Overview of Cornea Transplantation
Corneal disease, which affects the clear front surface of the eye, is the fifth largest global cause of blindness. To treat this, corneal transplantation, also known as keratoplasty, is commonly performed. This surgery was first successful in 1905 and it’s widely used to treat various eye disorders that may lead to blindness. Such disorders include dystrophic (growth-related), infectious, degenerative (breaking down over time) and inflammatory corneal diseases, which could be due to other conditions that affect the surface of the eye. Conditions like trachoma (an infection that damages the eyes), xerophthalmia (dry eyes), river blindness, and microbial keratitis (an infection of the cornea) are the second biggest cause of blindness worldwide, next to cataracts. These are more common in Africa and Asia where corneal scarring, especially in children, is a high cause of reversible blindness.
While corneal transplantation is the main way to rehabilitate vision, its success is limited by the availability of cornea donors and the required expertise, especially in developing countries. Unlike in Western countries, where inherited, degenerative, and iatrogenic (caused by treatment) conditions such as Fuchs corneal endothelial dystrophy, keratoconus, and pseudophakic bullous keratopathy are more prevalent, demand for treatment in developing countries often outweighs supply. This leads to long waiting periods.
Modern corneal transplantation evolved from centuries of exploration, testing and persistence. The concept of corneal transplant was first proposed in 1789, while the first successful human corneal transplant was performed in 1905 on a patient who had suffered lime burns. Over the years, advancements like lamellar corneal transplants (which layer different elements of the cornea instead of transplanting the entire cornea) reduced the risk of the transplant failing. For successful outcomes of this surgery, it is important to address factors like inflammation and prepare the ocular environment thoroughly, while also controlling wider health issues like diabetes, hypertension and heart, lung and kidney diseases.
Over the last 20 years, developments in our understanding of the immune system, surgery techniques and tissue storing practices have largely influenced corneal transplantation. For example, recent techniques now allow for transplantation of only certain parts of the cornea, leading to better results and reduced complications. Despite these advancements, there is still significant demand for traditional full-thickness corneal transplants, particularly in countries where eye diseases are more rampant.
Anatomy and Physiology of Cornea Transplantation
The cornea is a clear, blood vessel-free part of the eye that has a key duty in securing the eye’s inner elements and aids significantly in focusing the light entering the eye. The cornea which has a rounded bulge shape is found at the very front of the eyeball. It extends about 11.5 mm; it’s greater in the middle, at 550 to 565 μm, and thicker on the edges, from 610 to 640 μm. There is an extensive network of nerves spread across the cornea with fibers originating from the ophthalmic division of the trigeminal nerve, which is the primary nerve of the face.
The cornea consists of several layers:
– Epithelium: This is the outermost layer, first defense of the eye, supplying a smooth surface for the refraction of light, and also has immune response functions. This layer is made up of 5 to 7 cell layers in the middle and 7 to 10 cell layers around the edges, keeping a thickness of about 50 μm. Stem cells and others multiply on the edges and migrate towards the center, replacing surface cells. This layer contains squamous cells, wing cells, and basal cells.
– Bowman layer: This tough layer is composed of collagen types I and V, which acts as a resilient, cell-free support structure of the stroma, affecting the shape of the cornea. This layer can regenerate when damaged.
– Stroma: This layer gives strength and ability to refract, or bend, light. It has neatly arranged collagen fibers, glycosaminoglycans, and interlinked cells called keratocytes in tightly packed layers or lamellae. At the cornea’s center, there are 200 lamellae with more packed and interconnected in the front region than in the back.
– Dua layer: This layer is 10 to 15 μm thick and tightly adheres to the stromal fibers.
– DM: This 7- to 10-micrometer structure is mainly created from type IV collagen and laminin, secreted constantly by the endothelial cells. The DM serves as a base for endothelial cells and plays a significant role in keeping the cornea clear.
– Endothelium: This layer consisting of a single layer of cube-shaped cells, which are highly rich in mitochondria, is vital in preserving transparency.
Why do People Need Cornea Transplantation
If you have a scar on the back of your cornea but the part of your eye called the “endothelium” is still healthy, your doctor may recommend a surgery called DALK (Deep Anterior Lamellar Keratoplasty). However, if the front part of your cornea has a scar, and the endothelium isn’t working properly, the best option might be a surgery called full-thickness PKP (Penetrating Keratoplasty). If only the front part of the cornea is affected, doctors usually recommend surgeries called DSEK (Descemet’s Stripping Endothelial Keratoplasty) or DSAEK (Descemet’s Stripping Automated Endothelial Keratoplasty).
Doctors use different types of cornea transplants based on different factors like how the eyes look, how well they’re doing their job, and if there’s any damage to the eyes. Less tissue is used in lamellar transplants, which are a kind of cornea transplant surgery. This type of surgery carries less risk and can help keep the eye’s structure intact and avoid rejections after the surgery.
