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Corneal transplantation for keratoconus
The drawings above are sections through the cornea, which is the clear front part of the eye wall. Keratoconus is a condition in which the central part of the cornea is relatively thinned and has an abnormally steep curvature.
The reason some patients develop keratoconus is not fully understood. But affected patients are thought to be predisposed to developing an abnormal corneal shape by minor genetic defects in the molecular pathways governing corneal tissue maintenance and wound repair. Cells called keratocytes are responsible for replacing and repairing corneal tissue. This tissue maintenance is normally a very slow process, and the shape changes seen in keratoconus typically develop over many years.
Mild forms of keratoconus are common, and not normally associated with any visual problems. Laser corrections for myopia can make keratoconus worse, and patients are screened carefully prior to treatment to ensure that they do not have a pattern of corneal shape changes which is suggestive of very mild keratoconus.
Contact lens wear for keratoconus
Patients with more advanced corneal shape changes may have difficulty in correcting associated focussing problems with spectacles. At this stage, rigid gas permeable contact lenses are very effective. Rigid lenses tend to work better than soft lenses in keratoconus because their shape is not influenced by the regularity of the corneal surface. Most patients adapt very well to rigid gas permeable contact lens wear, and a variety of strategies is available to help patients who find these lenses uncomfortable. Moorfields has a specialised medical contact lens fitting service for patient with keratoconus and other corneal shape problems.
Emerging therapies in keratoconus
Corneal transplantation is normally only necessary where corneal shape changes in keratoconus have advanced to the point at which a stable, comfortable contact lens fit can no longer be obtained. Approximately 1 in 5 patients reaches this stage. Emerging new procedures such as corneal collagen cross-linking (CXL) and intracorneal ring segment (ICRS) implantation may help diminish the numbers of patients requiring corneal transplantation.
Corneal transplantation for keratoconus
Although most patients with keratoconus do not progress to the stage at which corneal transplantation is necessary, keratoconus remains the most common clinical indication for corneal transplantation. Two types of corneal transplant are commonly used to restore a normal corneal shape in advanced cases of keratoconus.
Conventional corneal transplantation - PK (Penetrating Keratoplasty)
In conventional corneal transplantation, the full thickness of the front of the eye wall is replaced by a disc shaped piece of donor corneal tissue which is sewn into place. The circular wound heals gradually, and the sutures are normally removed 1 to 2 years after surgery.
Otherwise known as full thickness corneal transplantation, PK is still commonly preferred in keratoconus, particularly where there has been scarring or damage to the back layers of the cornea in later stage disease. For many patients with keratoconus requiring corneal transplantation, the delicate pump cell layer lining the back of the cornea (the corneal endothelium) remains normal. Surgeons often now attempt to leave this normal layer in place, selectively replacing the abnormally shaped part of the cornea in front.
Partial thickness corneal transplantation - DALK (Deep Anterior Lamellar Keratoplasty)
In DALK, a central disc of corneal tissue including the front 95% of the eye wall is replaced with a reciprocal disc shape of transplant tissue. Importantly, the endothelial cell layer and a thin layer of supporting tissue is left in place. The endothelial cell layer is the main target of immunological attack in corneal transplant rejection reactions, and damage to this layer during graft rejection can cause the graft to fail (become cloudy). Rejection reactions directed against other layers of the cornea are less common and are relatively easy to treat. Because the endothelial cell layer is not transplanted in DALK, rejection reactions are much less problematic.
Outcomes of conventional corneal transplantation (PK) for keratoconus
- Although around 1 in 3 patients will experience at least one rejection episode after PK for keratoconus, 95% of corneal grafts remain clear for 5 years or longer, and 90% remain clear for 10 years or longer
- Problems with astigmatism and large differences in the degree of myopia between the right and left eyes are common after surgery, and 1 in 3 patients remain reliant on contact lenses for visual correction after transplantation. Further surgery to correct high levels of astigmatism or myopia is commonly required for the best results.
DALK vs PK
Advantages for DALK include:
- Fewer problems with rejection - as explained above, the endothelial cell layer is the principal target for immune attack in corneal transplant rejection reactions. Leaving the original endothelial layer in place with a DALK avoids most rejection problems.
- Increased wound strength - the strength of the eye wall is greater after partial thickness (DALK) than after full thickness (PK) grafting techniques. Corneal sutures are typically removed within 1 year of DLK; whereas removal at 18 months or later is normal after PK.
Disadvantages for DALK include
- Reduced visual clarity - some visual clarity is lost as a result of light scatter at the interface between the transplant and the host tissue in all partial thickness corneal grafting techniques. The amount of any visual degradation is usually small (typically less than one line on the test chart), and often reduces with continued healing in the first year after transplantation.
- Technical difficulty - the thin layer required created during DALK is very delicate and breaks during surgery in around 1 in 8 patients. If the perforation in this layer is small, a DALK is usually still possible, although revision procedures are then sometimes required to remove fluid which may accumulate between the perforated endothelial support layer and the new graft. The alternative is to convert to a conventional PK during surgery. Perforation of the posterior endothelial support layer is more likely in patients with advanced keratoconus and a very steep corneal profile.
Both PK and DALK produce excellent results in keratoconus. DALK has saftety advantages, but the chances of maintaining a clear graft after PK are very good. PK probably gives marginally superior visual results.
Recovery from surgery
Corneal transplantation (PK and DALK) can be performed under either general or local anaesthetic. An overnight stay after surgery is often preferred for patients travelling from outside London. The eye is usually sore and watery in the early days after surgery, but comfort quickly improves. Drops are used frequently to help improve comfort, protect from infection, reduce inflammation and promote healing. Patients can usually return to work after 2 weeks off to concentrate on putting eye drops in hourly or 2 hourly.
Vision is usually blurred in the early days after a corneal graft. 3 months are required for initial shape stabilisation before a spectacle test is performed to complete the first stage of visual rehabilitation. Vision then usually remains stable until sutures are removed 18 months to 2 years after surgery in PK patients and at 9 months to 1 year after DALK. Suture removal produces further corneal shape changes, and additional spectacle testing or revision surgery to correct the corneal shape is often necessary to get the best visual results at this stage.
Corneal transplant rejection
Anti rejection eye drops are usually continued for at least a year after PK. It is important to come straight to Moorfields Eye Casualty if, after corneal transplantation, the eye becomes blurred, red or painful at any stage (even many years) after surgery. Most rejection episodes can be reversed, but prompt treatment is necessary to avoid corneal transplant failure.
Rejection reactions are unusual after DALK, and those that occur are usually mild and easily reversed. Nonetheless, it is important to attend for emergency review if the eye becomes red and painful or if the vision is blurred.
For a corneal transplantation assessment with Mr Allan you will need to be eligible for NHS treatment and have a referral from your GP or Ophthalmic Surgeon. Referrals should be addressed to Mr Bruce Allan, Consultant Ophthalmic Surgeon, Moorfields Eye Hospital, City Rd, London EC1V 2PD
NHS Secretary - Barbara Stacey (email Barbara.Stacey@Moorfields.nhs.uk; telephone 02075662320)