortion results in a poorly focused image. It has 3 layers:
- The thin surface “skin” (or epithelium)
- The thick central layer (or stroma) and
- The single layer of cells on the back surface (or endothelium) – this last layer is made of cells that are not replaced through life (when damaged the place of the dead cells is taken by enlargement and movement of their healthier neighbours).
All of these layers must be clear and smooth for the cornea to work as a window. The cells of the back surface layer (endothelium) pump fluid out of the cornea to maintain its thickness at about 0.5mm – if this layer stops functioning normally the corneal thickness increases and when it reaches about 0.65mm it starts to become opaque (corneal failure or decompensation), at about 0.8mm the cornea becomes waterlogged resulting in blistering of the “skin” (bullous keratopathy) leading to pain in addition to blindness.
What can go wrong with the cornea?
All the layers can be affected individually or in combinations. None of these problems are very frequent within the population.
- The central layer (stroma) may become scarred (as a result of injury or infection) or irregular in shape as a result of conditions that are probably genetically determined, such as keratoconus, or due to dystrophies such as lattice, granular and macular dystrophy.
- The back surface layer (endothelium) may become inadequate to maintain its pumping action (corneal failure or decompensation) as a result of genetically determined conditions such as Fuch’s dystrophy or injury from trauma or surgery.
- It may develop a hole (perforation) in the central layer (stroma) as a result of inflammation or infection; if this is not treated quickly the eye usually becomes blind.
- It may become opaque due to damage to the “skin-making” tissue (epithelium) around the edge of the cornea (limbus) usually as a result of severe inflammation or a chemical injury.
Why have a corneal graft (transplant)?
To replace a damaged cornea (see the commoner causes of this above) with a donated cornea. It is the only available treatment for severely damaged corneas (apart from artificial corneal transplants (keratoprosthesis) which are only for very badly damaged eyes when conventional graft surgery is known to fail or to have already failed.)
This is only worth doing when the inside of the eye (retina and optic nerve) are still functioning adequately (the camera analogy is that there is no purpose in replacing the lens in the camera if the film is not working). Conditions that may have damaged the inside of the eye are glaucoma, optic nerve disease, retinal detachment, severe inflammation or infection inside the eye.
An eye with potential vision can always detect light well even when the cornea is completely opaque.
What type of corneal graft (transplant) should I have?
There are two principal types: partial thickness (= lamellar) or full thickness (= penetrating). Penetrating grafts are the most widely used but lamellar grafts are increasingly used as an alternative to penetrating grafts in many, but not all, situations.
Penetrating keratoplasty (PK / = full thickness corneal grafts) have been the most widely carried out for all types of corneal disease for 40 years. However this type of graft is only mandatory if there is deep corneal scarring OR when the corneal disease involves both the endothelium AND the stroma. For epithelial and stromal diseases it is carried out because it is easier to replace the whole cornea rather than a layer and because the vision is possibly better after a full graft; the alternative lamellar procedure is the deep anterior lamellar keratoplasty (DALK) – see below. The downside of the penetrating graft for stromal and epithelial disease is that it is the transplanted graft endothelium that is the principal stimulus for rejection, which is the commonest complication of this type of graft (about 20% for low-risk cases), and leads to graft failure in some. Also, the donor endothelium has a limited lifespan.
For endothelial disease penetrating grafts have been the only procedure available until this millennium. However for patients with a healthy stroma and epithelium, the deep endothelial lamellar keratoplasty have been developed as alternatives (see below).
Lamellar (partial thickness) corneal grafts have been very infrequently used in recent decades but are again increasing in popularity for reasons outlined below. They may be anterior OR posterior.
