The cornea is the clear window in the front of the eye. Light rays travel through the cornea before reaching the lens in the middle of the eye and then the retina in the back of the eye.
The cornea has five layers. The outer layer is known as the epithelium. The epithelium is attached to a thin layer known as the Bowman’s membrane. The attachment of epithelium to Bowman’s membrane may be thought of as a layer of paint on pprimer. The middle layer, also the thickest layer of the cornea, is called through stroma. A layer called Descemet’s membrane lines the internal portion of corneal stroma. The most inner layer of the cornea is a single layer of cells known as the endothelium. A chamber filled with clear fluid is behind the cornea. This chamber is called the anterior chamber.
Fluid from the anterior chamber flows naturally into the cornea. However, the corneal endothelium constantly pumps fluid in the cornea back into the anterior chamber. If the corneal endothelium does not function normally, then fluid moves into the cornea to cause swelling. Swelling of the cornea causes it to become cloudy and blurr vision. The degree of blurred vision is directly relateed to the amount of swelling in the cornea. In advanced cases of corneal swelling, the outer layer of the cornea – the corneal epithelium- also becomes swollen. Swelling of the corneal epithelium may cause a marked reduction of vision because the normal shape of the eye is changed and a diffuse haze in the vision. As the corneal epithelium becomes swollen, small blisters known as “bullae” develop. The corneal bullae cause a sandy feeling in the eye. If these bullae breakdown and burst, the sensitive nerves in the cornea are exposed, causing significant discomfort.
The health of endothelial cells can be determined by examining their shape and number. A slit lamp micrcoscope that magnfies many thousands of times can be used in a doctor’s office to examine the shape of endothelial cells. As endothelial cells age or degenerate, their size becomes less uniform. Special photographic methods known as specular microscopy may be used to count the number of endothelial cells in a specific area. Counting the number of endothelial cells may be difficult when the cornea is too swollen and cloudy. Most people are born with about 4,000 endothelial cells per square millimeter of the endothelial surface. Endothelial cells do not grow in number, divide, reproduce or replace themselves. As we age, we gradually lose our corneal endothelial cells. The average number of corneal endothelial cells as we age is shown in the following table:
|Age||Endothelial Cell Density|
|Infant||3,000 – 4,000 / square mm|
|Teenager||3,000 / square mm|
|Adult||2,500 – 3,000 / square mm|
|Elderly||2,000 – 2,500 / square mm|
Once lost, corneal endothelial cells do not grow back. The remaining corneal endothelial cells spread out to cover spaces from lost endothelial cells. Therefore, fewer endothelial cells serve to prump fluid out from a greater area of the cornea. The ability of the corneal endothelium to pump fluid out of the cornea becomes less efficient with defective or fewer endothelial cells. As the pumping system becomes less efficient, corneal swelling with clouding and reduced vision develops.
Fortunately, most people have enough endothelial cells throughout their life to prevent corneal swelling or edema. If the cells are healthy and functioning properly, as few as 500 endothelial cells per square millimeter can maintain a clear cornea. If the cells are not healthy, fewer than 1000 cells per square millimeter may result in corneal swelling or edema.
As the cornea swells it becomes thicker. An instrument known as a pachymeter may be used to measure corneal thickness. This serves as an indirect measure of the severity and progression of Fuch’s endothelial dystrophy.
Dr. Ernst Fuchs, an ophthalmologist from Vienna, first described a degenerative condition of the corneal endothelium that is named after him. Fuchs’ Dystrophy generally affects both eyes, although it is slightly more common in women than in men. In Fuchs Dystrophy the endothelial cells do not efficiently pump fluid out of the cornea. The endothelial cells in Fuchs’ Dystrophy age and degenerate fater than normal cells. The exact cause of Fuchs’ Dystrophy is unknown, although genetic and hormonal factors are believed to play a role.
Symptoms of Fuchs’ Dystrophy
In the early stages of Fuchs’ Dystrophy, patients may wake up with blurred vision that gradually improves during the day. That’s because during the day, fluid continuously evaporates from the surface of the eye, drawing out excessive fluid from within the inner compartments of the eye. Patients may comment that use of a hairdryer blowing towards the eye improves their vision.
Fluid from the cornea does not evaporate when closing our eyes as we sleep. Therefore, fluid tends to accumulate in the cornea overnight and cause blurred vision in the morning. Patients may complain about fluctuating vision, glare when looking at lights, and a sandy or gritty sensation.
In advanced cases, evaporation from the surface of the cornea is insufficient to remove the excess fluid within the cornea. Fluid accumulates within the cornea to cause the cornea to become swollen. Cloudy vision may last the entire day.
There is no absolute cure for Fuchs’ Dystrophy. Endothelial cells cannot be stimulated to function better or to proliferate. Blurred vision resulting from the corneal swelling can be controlled with medication. Hypertonic saline medications such as Muro 128 eyedrops or ointment may be prescribed to absorb fluid from the cornea and reduce swelling. Eyedrops with steroids may be helpful in specific situations. Patients who use steroid eyedrops need to be carefully monitored for side effects of steroids, including cataracts and glaucoma. A soft contact lens may be placed over the eye to alleviate discomfort, especially if bullae of the cornea develop.
Surgery of the cornea may be considered when vision deteriorates to the point that it interferes with daily activities. Surgery for Fuch’s Dystrophy consists of either DESK, a procedure that replaces diseases or missing endothelial cells, or corneal transplantation. A corneal transplanation, also known as penetratimng keratoplasty, replaces the full thickness of the cornea, and replaces it with a clear, healthy, tissue from a donor.
Fuchs’ Dystrophy in Patients with Cataracts
As people age, the lens within the eye often becomes cloudy. A cloudy lens is referred to as a cataract. When cataracts become visually significant, cataract surgery may be considered to restore clear vision. Cataract surgerey is one of the most common surgical procedures and has a very high rate of success. It is important to keep in mind that cataract surgery can effect the endothelial cells of the cornea. The cells may be temporarily shocked by cataract surgery, resulting in temporary swelling of the cornea and hazy vision. However, if the number of endothelial cells is insufficient following cataract surgery, permanent swelling of the cornea may develop, reducing vision. Corneal transplantation ort another type of surgical procedure may become necessary to recover vision.
In the presence of advanced Fuchs’ Dystrophy and cataract, it may be advisable to combine cataract surgery with corneal transplantation. One combined surgery may achieve what would otherwise require two separate surgeries with a longer post-operative recovery.
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