Penetrating keratoplasty for keratoconus

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THE CORNEAL RECIPIENT

The cornea is the clear part of the eye which lies in front of the coloured iris and the central pupil. It acts as a window to let light into the eye, and its smoothly curved shape helps to focus the light rays through the pupil, through the lens, and onto the retina at the back of the eye.

The cornea can become damaged by a number of diseases which may leave a scar or opacity that reduces the vision. Sometimes the normal function of the cornea fails, and then it becomes hazy and the eye may become painful and inflamed.

In the condition of keratoconus, the shape of the cornea becomes irregular and this causes distortion of vision.

When the cornea fails to function for any of these reasons, it may be necessary to replace the diseased cornea with a graft of healthy corneal tissue transplanted from a donor.

Because of the theoretical risk of disease transmission, corneal transplant recipients are no longer able to be blood or organ donors.


THE CORNEAL GRAFT

The operation can be performed under a general or local anaesthetic, depending on the condition of the patient. Using microsurgical techniques, the diseased area of the cornea is removed and replaced with an identical disc of donor tissue that is fastened with very fine sutures. Steroid drops must usually be put into the eye for some time after surgery to help prevent rejection.

When the cornea has healed the stitches can be painlessly removed following a drop of local anaesthetic. A change of spectacles is almost always required after a graft operation, but this is not generally worthwhile until the shape of the graft has stabilised, which can take from six months to a year.


Different types of graft operation

Full-Thickness Corneal Graft - (Penetrating Keratoplasty)

A full thickness corneal transplant is carried out when the endothelium fails. The central part of the patient's cornea is cut out with a circular blade (trephine) and replaced with a healthy living cornea obtained from a corneal donor. The success rate of this type of graft varies from 55% - 95% with an average graft survival of about 75% at five years post-operatively.

A common cause of failure of a full-thickness graft is rejection. When a graft is rejected the patient's cells attack and destroy the endothelium of the graft and this causes it to become hazy. If a penetrating graft fails it can be repeated, but the chance of success for a repeat-graft is not as good as for a first-time graft.

Tissue-matched graft

In patients where the risk of graft rejection is higher, for instance if they have had a previous graft that has failed, then a graft with a donor tissue type closely matched to their own may give a better chance of success. Because tissue types vary so greatly it can sometimes mean a long delay before a suitable donor cornea becomes available. For this reason tissue matching is not generally carried out for routine operations.


D
eep Anterior Lamellar Graft

In this operation, known as deep anterior lamellar keratoplasty or DALK, the outer layers of the patient's cornea are removed and replaced with a partial thickness of donor tissue. This operation has the advantage that the graft cannot fail from endothelial rejection, but is suitable only for people in whom the inner endothelial layer is still healthy.


Keratophakia graft

In keratoconus the shape of the patient's cornea is thin and distorted. Implantation a layer of donor cornea into the patient's cornea can restore the cornea to a normal thickness, and help stabilise the condition.


Epikeratophakia graft

'Epi' grafts can also be used in early keratoconus. These grafts are placed on top of the patient's cornea. 'Epi' grafts have also been used to correct severe long or short sight.



THE CORNEAL DONOR

Many people carry organ donor cards or have suggested that after their death they would like to become a corneal donor. If this is the case their corneas can be used, with the consent of close relatives or friends, for transplant operations.

If you wish to register as a corneal donor, registration forms are available from doctors' surgeries, chemists, libraries, or by telephoning free of charge on 0800 555 777.

Corneal donation is entirely voluntary, and without the generosity of donors and their families many patients suffering from blindness would be untreatable. By giving consent to donation the donor's family can enable two patients to have their sight restored.

To avoid transmission of infection, a donor's medical history is checked to exclude conditions including rabies, Creutzfeld-Jacob disease (CJD), and diseases of the nervous system of unknown cause. Enquiries are made about possible exposure to the AIDS virus, and blood is tested for HIV, hepatitis B and C, and syphilis.

After transplantation has been carried out, many patients are keen to express their thanks. They may do this only indirectly because, for reasons of confidentiality and respect for privacy, the identity of neither donor nor recipient is disclosed.


CORNEAL STORAGE

Unlike many types of transplant operation that have to be carried out as an emergency, it is possible to perform graft operations as part of a routine schedule because corneas can be stored.

There are half a dozen cornea banks in the UK, and there is a central transplant co-ordinating organisation that regulates the supply of corneas from the cornea banks to the eye surgeons carrying out the graft operations. Corneas are preserved in culture medium and this keeps the tissue healthy for several weeks. This allows enough time for the necessary tests to be carried out on the donor and the cornea to ensure that the cornea is suitable for transplantation. When all the tests are completed, the patient and surgeon are notified and the operation can go ahead.


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