Deep Anterior Lamellar Keratoplasty (DALK / = anterior grafts) have become more widely used because of the benefits of a greatly reduced risk of rejection and late graft failure and the development of better techniques for doing the surgery. They are only suitable for use in conditions affecting the front layer (epithelium) and central layer (stroma) of the cornea. A lamellar graft will not become “clear” if the posterior layer of the cornea (endothelium) is diseased or damaged. The down side is that the technique of deep lamellar keratoplasty is technically difficult and if the endothelium is perforated during the surgery the vision may not recover without a penetrating graft – the surgeon can convert to a full (or penetrating) graft at the time if this happens. Also the vision is not as good following a successful lamellar graft as after a successful penetrating graft although the difference is small and patients can expect to meet the driving standard after both types.
Posterior lamellar grafts (Endothelial keratoplasty) are a recent innovation. In this type of graft only the posterior 10% of the cornea is transplanted. This type of graft is known as Descemet’s stripping automated endothelial keratoplasty (DSAEK).
Advantages for endothelial keratoplasty (EK) include:
- Greater strength – after EK, an injury to the eye is unlikely to cause eye wall rupture and permanent loss of sight. This means fewer restrictions on activity than after PK
- Better shape – after EK, changes in the eye wall shape are greatly reduced in comparison with PK. Problems with astigmatism are uncommon, visual recovery is faster, and patients are less dependent on spectacles or contact lenses for good vision.
- No suture problems – two of the commonest reasons for graft failure in PK are rejection and infection. Both problems may be precipitated by suture loosening or breakage. Because there are no sutures on the corneal surface after EK, these problems are avoided.
Disadvantages for endothelial keratoplasty are:
- An extra optical interface – some light is scattered at the junction between the eye wall and the thin layer of tissue which supports the new endothelial cell layer. This may degrade vision, particularly in the early months after surgery
- Failure to adhere – up to 20% of EK patients (1 in 5) require a revision procedure if the graft fails to adhere at the first operation. This is usually performed under local anesthetic. A new air bubble is injected into the eye and the graft is floated back into place.
It is an unsuitable technique for a patient with corneal scarring for whom the penetrating graft will treat both the scarring and the endothelial failure.
My recommendations for lamellar or penetrating grafts: Generally I will recommend a deep anterior lamellar graft (DALK) if your cornea has a normal posterior layer (endothelium) and scarring or thinning that is limited to the anterior half of the stroma. I recommend an endothelial keratoplasty (DSAEK) for patients with endothelial disease only (Fuch’s dystrophy or pseudophakic bullous keratopathy). I do penetrating grafts (PK) if the cornea is very thin and/or scarred or if posterior layer is diseased in the presence of diseased anterior layers.
Why not to have a corneal graft (transplant)?
- If you are not prepared for a long recovery period and numerous follow up visits: a corneal graft operation is a major procedure for the eye (although not for you) and the recovery period for good vision is very prolonged (18-24 months) for PK and DALK although most patients will notice an improvement within a few days of surgery. A minimum of 10 visits is needed after surgery and the average is higher.
- 25% of patients need contact lenses for best vision and
- 10% require astigmatism correction with surgery
- Patients having DSAEK can expect stable vision within 2-4 months after surgery but will still require eye drops for 6 – 24 months or more.
- If your other eye is healthy you should think very carefully about having a corneal graft as the quality of vision will seldom be as good in the grafted eye.
- If you are forgetful about your treatment: you must be able to take eye drops for a minimum of 4-6 months (depending on the type of graft); forgetting to take medication is a frequent cause of graft failure.
Where does the corneal transplant come from and how safe is it?
- The cornea is human tissue and comes from a donor.
- Unlike other whole organ transplants the cornea can be removed several hours after death – only a small proportion are taken from “brain dead” donors.
- Donor corneas are provided by eye banks who are members of Eye Bank Associations with agreed national and international standards. My patients will have transplants provided by the Colorado Eye Bank, USA.
- Donor transplants are not released for use until the donors have been shown to have:
- no antibodies for hepatitis or AIDS
- no medical record of an undiagnosed neurological disease or degenerative neurological disorder such as Parkinson’s disease or Creutzfeld-Jacob disease
- The donor material is assessed for quality including clarity, and the health of the posterior layer (endothelium).
- The donor transplant can be stored for 7-10 days in the refrigerator in a purpose-designed medium.