The Cornea and Corneal Transplantation
The cornea is the clear, transparent tissue at the front of the eye, that functions as a 'window' for the eye to view the world. It also has the very important function of focusing light that enters the eye. The cornea is only about 11-12 mm in diameter, and 0.5mm thick in the centre, but if the cornea becomes opaque a patient would be functionally blind.
A number of diseases affect the cornea. Fortunately, many can be treated medically or with the help of contact lenses. With more serious diseases, corneal transplantation may be needed. In general, transplantation is required for optical, tectonic or therapeutic reasons. In the first instance, the cornea may become opacified. This can happen if, for instance, a severe corneal infection and corneal ulcer develops, followed by corneal scarring. In other patients, the cornea may become very thin as a result of infection or degenerative processes-so much so that the cornea is in danger of perforating or has already perforated. In such cases an emergency corneal graft is required to patch up the eye. Finally, some patients develop very severe infections which stop responding to medication. They require surgery to remove the diseased cornea, and replace it with healthy, new tissue.
Severe corneal distortion from keratoconus
A severe corneal ulcer with pus in the eye (hypopyon)
Forms of Corneal Transplantation
Even though the cornea is only 0.5mm thick in the centre, it consists of 3 important functional layers. The first is the 'epithelium'-the skin of the cornea. The middle layer forms the majority of the corneal thickness and needs to be kept in a dehydrated state to maintain transparency. It is also the layer that tends to develop scarring if it becomes damaged. The innermost layer is the 'endothelium' which is very important for maintaining dehydration and transparency of the rest of the cornea. Corneal transplantation may now be performed to replace any or all of these corneal layers.
Anterior Lamellar Keratoplasty
Posterior Lamellar Keratoplasty (DSAEK)
Penetrating Keratoplasty refers to surgery that replaces the entire thickness of the cornea. The entire thickness of the central, diseased cornea is removed, and healthy cornea is then stitched in place with up to 16 fine stitches. This surgery may be required if the entire cornea is diseased.
Lamellar Keratoplasty(LK) refers to surgery that replaces only part of the corneal thickness. If the outer/front part of the cornea is replaced, it is called 'anterior LK'. If the inner/back part of the cornea is replaced, it is called 'posterior LK'.
In conditions like corneal scarring and keratoconus, where only the front part of the cornea is diseased, anterior LK is performed. By cutting through only part of the cornea, the eye is physically stronger. However, the most important factor is that the inner corneal layer-the endothelium, remains the patient's own. This prevents the occurrence of sight threatening rejection.
On the other hand, where only the back part of the cornea is damaged, posterior LK is performed instead of penetrating keratoplasty. Descemet's Stripping Automated Endothelial Keratoplasty (DSAEK) is the main form of posterior LK currently performed and represents a revolutionary new procedure for diseases affecting the inner layer of the cornea.
- Only the thin inner lining of the cornea is removed, and is replaced with a thin layer of donor cornea through a small incision.
- Visual recovery is faster, and little astigmatism is caused as there are few or no corneal stitches.
- The cornea is also stronger in the long term with less chance of serious complications if the eye is accidentally hit.
Potential Problems After Corneal Transplantation
In most forms of keratoplasty, rejection may occur at any time after the first month after surgery. If rejection affects the inner corneal layer, the corneal graft may swell and lose its transparency. If a corneal transplant patient develops blurring of vision, he/she needs to be seen straight away, since rejection, if treated early, can be reversed. Overall, the chances of rejection are reduced to <10% by the use of steroid eyedrops. Sometimes, stronger anti rejection medication may be needed if the cornea is at higher risk due to the presence of blood vessels or multiple previous grafts.
Most patients after corneal transplantation (except anterior LK) remain on steroid eyedrops for life. This, together with the presence of stitches, may put the eye at slightly higher risk of infections. If the eye becomes red, painful or develop blurred vision after surgery, the patient should seek an urgent eye assessment.
In the early stages after penetrating keratoplasty or anterior lamellar keratoplasty, the stitches may induce a degree of astigmatism. The refractive power of the eye is also not stable yet, causing some blurred vision. As the eye heals, some of the stitches may be removed from 9-12 months after surgery, thus reducing the astigmatism and improving vision. When astigmatism has been minimised and refraction is stable, glasses may be worn for best vision.