Corneal Transplant
The cornea is the transparent, dome-shaped part located at the very front centre of the eye. It is the outermost layer of the eye and serves to protect it. Since the cornea is transparent, it can focus light onto the retina, allowing us to see. The cornea is composed of five layers, from the outermost to the innermost layers covering the eye. Each layer has different thickness. The innermost layer, which is closest to the inside of the eye, is known as the endothelium, consisting of a single layer of cells. The function of the corneal endothelium is to remove excess water from the cornea to maintain its transparency. This layer of cells is very precious because it is not replaceable. Once cells are lost or damaged, they do not regenerate.
Q2. What are the causes of corneal diseases?
There are various causes for corneal diseases, including trauma, scratches, damage, or chemical burns. Infection is also a common cause; bacterial, viral, fungal, and amoebic infections can all lead to corneal ulcers. Even if the ulcer heals, it may leave a scar, significantly reducing corneal clarity. Structural change of the cornea, such as keratoconus, can also cause the cornea to deform. It is difficult to refract the light correctly on the retina and form a clear image. Another common condition is the loss of endothelial cells. It can occur if the patient has undergone eye surgery, had chronic inflammation, or got high intraocular pressure due to glaucoma. When endothelial cells are lost or become insufficient, the cornea becomes swollen and cloudy. These are typical causes of corneal disease. Eventually, a corneal transplant may be required to improve vision.
Q3. What are the types of corneal transplants? What is penetrating keratoplasty?
There are two main types of corneal transplants. The first type is the traditional penetrating keratoplasty, and the second type is lamellar keratoplasty. Lamellar keratoplasty can be further subdivided into anterior lamellar keratoplasty and posterior lamellar keratoplasty.
Penetrating keratoplasty involves removing the patient’s entire cornea. The donor’s cornea is trimmed to an appropriate size before it is sutured onto the patient's eye surface. This procedure results in a larger wound and requires stitches. It takes a longer recovery time and can affect the visual outcome. Since the surgery involves transplanting the entire cornea, there is a higher risk of rejection.
Q4. What is lamellar keratoplasty? What are the new development?
For the type of lamellar keratolplasty, it depends on which layer of the cornea is affected by the disease. If it involves the part above the stromal layer, deep anterior lamellar keratoplasty is considered. It allows the preservation of the patient’s Descemet’s membrane and endothelium, therefore, it greatly reduces the chance of postoperative rejection. However, since anterior lamellar keratoplasty requires stitches, it takes longer time for vision improvement.
If the pathology affects the endothelial cell layer, posterior lamellar keratoplasty may be considered. Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) is performed, where parts of the stromal layer, Descemet's membrane, and endothelium are transplanted into the patient's eye. It is a minimally invasive surgery with an incision of about 4 to 5 mm. The transplanted cornea does not require sutures. It is secured with gas, leading to quicker and better vision improvement and faster recovery. Due to the relatively small amount of transplanted tissue, the rejection risk is much lower.
Worldwide and in Hong Kong, the development trends in corneal transplant technology have focused on posterior lamellar keratoplasty. It aims to transplant fewer donor tissues to significantly reduce rejection risk. As a result, Descemet’s Membrane Endothelial Keratoplasty (DMEK) has been developed. It involves transplanting only the Descemet’s membrane and endothelial cell layer, as indicated by the green area in the graphic. The endothelial cell layer is around 0.02 mm thick, with an incision of just 3 mm. It leads to faster recovery and higher potential to improve the vision to the pre-disease state, making this a major trend worldwide and in Hong Kong.
Q5. What should patients take note of before and after the corneal transplant surgeries?
Patients should be mindful of several things before and after surgery. Prior to undergoing a corneal transplant, the condition of the eye to be operated on should be stable, such as having normal eye pressure or controlled inflammation. It helps increase the success rate of the transplant. Prior to the surgery, it is important to keep the eyelids clean to reduce the risk of infection. It also helps lower the risk of post-surgical wound infection. To minimise bleeding during surgery, doctors may require patients to stop using anticoagulant medications. It helps increase the chance of surgical success and minimise associated risks. After the surgery, patients should follow the doctor’s advice, using medication regularly and attending follow-up appointments as scheduled.
For posterior lamellar corneal transplant surgery, as sutures are not used to secure the cornea. Gas is used to hold the cornea in place inside the eye. Therefore, patients need to lie flat with a face-up position for at least 24 hours after surgery. Patient’s compliance in keeping the position is crucial to improve the surgical outcome. After the surgery, patients should avoid swimming and strenuous exercise, and take more rest to speed up recovery.
If a patient has history of a corneal ulcer due to viral infection before the surgery, doctors will prescribe antiviral medication after the transplant to reduce recurrence of viral infection in the new cornea. This medication must be taken for more than a year, and patient’s compliance is crucial.
Q6. It takes time to adapt after the surgery. However, in which situations should I seek medical attention immediately?
After the surgery, patients need to attend follow-up appointments. However, before these appointments, if any of the following symptoms occur, patients should not hesitate to seek medical attention immediately: extreme eye pain, sudden visual loss, persistent redness of the eye, tearing, light sensitivity, etc. Do not wait until the follow-up appointment if these symptoms occur. Patients should promptly consult a doctor to check the condition of their cornea. After the surgery, patients need to use steroid eye drops for an extended period to reduce rejection. These steroid drops might cause some patients to experience increased eye pressure, which can lead to headaches. Therefore, if a headache occurs after surgery, a consultation with a doctor for an eye pressure check is recommended.
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