What is the Cornea?
The cornea is a transparent tissue, resembling a watch crystal, that makes up the front surface of the eye. In the normal eye, images are first focused through the cornea. As light continues into the eye, it is again focused by a tiny lens located just behind the pupil. Light continues to the retina at the back of the eye where it is transformed into that marvel called sight. The cornea’s role in focusing exceeds that of any other structure of the eye. For this reason, even minor defects can have a profound effect upon vision.
Corneal Structure
The layers of the cornea are critical to good vision. The outermost layer is called the epithelium. This layer is much like the skin except that it is transparent. Most of the nerve endings of the cornea are located in the epithelium. This explains why it is so sensitive to the touch. If the epithelium is damaged, although painful, it grows back within a few days without any scarring.
Bowman’s layer is located just below the epithelium. It is composed of strong collagen fibers, which help the corneal maintain its shape. Although Bowman’s layer is resistant to injury and disease, if it is damaged, a scar will usually result.
The middle layer of the cornea is called the stroma. This is the thickest layer of the cornea and is approximately 78% water. In fact, because it contains specialized sugar molecules, it actually attracts water. The regular spacing of these fibers is critical to the clarity of the cornea. Anything that disrupts the arrangement of these fibers, such as swelling or scarring will cause light to scatter haphazardly as it passes through.
Lying beneath the stroma is Descemet’s membrane. This layer serves as a barrier to infectious organisms and blood vessels, but at the same time allows water and nutrients to pass through. Descemet’s membrane also ‘glues’ the innermost layer, the endothelium, to the cornea.
The endothelium is only one cell layer thick. This delicate layer plays an essential role in maintaining corneal clarity. It acts as a fluid regulator for the stroma. The endothelium allows fluid to pass into the stroma. Endothelial cells do not regenerate, therefore once they are damaged or destroyed they are gone forever. Injuries, surgeries and some eye diseases can destroy endothelial cells. Sometimes it is simply the process of aging that reduces the number of cells.
Bullous Keratopathy
Bullous keratopathy is a disease of the cornea. It is one of the most common conditions that effect the clearness of the cornea. This disease progresses very slowly and may be treated for many years before the cornea becomes so cloudy that surgery is necessary to restore clear vision.
Bullous keratopathy occurs when a large number of endothelial cells are lost and excess fluid builds up inside the cornea. This painful swelling disrupts the arrangement of the stromal fibers and results in cloudiness of the cornea.
Persons who have bullous keratopathy often experience blurred vision that is worst in the morning but improves as the day progresses. As we sleep with our eyes closed, fluid is trapped under the lids. This in turn causes the cornea to become wetter. When the eyes are open, the excess fluid evaporates into the air along with the tears.
As the disease progresses, fluid filled blisters develop under the epithelium. When one of these blisters ruptures it is painful, and precautions should be taken to prevent infection. When the doctor performs a microscopic slit lamp examination, he might find that the epithelium has separated from Bowman’s layer and that blisters have formed. He might also find swelling and blood vessels present in the stroma. A special examination called specular microscopy can actually count the diminished endothelial cells. Guttata can be seen as blank spaces on this test and represent elevations of dead endothelial cells.
Treatment
The main goals in the treatment of bullous keratopathy are to reduce swelling, provide comfort for the patient and to restore useful vision. Swelling can often be controlled using eye drops of saline and steroid antibiotic solutions. The salt draws excess fluid out of the cornea, thus providing temporarily improved vision and comfort. A contact lens may be used temporarily for comfort, but it will not improve vision. Increased pressure within the eye may worsen corneal swelling and eye drops may be used to control eye pressure.
The treatment of choice for relief of pain and restoration of good vision is called corneal transplantation. In this procedure, the diseased portion of the cornea is removed and replaced with a high quality donor cornea. The corneal transplant is the most successful of all human organ replacement operations.
