Author: Sherman Winston Reeves, M.D., M.P.H.
Overview:
The cornea is a thin, clear tissue which serves as the front wall of the eye and plays a critical role in transmitting light from the outside world into the eye. Several eye diseases can damage the clarity or shape of the cornea and prevent the uninterrupted passage of light through it. Additionally, the cornea may play a role in a refractive error of the eye, resulting in an improperly focused image on the retina. Several surgical options are available for treating corneal diseases and correcting refractive errors.
Corneal anatomy:
In order to understand how corneal surgery can help treat corneal disease and refractive error, it is helpful to review the important parts of the corneal anatomy.
The cornea, which is about the thickness of a credit card, is comprised of several layers of tissue. The surface of the cornea is covered by a very thin layer of skin cells called the epithelium, which rest on a fine collagen membrane called Bowman’s membrane. The bulk of the cornea, which lies underneath the epithelium and Bowman’s membrane, is called the stroma. Underneath the stroma lies Descemet’s membrane, which separates the stroma from the innermost lining of the cornea. The innermost portion of the cornea is a thin layer of cells called the endothelium, which are responsible for keeping the fluid inside the eye out of the cornea.
The cornea plays an important role in focusing light that enters the eye. When light passes through the cornea from the outside world, it is bent, or refracted, as if passing through a lens. In fact, the cornea works in concert with the crystalline lens inside to help focus light on the retina , much like the lens system of a camera.
The unhealthy cornea
A cornea can become unhealthy in several ways. First, injury to the cornea can result in scarring. A scar, which is dense and white, can directly impede the passage of light through the cornea. Further, a scar can cause the shape of the cornea to become abnormal and warped, which can cause incoming light to be bent in an irregular fashion as it passes through the cornea. Warping of the cornea can also occur in certain diseases, such as keratoconus, where portions of the cornea becomes abnormally thin or irregularly shaped. Disease or injury can also cause the cornea to become swollen. When the cornea swells, it also becomes cloudy, impeding the clear passage of light. Lastly, though a cornea may otherwise be clear and regularly shaped, it must work in concert with the crystalline lens of the eye to focus light crisply on the retina. If these two structures are not correctly balanced, refractive error occurs. As such, a cornea could be to flat or steep for an eye and its lens, resulting in hyperopic or myopic refractive error and blurry vision without the use of glasses or contacts.
Penetrating Keratoplasty: The complete corneal transplant
When the cornea becomes extremely swollen, scarred or warped, light cannot easily pass through to the retina, and vision is lost. Several diseases can result in a cloudy or warped cornea, such as corneal swelling after eye surgery, Fuchs Dystrophy, Keratoconus or scarring after corneal trauma. In these cases, a complete replacement of the cornea may be needed. The surgical procedure to accomplish a complete corneal transplant is called Penetrating keratoplasty(PK). During a PK, the entire cornea is removed, much like a man-hole cover, and replaced with a complete donor cornea. The donor tissue is secured in place with many sutures which are finer than a human hair, usually placed in a spoke like fashion around the outside edge of the transplant. These sutures remain in place for several months, and then are slowly removed over the course of one to two years. Many eye medications are required after a corneal transplantation, and a lifetime commitment to eye drops and regular eye examinations is required in order to keep the transplant healthy and prevent rejection.
DSEK: The partial corneal transplant
Some disease processes, like forms of Fuchs Dystrophy and select cases of corneal swelling after eye surgery, are due to dysfunction of the inside lining cells of the cornea. This layer of cells, called the endothelium, can be replaced surgically in a procedure called Descemets Stripping Endothelial Keratoplasty (DSEK). In DSEK, the endothelial lining of the cornea is removed through a small incision in the eye and replaced with a small disc of endothelial cells from a donor. The replacement of the malfunctioning endothelial cells allows the cornea to once again become clear by restoring the lost function of the endothelial cells. DSEK is performed through small incisions and has a relatively fast recovery period. Usually, the cornea can be greatly cleared by 4 to 6 weeks after the procedure. Occasionally, however, the donor endothelial cells become dislodged from the inside of the cornea and have to be repositioned surgically or replaced entirely. Like penetrating keratoplasty, DSEK is a corneal transplant procedure, so a lifetime of eye drops and eye examinations are needed afterwards.
Superficial and Phototherapeutic keratectomy: Removal of small corneal scars
When scar tissue forms on the surface of the cornea, it can be removed by a procedure called superficial keratectomy. In this procedure, the eye is numbed using eye drops. The corneal epithelial surface cells are first removed, then scar tissue is then peeled off the front of the cornea. The epithelial cells then heal over the wound, usually in a bout 5 to 7 days, just like the healing of a cut on the skin.
When the corneal scar tissue extends into the upper parts of the corneal stroma, or is very irregular in nature, the excimer laser can be used to sand down the scar in a procedure called phototherapeutic keratectomy. A similar process is involved as in superficial keratectomy, though the excimer laser removes the scar tissue, rather than the surgeon manually dissecting it from the cornea.
Pterygium Excision: Removal of abnormal conjunctival tissue from the cornea
In the disease called pterygium, the conjunctiva of the eye begins to grow in an abnormal fashion across the surface of the cornea. As the conjunctiva is not clear, light is impeded from passing through the cornea in the area where the pterygium has grown. Pterygia can also bring scar tissue onto the surface of the cornea as they grow, further impeding the passage of light and causing astigmatism to occur. Pterygia can be removed surgically and the scar tissue sanded off the cornea, usually with a fine diamond-tipped corneal sander. As pterygia can recur after surgical removal, several additional techniques are used during the surgery to help prevent the abnormal tissue from growing again. Most commonly, a small amount of normal conjunctiva is moved into position over the sclera area where the pterygium was removed. A medication called mitomycin-C is also sometimes used to help prevent abnormal pterygium cells from recurring.
INTACS: Support for a warped cornea and correction of myopia
In keratoconus, sometimes the main cause of poor vision is warping of the cornea. In keratoconus, the cornea starts to thin and protrude forward at a point just below its center. This results irregularly curved corneas and high levels of astigmatism that are difficult to correct with ordinary glasses and contact lenses. To help correct this warping, small plastic ring segments, called Intacs can sometimes help normalize the central corneal shape and improve vision by making the corneal shape more regular. Intacs can also be used to correct low levels of myopia .
LASIK and PRK: Correction of refractive error
The most common corneal surgery performed in the United States today is excimer laser corneal refractive surgery, such as LASIK and PRK . In these procedures a small amount of corneal tissue is removed to reshape the cornea and help focus light more crisply on the retina.
Conductive keratoplasty: Corneal correction of hyperopia and presbyopia.
In hyperopic refractive error, the cornea may be too flatly curved for the eye to allow incoming light to form a clear image on the retina. With Conductive Keratoplasty (CK) surgery, however, the shape of the cornea can be changed in order to help light focus more crisply on the retina. In this surgery, a small probe is used to apply radiofrequency energy to the outside edge of the cornea, resulting in a steeper central shape of the cornea. CK can also be used to create monovision in one eye and help relieve the symptoms of presbyopia .