There are many reasons as to why a person may need a cornea transplant across the world. It largely depends on where you live. In developed countries, a disease called “bullous keratopathy” is a common reason for needing a cornea transplant. However, in developing countries, infections (infective keratitis) and corneal scars are more common reasons.
One study showed that the number of corneal grafts, which are used in transplant surgeries, had significantly increased between 2005 and 2014. Reasons for needing a cornea transplant can include a disease called Fuchs endothelial dystrophy, a condition called keratoconus, bullous keratopathy, and failed prior transplants. Recently, there’s been a rise in cornea transplants for conditions like congenital opacities (or birth defects in the eye), and Keratoconus.
There are various reasons why someone might need a cornea transplant. Some of these include eye conditions such as Keratoconus, corneal degeneration, inflammation of the cornea (keratitis), birth defects that affect the cornea, injuries to the cornea, and failed previous cornea transplants. The exact cause can vary, and is often influenced by various factors such as socioeconomic conditions, geography, and economic status.
There are different reasons why you might get superficial corneal scars. This can be caused by infections, eye diseases called dystrophies that affect the front of the cornea, or degenerations. If you have any of these conditions and it’s causing problems with your endothelium, you might need a type of surgery called EK or “Endothelial Keratoplasty”. Different types of EK procedures, including PLK, DLEK, DSEK, DSAEK, DMEK, and DMAEK, could be used based on your specific situation. However, these EK procedures are not suitable if your endothelium is healthy.
Another type of surgery called DALK, focuses on the front part of the cornea. This might be the best choice for patients with conditions like keratoconus or corneal scars. You might need a DALK procedure for deep scars on your cornea caused by infections or chemical injuries that have healed, keratoconus, or a type of eye disease called stromal dystrophies.
When a Person Should Avoid Cornea Transplantation
The reasons why a corneal transplant might not be suitable vary from place to place. For example, in the United Kingdom, the National Health Services Blood and Transplant Agency says a corneal transplant (replacing the clear front part of the eye) should not be done if:
1. The transplant isn’t likely to improve the way the cornea works or its structure.
2. The transplant doesn’t remove tissue that could otherwise harm the eye more.
Equipment used for Cornea Transplantation
For the surgery to go smoothly, it’s important to have the right equipment by hand in the operating room. Apart from this, the surgeon should also have the skills to use local anesthesia – for example, a retrobulbar block, which is a common way to numb the eyes before surgery. Sometimes, this local anesthesia might be used in combination with anesthesia to get you completely unconscious (general anesthesia) or a medication that will make you relaxed and sleepy (intravenous sedation).
Who is needed to perform Cornea Transplantation?
The medical team that does your operation is made up of several people. This includes the main doctor, who is the surgeon. The surgeon is responsible for doing the procedure. They’re supported by another doctor, the assistant, who helps them during the operation. An anesthesiologist is also important. This is a specialized doctor who is responsible for making you sleep during the procedure, so you don’t feel any pain. Also, there are nurses who provide additional support and care to ensure everything goes smoothly.
Preparing for Cornea Transplantation
Doctors need to clearly explain the details of the procedure to the patient, giving them plenty of time to understand and weigh the potential risks and benefits. This open and honest conversation is important for gaining the patient’s informed consent, meaning they know what to expect and agree to the procedure. Before the procedure, doctors will record the patient’s complete health history, including any medicines they’re currently taking and any known allergies. They will also conduct an eye examination.
Since many patients who need this procedure may be older and may have other health conditions, it’s important to give tailored advice on managing their medications, such as blood thinners, high blood pressure drugs, and diabetes drugs. Healthcare organizations have helpful guidelines on how to manage these drugs before surgery, advising patients whether they should keep taking them or temporarily stop.
Clear communication with the patient or their caregivers is key to identify any possible complications as early as possible, and to manage time effectively. Patients should be given contact details for immediate help, keeping in mind any issues that might come up after the operation. Plans for getting the patient home and taking care of them after the procedure should be discussed with a close family member or friend, recognizing the need for a supportive environment after surgery. This approach focuses not just on the success of the procedure, but also on the patient’s overall well-being and support throughout the entire process.
How is Cornea Transplantation performed
The process for a Penetrating Keratoplasty (PKP) or full corneal transplant surgery involves a few key steps. First, your eye will be numbed with anaesthesia. Then the surgeon will hold the eye in place using a ring or stitches, and a device will keep your eyelid open. The exact size for the new cornea (the clear covering over the front of your eye) will be carefully measured. The new cornea will be cut to be slightly larger than the size of your old cornea.
A special tool called a trephine will be used to cut out a round piece of your old cornea. The surgeon will then place the new cornea in this space and secure it with non-dissolving stitches. A gel-like substance may be used to stop the coloured part of your eye (the iris) from sticking to the stitches. After the surgery, you will receive antibiotic treatment to prevent infection, and your eye will be protected with a shield.
