corneal disease, fuchs endothelial dystrophy and corneal transplantation
what is the cornea?
The cornea is the clear window at the front of the eye and is the main focusing element. Uniquely, it is the only transparent structure in the human body and can be thought of much like the crystal face of a watch. Behind the cornea lies the coloured iris and in the middle, the dark pupil of the eye.
There are many different reasons why the cornea may be damaged and lose its clarity and transparency leading to loss of vision. these can include infective causes such as the herpes simplex virus (cold sore virus) or contact lens related infection, traumatic injuries, inherited diseases such as Fuchs' dystrophy or lattice dystrophy, or even due to previous cataract or glaucoma surgery.
The optical function of the cornea may also be affected by conditions that lead to a change in it's shape including keratoconus or pellucid marginal degeneration, or due to prior infection or injury.
The cornea is a layered structured. The main layers, the epithelium at the surface, the stroma in the middle and the endothelium at the back of the cornea consists of different cell types. Corneal diseases can affect selective layers of the cornea or may affect all layers of the cornea, each of which can lead to lost vision which may require surgery.
Corneal transplantation has been performed for over a century and is the only mode of treatment for some patients that have lost transparency or function of their cornea leading to loss of vision.
types of corneal transplant
During all types of corneal transplantation the diseased part of the of the patient's cornea is removed and replaced with healthy corneal tissue from a human donor. For many years the predominant type of corneal transplantation was penetrating keratoplasty (PK) which involved removing a circular full thickness area of cornea from the patient and replacing it with full thickness donor tissue. More recently, due to advances in technology and technique, it has been possible to remove a specific layer of corneal tissue and replace just that layer. This type of surgery is called lamellar (layered) surgery. PK is still be performed in cases where all layers of the cornea are involved.
Lamellar surgery is technically demanding and requires that your surgeon have completed advanced specialisation in these techniques. Lamellar surgery is broadly divided into anterior lamellar (the front layer) or posterior lamellar (the endothelial layer) surgery. The diagram below illustrates the various types of corneal transplant that are now possible.
Lamellar corneal transplants are associated with lower rejection rates and preserve the structural integrity (strength) of the whole eye.
different types of Corneal Transplants
(A) The cornea consists of five main layers: the superficial multilayered epithelial cell layer, Bowman's membrane, which is a condensation of the anterior stromal layer; the corneal stromal layer (making up the major thickness of the cornea) consisting largely of collagen with relatively low numbers of stromal keratocytes; Descemet's membrane; and most posteriorly, the endothelial cell monolayer that is responsible for deturgescing the stroma to maintain corneal transparency. (B) Penetrating keratoplasty. (C) Anterior lamellar keratoplasty. (D) Deep lamellar endothelial keratoplasty. (E) Descemet's stripping automated endothelial keratoplasty. (F) Descemet's membrane endothelial keratoplasty.
where do donor corneas come from?
Human corneas used for transplantation are obtained from human donors that have given their eyes for medical use after their death. In the United Kingdom the majority of donor organs including hearts, kidneys and eyes are processed by the UK Transplant Service and undergo stringent tests to exclude risk of transmissible diseases and infection, and to ensure sufficient cell numbers for transplantation. Moorfields Lions Eye Bank also provides donor tissue to the hospital and works under similar stringent national guidelines. In the UK just over 4000 human eyes are retrieved from donors for transplantation each year. These can be stored for up to 28 days before use.
what is Fuchs Dystrophy?
A healthy cornea is clear and bright. In advanced Fuchs' dystrophy the cornea becomes hazy and dull due to swelling
Fuchs' endothelial dystrophy is a progressive disease of the corneal endothelial cells that leads to clouding of the cornea and eventual reduction in vision.
The endothelial cells of the cornea are a specialised single layer of cells on the back surface of the cornea that pump water out of the cornea. This keeps the cornea thin and clear. Endothelial cells do not have the ability to regenerate and hence if they are damaged the cornea gradually begins to swell with water, making it hazy and leading to loss of vision. You may notice this as foggy vision, particularly in the morning when you wake up. This can be like looking through a misted window. Typically, vision begins to improve toward the end of the day, but this may not happen in advanced cases.
A baby is born with roughly 4000 endothelial cells per square millimetre. In an 80-year-old without Fuchs' dystrophy the endothelial cell count is usually between 1500-2500 per square millimetre. This reduction in cell numbers is normal and happens because of ageing. The cornea remains clear even up to around 800 cells per square millimetre. In a patient with Fuchs' there is an accelerated rate of cell loss, such that a 50 years old may have less than 500 cells per square millimetre. In addition, endothelial cells in Fuchs patients are abnormal and can accumulate abnormal proteins resulting in small bumps on the back of the cornea called guttata. These changes can result in symptoms of glare and haloes around bright lights and a decrease in the quality of vision.
Whilst early signs of Fuchs' dystrophy can be detected by an eye surgeon even in patients in their thirties, it is relatively uncommon for patients to become symptomatic and require surgery till they are in their fifties.
Fuchs' dystrophy occurs as a result of abnormal genes that affects the function of endothelial cells. Not all of these are well understood yet. Whilst Fuchs' can be expressed by different generations of the same family, it can also commonly occur sporadically where there is no previous family history. Inheritance of the Fuchs' gene is also variable and the fact that you have Fuchs' doesn't always mean that your offspring will express Fuchs' endothelial dystrophy.
Medical Treatment of Fuchs' Dystrophy
During the early stage of Fuchs dystrophy when swelling of the cornea is still mild, your doctor may prescribe you sodium chloride (salt) drops or ointment. By increasing the salt content of the tears water is drawn out of the cornea by osmosis.
Salt drops do not cure Fuchs' Dystrophy and simply delay the need for surgery in early cases.
External links: For more information about Fuchs' Dystrophy
External links: Fuchs - a patient's perspective
medications after corneal transplantation
Regardless of the type of corneal transplant that you have you will need to use a combination of antibiotic and steroid drops for the first few weeks after surgery. You may also be given steroid tablets to reduce the risk of rejection in some cases.
In the longer term you are likely to remain on a steroid drop at least once a day. This can signficiantly reduce the risk of rejection occurring.
removal of stitches
Depending on the type of transplant that you have, the donor cornea may need to be stitched into place. Stitches usually come out at 9 to 12 months for lamellar grafts and at around 18 months for full thickness penetrating grafts.
On some occasions stitches may be left in place.
Mr Guerin will see you a few days after your surgery. If your eye is healing normally at this stage your next follow up will be in a months time. Subsequent follow up is every 3 to 6 months. Your appointments will become less frequent as the length of time from your surgery increases.