Moving forward into 2022, we are starting a series regarding corneal transplants and reasons as to why a corneal transplant may be necessary. We’ll start with articles regarding several different diseases leading up to an article describing different types of corneal transplants and how these procedures work.
To start off the new year, we are going to discuss Fuch’s Endothelial Dystrophy.
Fuch’s Endothelial Dystrophy is a condition that affects the eyes resulting in poor vision secondary to an accumulation of fluid within the front most structure of the eye—the cornea.
Fuch’s Endothelial Dystrophy is one of the more common diseases that requires corneal transplantation as treatment in the end-stage of the disease.
Anatomy of the Cornea
To best understand what Fuch’s Endothelial Dystrophy is, it will be helpful to understand how the cornea works.
As briefly mentioned above, the cornea is the front–most structure of the eye.
The cornea is responsible for several different things, but its main job is to refract, or bend, light rays so that incoming light lands properly on the back–most structure of the eye, the retina (where light is detected and then transmitted to the brain for processing).
The cornea is made up of 5 separate layers—epithelium, Bowman’s layer, stroma, Descemet’s layer, and endothelium.
The epithelium is the outermost layer, coming into contact with the tear film. The epithelium’s primary job is to act as a barrier to keep bacteria and other foreign objects out of the eye.
Bowman’s layer is a bit stronger than the epithelium. It helps the cornea to maintain its shape and structure and separates the outer epithelium layer from the inner stromal layer.
The stroma makes up the majority of the cornea. It is a thick layer containing the entrance point for corneal nerves. The stroma is responsible for clear, uninterrupted vision by keeping cells regularly spaced and healthy.
To maintain clear vision, the stroma must be kept slightly dehydrated, meaning the water content of the stroma must be less than surrounding tissues. If too much water is retained by the stroma, it will swell causing corneal edema. Corneal edema creates blurred vision with excessive amounts of glare, halos around lights, and sometimes even light sensitivity.
The endothelium is the thinnest layer of the cornea consisting of just one layer of cells. These cells contain specialized pumps that pump water out of the cornea to maintain stromal dehydration.
What is important to note about the endothelium is that the cells of the endothelium cannot regenerate. The endothelial cells you are born with are the ones you die with.
However, endothelial cells do become damaged over time. When endothelial cells die, surrounding endothelial cells grow in size and become irregular in shape to try and “pick up the slack”.
While death of a few endothelial cells here and there is not cause for alarm (in fact loss of some endothelial cells is expected throughout life!) excessive loss of endothelial cells will alter the dehydration mechanism, resulting in the accumulation of water within the stroma—or stromal edema.
As you can see, while there are many layers, all parts of the cornea must be healthy for proper function, which is an essential part of clear vision.
Fuch’s Endothelial Dystrophy: The Basics
The exact underlying mechanism of Fuch’s Endothelial Dystrophy is still a bit debated in a “what came first the chicken or the egg?” type of debate.
Some say Fuch’s is due to idiopathic (unknown) death of endothelial cells resulting in secondary damage to Descemet’s membrane.
Alternatively, others believe that Fuch’s is due to irregular, excessive growth of Descemet’s membrane producing an abnormally thick and irregular Descemet’s membrane, resulting in secondary damage to endothelial cells.
As you can recall, Descemet’s membrane is composed of collagen. When Descemet’s membrane becomes damaged, it repairs itself by making little collagen plugs to stabilize the integrity of the layer.
Regardless of the exact mechanism, what is seen in Fuch’s is an irregular, thicker than normal Descemet’s membrane with endothelial cell loss. The question at this time is what came first, the endothelial cell loss or Descemet membrane growth?
The same happens to the cornea if Descemet’s layer is not smooth and equal! Excessive growth of Descemet’s layer (whether this be from initial abnormal growth or abnormal growth secondary to the collagen plug repair system) pushes on and knocks out endothelial cells (which are likely already in a compromised state themselves due to the disease).
As more and more endothelial cells are lost, Descemet’s membrane will become more and more irregular, and the cornea becomes less and less able to pump out adequate amounts of water, leading to fluid accumulation in the stroma.
When the stroma swells, it puts more pressure on endothelial cells and even more die off, creating a never-ending cycle of endothelial cell damage and excessive corneal swelling.
Therefore, while the ultimate problem is “endothelial dystrophy” (or death of endothelial cells), essentially 3 layers—Stroma, Descemet’s, and Endothelium–are involved in Fuch’s Endothelial Dystrophy.
Fuch’s Endothelial Dystrophy: Progression of Disease
Fuch’s Endothelial Dystrophy is a slower progressing disease—it normally takes many years of endothelial cell loss to begin to notice symptoms of blurry vision, and many more years before later-stage disease findings appear (if they appear at all!).
Guttata—irregular “bumpy” appearance of Descemet’s membrane—are typically the first noticed sign of Fuch’s. Guttata are the pin-point areas of irregular collagen growth within Descemet’s membrane.
While guttata are seen in individual’s with Fuch’s Endothelial Dystrophy, they can also be seen in normal healthy individuals, as guttata can be a part of the natural aging process. For that reason, guttata are a sign of Fuch’s, but not a diagnostic factor.
The amount and location of guttata is what may indicate further testing for Fuch’s, which requires imaging of the cornea to measure the density and uniformity of the endothelial cells.
In the moderate stages of Fuch’s, an optometrist or ophthalmologist can begin to see noticeable swelling of the cornea in addition to guttata, as more and more endothelial cells are lost.
These epithelial blisters are called bullae. Bullae are extremely painful as the corneal epithelium is full of pain-detecting nerves. Like regular blisters, bullae can pop, resulting in excruciating pain.
How is Fuch’s Dystrophy Treated?
As mentioned earlier, Fuch’s is typically a slower progressing disease that will likely only require observation by your eye doctor for many years. Yearly eye exams are extremely important for those with Fuch’s as we want to prevent the disease from progressing to severe vision loss and bullae formation.
Additionally, your doctor may prescribe an over-the-counter medication called Muro-128 ointment. Muro-128 is a hypertonic solution, meaning it pulls excess water from the eye to help reduce stromal swelling and alleviate some of the stress on the endothelial cells.
Muro-128 is not a long-term fix, however, and is only used for temporary relief of symptoms in those in the mild to moderate stages of the disease. It is most often prescribed to be used at bedtime to help reduce blur upon waking in the morning.
The ultimate treatment for Fuch’s Endothelial Dystrophy is a corneal transplant in which the host’s corneal tissue is replaced with a donor’s corneal tissue.
There are several different types of transplants available to date, which will be discussed in the future towards the end of this article series. The most common types used for Fuch’s treatment are DSAEK and DMEK.
Again, annual eye exams are very important for those with Fuch’s, as your doctor will be able to watch the progression of the disease and arrange for a corneal transplant prior to the disease advancing the bullae stage, as transplant surgeries need to be scheduled months in advance and proper preparatory measurements and precautions will be necessary.
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