Overview of Cornea Transplantation
Corneal disease, which affects the clear front surface of the eye, is the fifth most common cause of blindness worldwide. The main treatment for this is a procedure called keratoplasty, also known as a cornea transplant. This treatment is often necessary for a variety of corneal disorders, which can be caused by factors such as infections, nutritional disorders, and degenerative conditions. These disorders are especially prevalent in developing countries, particularly in Africa and Asia, and can lead to a particular type of blindness called corneal blindness.
The first successful cornea transplant was performed way back in 1905. Since then, this procedure has been used globally to treat eye conditions such as keratoconus (a condition where the cornea becomes cone-shaped), dystrophies (which cause clouding of the cornea), and microbial keratitis (an infection of the cornea). This is especially important because treating these conditions can often prevent blindness. However, cornea transplants are often limited by a lack of expert surgeons and donor corneas, particularly in developing countries.
The history of cornea transplant goes back to 1800 but saw significant improvements in the last 20 years. The first successful transplant of a human cornea was performed by Eduard Zirm in 1905. Since then, the technique has been further developed, and now there are different types of cornea transplants. Some of these involve transplanting all the layers of the cornea called a penetrating keratoplasty (PKP), and others, like lamellar transplants, involve replacing only some layers of the cornea.
Recent advancements in cornea transplant have been mainly in improving surgical techniques, tissue banking, and understanding of immune reactions in the body. One of the main advancements in the last 20 years has been the ability to replace only the specific layers of the cornea affected by disease, such as with posterior lamellar keratoplasty (PLK).
There are five layers to the cornea, and depending on the condition, surgeons can replace any of these during a transplant. This is important because this selective replacement can lead to better sight outcomes for patients and reduce the risk of complications. For example, in full-thickness transplant or PKP, all five layers of the cornea are replaced. But in other types such as deep lamellar endothelial keratoplasty (DLEK) and Descemet stripping endothelial keratoplasty (DSEK), only the innermost layers may be replaced.
Anatomy and Physiology of Cornea Transplantation
The cornea is a clear and bloodless part of the eye that is critical for protecting the eye’s inner components. It also makes a significant contribution to the eye’s ability to focus light, accounting for about two-thirds of this function. The cornea is curved and is the outermost part of the eyeball. It’s about the size of a large marble, and its thickness changes from the middle to the edge, being slightly thinner in the center and thicker on the sides.
This part of the eye has a rich supply of nerves coming from a main nerve called the trigeminal nerve. These nerves form an extensive network, similar to a highway system, providing the cornea with sensitivity.
The cornea is made up of various layers, each having different functions:
The epithelium is the uppermost layer, serving as a protective barrier for the eye, ensuring a smooth surface for light reflection, and playing a role in immune responses. You can think of it like the skin of the eyeball. It’s made up of 5 to 7 cell layers in the middle and 7 to 10 cell layers on the edges. These cells constantly renew from the edges to the center.
The Bowman layer is a tough layer, made up of types I and V of a protein called collagen, which helps maintain the shape of the cornea. Interestingly, this layer has the ability to regenerate, quite like when your skin heals after a cut.
The stroma is the next layer, providing strength and focusing ability to the cornea. It contains regularly arranged fibers made of collagen, a jelly-like substance called glycosaminoglycans, and cells called keratocytes arranged in flat layers or lamellae. These substances are similar to those found in your tendons and ligaments.
The Dua layer is comparatively thinner, tightly attached to the stromal fibers. There’s also the DM layer made up primarily of collagen and another protein called laminin. This layer is continuously renewed by a layer of cells below it called endothelial cells. The DM layer forms the base for these cells, and contributes significantly to keeping the cornea transparent.
The endothelium is the last layer, made up of a single layer of cube-like cells full of mitochondria – the powerhouses of cells. This layer plays a key role in keeping the cornea clear.
Why do People Need Cornea Transplantation
DALK is a type of eye surgery that’s best for people who have a scar on the back part of their cornea, but the innermost layer of the cornea, called the endothelium, is healthy. If the endothelium is not in a good state and there is a scar on the front part of the cornea, then a full-thickness cornea transplant, known as PKP, might be more suitable. However, if the front part of your cornea is in good shape, DSEK or DSAEK surgeries could be the better choice.
Eye doctors carry out cornea transplants for various reasons, such as improving your eye’s appearance (cosmetic reasons), strengthening the eye’s structure (tectonic reasons), improving your eyesight (optical reasons), and treating eye diseases (therapeutic reasons). Having a type of cornea transplant called a lamellar transplant has several benefits, such as a smaller chance of serious eye bleeding, keeping the eye’s structure intact, and a lower chance of the transplanted tissue being rejected after surgery.
Worldwide, one of the main causes of blindness is problems with the cornea. The reasons for needing a cornea transplant can differ depending on the country. Developed countries often perform cornea transplants due to a disease causing fluid to build up on the cornea, known as bullous keratopathy. In contrast, in developing countries, infections and corneal scars are more commonly responsible. Interestingly, the reason for cornea transplants is changing in developing countries, moving from mainly bullous keratopathy to other causes of cornea deterioration.