Nowadays, there are other ways to perform corneal transplants that may give better results and have fewer complications than a full corneal transplant. These are called Anterior Lamellar Keratoplasty (ALK) and Endothelial Keratoplasty (EK) techniques.
However, PKP can still be the best choice for certain conditions such as deep cornea scars, and it remains the globally recognised standard.
ALK involves replacing just the damaged front part of the cornea with a piece of a healthy donor cornea, while leaving the back part of the recipient’s cornea untouched. This technique can be done in different ways using various tools and can offer many benefits. However, it requires great skill from the surgeon as it involves delicate layer-by-layer removal of corneal tissue.
Deep Anterior Lamellar Keratoplasty (DALK) is a type of ALK technique that is used when the entire front part of the cornea is damaged, but the back part is still okay. DALK can be performed using different methods and can offer great results. However, complications, including small and large tears in the cornea, can occur.
Superficial Anterior Lamellar Keratoplasty (SALK) is another technique that is used for treating shallow corneal scars. In this procedure, a thin layer of corneal tissue is fixed with glue. Automated Lamellar Therapeutic Keratoplasty (ALTK) is recommended when corneal scars reach the middle layer of the cornea. It involves the use of a device that cuts the cornea evenly for better fitting of the donor tissue.
Posterior Lamellar Keratoplasty (PLK) involves replacing only the damaged back part of the cornea with healthy donor tissue. It’s typically used to treat conditions that affect the back part of the cornea, like Fuchs endothelial dystrophy, Posterior polymorphous corneal dystrophy, cultural thinning, and other conditions.
Endothelial Keratoplasty (EK), which includes Descemet’s Stripping Endothelial Keratoplasty (DSEK) and Descemet’s Membrane Endothelial Keratoplasty (DMEK), is now commonly favoured over full corneal transplant. This preference is due to its safety and it’s easier for patients to recover from.
Possible Complications of Cornea Transplantation
After surgery, some people might experience problems or changes, these are called complications. These complications can happen at different times. Early complications are those that happen within a few days to weeks after the surgery. These might include leakage from the surgical wound, pressure build-up in the eye, bleeding, and infection.
Late complications are those that occur months to years after the surgery. These might include:
* Swelling of the cornea, which is the clear front surface of the eye
* Cataracts, which are cloudy areas in the eye’s lens causing blurry vision
* Graft failure, which means the transplanted tissue isn’t working well
* Graft dislocation, where the transplanted tissue moves from where it was placed
* Graft rejection, when the body’s immune system attacks the transplanted tissue
* Pupillary block glaucoma, a type of eye damage where fluid can’t flow properly, causing pressure in the eye
* Graft infection, where the transplanted tissue gets infected
* Graft infiltrates, where cells move into the graft and can damage it
* Endophthalmitis and panophthalmitis, severe eye infections that can cause pain, redness, pus, and vision loss
* Epithelial ingrowth, where cells grow into areas they shouldn’t
* Double anterior chamber, where there’s a split in the front section of the eye
* Eccentric graft, where the graft is off-center
* Astigmatism, a common vision condition that causes blurred vision
* DM detachment and DM perforation, problems with the thin inner layer of the cornea
* Secondary glaucoma, glaucoma that’s a side effect of another eye condition
* Lens expulsion, where the lens of the eye is pushed out of position
* Expulsive choroidal hemorrhage, a severe bleeding in the eye
* Recurrence of the original disease, which means the disease the surgery was meant to treat comes back
Survival rates for grafts – the transplanted tissue – after eye surgery are about 70% after 5 years and 50% after 15 years. This means that many people still see improvement years after their surgeries.
What Else Should I Know About Cornea Transplantation?
Corneal transplants, which replace damaged or diseased cornea in the eye, have been performed for over 110 years and are now one of the most common types of transplants worldwide. Particularly in the US, this procedure is increasingly popular and constitutes around 60% of all transplants.
There’s a type of corneal transplant known as endothelial keratoplasty (EK) that’s becoming more widespread. This approach selectively replaces only the sick part of the cornea, instead of the whole thing, which is done in the traditional approach, penetrating keratoplasty (PKP). However, despite its benefits such as improved graft survival and vision, it’s not accessible in many countries around the globe.
With the advancement of these selective transplantation techniques, the criteria for performing surgery have widened. This has consequently placed a higher demand on corneas from deceased donors that can be used for transplants. Currently, the supply can only meet about 60 to 70% of the growing need.
A significant challenge lies in expanding the cells from the inner layer of the cornea (Corneal Endothelial Cells, or CECs) in a laboratory setting after they’ve been taken from the body. To overcome this, scientists are looking into other types of cells that could potentially serve as replacements. Recent studies show promising results in generating CECs in the lab by using stem cells – cells that can develop into many different types of cells – and specially tailored chemicals. Further improvements in this method could lead to new treatments for conditions like Fuchs endothelial corneal dystrophy, where the CECs aren’t working as they should.
These research endeavors could lead to transformative changes in the way we perform corneal transplants, providing great solutions for challenging medical cases.
