Surgery: corneal transplantation

The cornea is maintained in a state of relative dehydration by an important layer of pump cells lining the back of the cornea called the corneal endothelium. We are born with an ample supply of these cells, but endothelial cell density decreases slowly with age. If cell density drops below a safe minimum level, the pump action of the endothelial cell layer becomes insufficient, and the cornea starts to become waterlogged and misty. The medical term for this waterlogging is corneal oedema. In the early stages of corneal endothelial failure, patients typically experience morning misting - cloudy vision which tends to clear later in the day.

Fuch’s Corneal Endothelial Dystrophy is a relatively common inherited condition in which endothelial function declines at a faster than normal rate. Most of us are unlikely to live long enough to experience any problems with corneal endothelial failure; but some patients with Fuch’s Dystrophy require a corneal transplant to restore a healthy corneal endothelium and clear vision. The age at which morning misting becomes a problem varies greatly, and many patients with Fuch’s Dystrophy are either never affected, or only affected late in life.

Other common causes of endothelial cell loss are eye injury and any form of surgery on the inside of the eye. After cataract surgery for example, we typically lose at least 5% of our corneal endothelial cells. This is usually not problematic; but if cataract surgery is carried out in patients with a diminished reserve of corneal endothelial function, the cornea may become misty as a result. Eye surgeons will often warn patients with Fuch’s Dystrophy that cataract surgery may result in the need for a corneal transplant. If morning misting is already a problem, or if microscopic blisters are present on the eye surface (a sign of corneal waterlogging/swelling) corneal transplantation may be combined with cataract surgery to avoid a delayed visual recovery.

Different types of corneal transplant for corneal endothelial failure
Endothelial cells continuously pump fluid out of the cornea. If these cells are not working well, the cornea becomes oedematous (swollen, waterlogged) and cloudy. A corneal transplant is then required to replace the endothelium. This transplant can be done in two ways.
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.
Endothelial Keratoplasty (EK)
Many different names and acronyms have devised for the same procedure.
Some of the common abbreviations for endothelial keratoplasty are:
  1. Posterior Lamellar Keratoplasty – PLK
  2. Descemets Stripping Endothelial Keratoplasty – DSEK
  3. Descemets Stripping Automated Endothelial Keratoplasty – DSAEK

    In DSAEK (EK), a new method of corneal transplantation, a healthy new endothelium is transplanted on a thin layer of donor corneal tissue which is floated into place and adheres without sutures. The eye wall is left intact. This helps to preserve a normal corneal shape, normal strength and a normal focussing power for the eye.
Outcomes for Conventional Corneal Transplantation
  1. Around 66% of corneal grafts (2 out of 3) performed for corneal endothelial failure remain clear for 5 years or more.
  2. If the corneal transplant fails (goes cloudy) repeat corneal transplantation is usually possible. But the chances of success diminish with successive grafts. Around 50% of second corneal grafts (1 in 2) remain clear for 5 years or more.
  3. Problems leading to graft failure include: inflammation caused by an immune reaction to the transplant (graft rejection), infection, and raised fluid pressure in the eye (glaucoma). Rejection episodes are more common in repeat corneal grafts.
  4. With a clear graft, around 75% of patients (3 out of 4) have improved vision. Vision stays at the same level as before surgery in around 12.5% (1 in 8) and is worse in about 12.5%.
  5. The commonest reason for a disappointing visual result if the graft remains clear is astigmatism (irregular corneal shape giving a blurred focus). Surgery and suture manipulations to correct astigmatism are often required after conventional corneal transplantation.
Endothelial Keratoplasty (EK) vs Conventional corneal transplantation (PK)
EK, like modern cataract surgery, can be performed through a small self-sealing incision.
Advantages for EK include:
  1. 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.
  2. 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.
  3. 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 EK are:
  1. 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. Research in progress indicated that the amount of vision lost in comparison with a full thickness graft (PK) is small and any loss here should be balanced with gains associated with reduced astigmatism.
  2. 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.
Combined cataract and corneal graft surgery
Cataract surgery is commonly combined with both PK and EK. This is because cataract surgery can commonly precipitate corneal endothelial failure in patients with Fuch’s Dystrophy. In some patients, the need for corneal transplantation is clear. But where doubt exists, cataract surgery is commonly performed first with the proviso that corneal transplantation may be necessary later if the cornea becomes cloudy.

Recovery from eye surgery
Corneal transplantation can be performed under either general or local anaesthetic. Patients usually can go home on the same day after surgery but can stay overnight if required. The eye is usually uncomfortable 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. For patients with a conventional graft (PK), 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. Suture removal produces further corneal shape changes, and further spectacle testing, or revision surgery to correct the corneal shape are often necessary to get the best visual results at this stage.

Visual recovery after EK is usually faster, with the graft clearing rapidly over the first 6 weeks after surgery and relatively early shape stabilisation. Further gains in graft clarity are common in the first 6 months after surgery.

Corneal transplant rejection
Anti rejection eye drops are usually continued for at least a 18 months after EK and PK. It is important to contact an eye specialist or to present yourself at an 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.


DEEP LAMELLAR KERATOPLASTY
KERATOCONUS

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. Johannesburg Eye Hospital is associated with optometrists specialized in medical contact lens fitting for patients 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. Relatively few patients reach this stage, and emerging new procedures such as corneal ring segment implantation / collagen cross-linking may help diminish the numbers of patients requiring corneal transplantation still further.

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, a disc shaped piece of donor corneal tissue that is sewn into place replaces the full thickness of the front of the eye wall. 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 the commonest type of corneal graft. PK is also used to treat conditions in which the cornea becomes waterlogged and cloudy as a result of poor function in an important layer of pump cells which lines the back of the cornea called the corneal endothelium. Because this cell layer remains healthy in keratoconus, partial thickness, or lamellar, corneal grafts in which the healthy endothelial layer is left in situ are often performed as an alternative to PK in keratoconus.

Partial thickness corneal transplantation - DALK
(Deep Anterior Lamellar Keratoplasty)
In DALK, a central disc of tissue including the front 95% of the eye is replaced with a reciprocal disc shape of transplant tissue. Importantly, the endothelial cell layer and a thin layer of supporting tissue are 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
  1. 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
  2. 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:
  1. 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 DLK avoids most rejection problems.
  2. 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 DALK; whereas removal at 18 months or later is normal after PK.
Disadvantages for DALK include
  1. 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.
  2. 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 that 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.
Summary
Both PK and DALK produce excellent results in keratoconus. DALK has safety advantages, but the chances of maintaining a clear graft after PK are very good. PK probably gives marginally superior visual results.


Recovery from eye surgery
Corneal transplantation (PK and DALK) can be performed under either general or local anaesthetic. Patients usually can go home on the day of surgery but can stay overnight if required. The eye is usually uncomfortable 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 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 becomes blurred.

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