A study found that the amount of cornea transplants has hugely increased, from under 1,000 in 2005 to 24,400 in 2014. The main reasons for having these transplants were largely bullous keratopathy, a condition causing the cornea to thin and change shape named keratoconus, a disease causing cells on the cornea to die known as Fuchs endothelial dystrophy, and failed grafts. Interestingly, by 2014, keratoconus and Fuchs had overtaken bullous keratopathy as the main cause for transplant.
More cornea transplants are now being done for people with conditions they were born with, such as Peter anomaly, congenital hereditary endothelial dystrophy (CHED), sclerocornea, and corneal dermoid, which all cause parts of the eye to be opaque. Meanwhile, a different type of cornea transplant called lamellar keratoplasty is commonly used for treating cornea deterioration, peripheral ulcerative keratitis (PUK), keratoconus, and general front cornea scarring. For people with keratoconus, the preferred course of action is now often DALK over the traditional PKP.
There are several reasons you might need a cornea transplant, such as keratoconus, ectasias, corneal deterioration, dystrophies, keratitis, congenital opacities, chemical or mechanical corneal trauma, and regrafts. However, the most common reasons can vary depending on other factors, like where you live or how wealthy your region is. What’s more, these common reasons may not represent the most common causes of cornea disease.
Damage to the cornea can happen due to several reasons, such as healed infections, trachoma infection, trauma, and conditions that affect the anterior (front part) of the cornea and deteriorate it. There’s a type of surgery called endothelial keratoplasty (EK) that involves changing different parts of the cornea. However, if your corneal endothelium is healthy, you will not be recommended this type of surgery. Instead, EK procedures are best for people with keratoconus, Fuchs’s endothelial dystrophy, posterior polymorphous dystrophy, CHED, bullous keratopathy, iridocorneal endothelial syndrome, and failed EKs. DALK, on the other hand, concentrates on the front part of the cornea and is recommended for people with keratoconus and corneal scars.
When a Person Should Avoid Cornea Transplantation
When it comes to getting a corneal transplant, which is a surgical procedure that replaces the clear front part of the eye, different countries and regions have different rules about who can and can’t get the surgery.
For example, in the United Kingdom, the National Health Services Blood and Transplant Agency says that there are certain situations where a person absolutely should not get a corneal transplant:
* If the transplant isn’t expected to improve the way the cornea works or looks, then it should not be done.
* If the transplant doesn’t get rid of tissue that could cause the eye to get more damaged, the person should not get a corneal transplant.
Equipment used for Cornea Transplantation
Having fundamental surgical tools is necessary. Along with this, the ability to use a type of local numbing (like a retrobulbar block, which numbs the eye area) is also crucial. This can be used alongside intravenous sedation (a drug delivered into a vein to make you relaxed and sleepy) or general anesthesia (a medicine which makes you unconscious or ‘asleep’ during surgery).
Who is needed to perform Cornea Transplantation?
When you have an operation, there’s a whole team that works together to take care of you. This team includes the surgeon – a doctor who specializes in doing operations. They’re the one who will perform the operation.
There’s also an assistant who helps the surgeon during the operation. The anesthesiologist is another important member of the team. This is a doctor who makes sure you stay asleep and don’t feel any pain during your operation.
Last but not least, there are also nurses as part of the team. They help before, during, and after your operation. They may get you ready for your operation, assist during it, and take care of you afterwards. Every member of this team is there to help you get through your operation safely and comfortably.
Preparing for Cornea Transplantation
Before any medical procedure, it is important for the doctor to thoroughly explain what will happen. This includes any risks and benefits, to make sure the patient fully understands. After this, the patient will formally agree to have the procedure, which is known as giving “informed consent”.
Prior to the procedure, the doctor will ask about the patient’s current medications and any allergies they might have. An eye exam will also be performed. In many cases, the people getting this procedure may be older and have existing health conditions. This means special attention needs to be given to the medicines they’re taking. This could include medicines to thin the blood, control blood pressure, or manage diabetes. There are often specific rules regarding if and when such medications should be continued or stopped before the procedure, so the healthcare team will clearly explain these details.
Clear communication between the patient, their caregivers and the doctors, is crucial. This can help identify any problems early and manage everyone’s time effectively. Patients should be given contact numbers to call for immediate help if they have any problems after the procedure. It is also important to arrange for transportation home and care after the procedure, ideally with a family member or friend. The goal is to provide a positive and supportive environment for recovery. This comprehensive approach not only helps the procedure go smoothly, but also supports the patient’s well-being throughout the process.
How is Cornea Transplantation performed
Penetrating Keratoplasty (PKP)
This is a type of eye surgery where a small, round piece of your cornea is replaced with a similar piece from a donor. To achieve this, you will be put to sleep with anesthesia. Then, your eyelid is gently opened and held in place using a special device. Using precision tools, the doctors measure the size of your cornea to decide how big the donated piece needs to be. This donated piece is then carefully inserted and held in place with special thread that does not dissolve, to ensure a secure fit. A special gel-like substance (hyaluronic acid) may be used to keep the colored part of your eye, the iris, from sticking to the stitch line. After the surgery, you will be given eye drops and the eye will be covered properly.
Nowadays, other techniques like Anterior Lamellar Keratoplasty (ALK) which only replaces the front layer of the cornea, has been gaining more preference due to lesser complications. However, PKP is still the gold standard worldwide. It has shown to consistently give great results especially in cases of deep scars, bulging of the cornea (keratoconus), infection, and trauma. For microbial keratitis, PKP continues to be the primary technique with excellent results. In some cases, Deep Anterior Lamellar Keratoplasty (DALK), which is another type of ALK, can be a successful alternative to PKP.
Deep Lamellar Keratoplasty (DALK)
This is another type of corneal transplant surgery that involves removing all or part of the middle and front layers (stroma) of the cornea while leaving the back layer (endothelium) intact. This surgery minimizes the chances of the new cornea being rejected, a common cause of graft failure. However, this surgery can be technically challenging and requires skill to successfully separate the layers of the cornea.
Superficial Anterior Lamellar Keratoplasty (SALK), Automated Lamellar Therapeutic Keratoplasty (ALTK) and DALK are various ways of treating corneal opacity (clouding of the cornea). In both predescemetic and descemetic DALK, affected portions of the cornea are removed and replaced, ensuring healthier parts of the cornea remain intact. These surgeries, specifically DALK, can come with complications such as small and large perforations, double anterior chamber, and irregularity of the area between the cornea layers.
Posterior Lamellar Keratoplasty (PLK)
PLK focuses on replacing only the diseased back layer (endothelium) of the cornea with a healthy donor tissue. This surgical technique is commonly employed in conditions like Fuchs endothelial dystrophy (a swelling of the cornea), posterior polymorphous corneal dystrophy, bulging of the cornea due to fluid accumulation, and iridocorneal endothelial syndrome which affects the eyes’ drainage system. Older versions of this surgery did not gain much popularity due to suboptimal outcomes. However, newer techniques that utilize an air tamponade (a bubble of air to apply pressure) instead of sutures has greatly improved the results. Descemet Stripping Endothelial Keratoplasty (DSEK) and Descemet Membrane Endothelial Keratoplasty (DMEK) are now widely preferred as alternatives to PKP, given the lesser complications observed. These newer techniques only replace the diseased endothelium, leading to improved visual outcomes compared to standard PKP.
Possible Complications of Cornea Transplantation
Problems after surgery can happen either soon after the surgery or much later on. Issues that happen soon after, or “early complications,” can happen over a few days up to several weeks after the operation. These may include something like leaking from the wound, increased eye pressure, bleeding, or infection.
Other problems can happen a long time after surgery, from months to years later. These are called “late complications,” and they can include a wide range of problems. This list could also include something like swelling of the front surface of the eye (corneal swelling), clouding of the eye lens (cataracts), failure or displacement of the graft, or rejection of the graft. Other potential issues include blockage of fluid in the eye causing pressure build up (Pupillary block glaucoma), an infection of the graft, damage to the graft tissue, severe eye infections called Endophthalmitis and Panophthalmitis, growth of outermost layer of the eye on the cornea (Epithelial ingrowth), a double front chamber of the eye, an off-center graft, irregular curvature of the eye lens (Astigmatism), detachment or perforation of Descemet’s membrane (the thin layer at the back of the cornea), an increase in eye pressure again called Secondary Glaucoma, expulsion of eye lens, severe eye bleeding, or a return of the original disease that required surgery.
According to research, the success rate of the graft staying healthy and working is about 70% after 5 years and 50% after 15 years.
What Else Should I Know About Cornea Transplantation?
Corneal transplants, surgeries to replace a damaged or diseased cornea in the eye, have been successfully performed for over 110 years and are now one of the most common transplant operations around the world. The US leads in corneal transplants, especially in a specific kind known as EK (Endothelial Keratoplasty), which makes up about 60% of all transplants in the country. It’s important to note though, this is not the case everywhere, with a third of countries globally not conducting any EK operations at all.
There have been big changes in how corneal transplants are done, with new techniques focusing on only replacing the part of the cornea that is damaged. These advancements have improved graft survival (how well the transplant works), visual sharpness, and the ability to focus, which has prompted more people to consider surgery. However, this also means the need for human corneas for transplantation has grown, and currently, there’s only enough supply to meet about 30% to 40% of this demand.
A significant challenge is the fact that corneal endothelial cells (CECs), which are cells on the inner layer of the cornea, do not multiply well once removed for examination in a lab. As such, scientists are studying other types of stem cells as possible sources of new corneal cells. Some promising research has shown that certain stem cells can be guided to become CECs in a lab. If this method can be fine-tuned, it may serve as a different method for replacing damaged corneas, mainly when dealing with CEC dysfunction, like in the case of an eye disease called Fuchs endothelial corneal dystrophy. As research in this area continues to progress, it could present exciting new possibilities for corneal transplantation in the future.