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Corneal Cross-Linking: An Overview
In 2016, the the Food and Drug Administration (FDA) approved corneal cross-linking. The approval of this procedure marked a new era in the treatment of corneal disease that causes distortions in the shape of the cornea. Such changes lead to nearsightedness and blurry vision. But now, a safe and effective treatment is available to halt the progressive distortion of the cornea.
What Is Corneal Cross-Linking?
Corneal cross-linking is a procedure used to halt the progression of two corneal diseases — keratoconus and post-LASIK corneal ectasia. The procedure strengthens a misshapen cornea by restoring the cornea's underlying support beams, called collagen fibers which are no longer giving the structure its natural, rounded shape.
During a corneal cross-linking procedure, vitamin B2 eyedrops are placed on the surface of the eye and absorbed in the cornea before the eye is exposed to a small amount of ultraviolet (UV) light. Together, the drops and UV light cause the collagen fibers in the cornea to "crosslink" or bond more tightly. This process strengthens the middle layer of tissue. It stabilizes the structure and halts any progressive vision loss.
The two corneal diseases that are treated with corneal cross-linking will be discussed in the text that follows. But, let's first understand the function and structure of the cornea and the role it plays in vision.
The illustration on the left represents weakened collegen fibers that occur in keratoconus. The illustration on the right shows collegen fibers that have been strengthened with cornea cross-linking.
Function of the Cornea
The cornea is the clear covering or outermost layer of the eye. The cornea is often thought of as a "windshield" for the eyeball because it's both a protective barrier against harmful particles and a filter for screening out damaging ultraviolet (UV) light from the sun. The cornea's primary role, however, is to help focus the incoming light your eyes need to see. Your ability to see clearly depends on how well your eyes convert light rays into "pictures" in your brain. It's the first — and most important — step in that process.
Although the lens of your eye makes it possible for you to focus on objects, the cornea is responsible for about 70 percent of your eye's refractive, or focusing, power. Accordingly, the shape and surface of your cornea are critical for quality vision.
Normal 20/20 vision relies on the eyeball being perfectly round and the cornea curving naturally into its dome shape. When light beams strike its surface, the cornea refracts or bends the rays evenly so that they point through the pupil, through the lens, and onto the retina. That's where light converts into electrical impulses the brain interprets as clear images.
However, if the cornea is too flat, too steep, or unevenly shaped, light hits and reflects images onto the retina in an irregular fashion. This results in what's called refractive error, the inability of the eye to focus correctly. Even people with normal eyes experience refractive errors. You've heard of these errors: nearsightedness (myopia) which is difficulty seeing far objects; farsightedness (hyperopia) — difficulty seeing objects close up; and astigmatism, or blurred vision caused by parts of the eyeball being shaped like the back of a spoon, which causes light to bend more in one direction than another. In normally sighted individuals, these deficiencies can be corrected easily with eyeglasses or contact lenses.
Structure of the Cornea
Unlike other parts of the body that are fed with blood vessels, the cornea gets nourishment from the aqueous humor — the fluid inside the front of the eye — that continually bathes the front of the eye. The cornea consists of three basic tissue layers separated by two thin membranes. Each has a critical function:
Epithelium. As the outermost layer of the cornea, its primary role is to block dust, water, bacteria, and other foreign materials from entering the eye. It also absorbs oxygen and nutrients to distribute to other corneal layers.
Descemet's membrane. A thin film of tissue, the Descemet's membrane separates the outermost layer — the epithelium — from the primary layer — the stroma — of the cornea.
Stroma. This primary layer of the cornea accounts for about 90 percent of the structure's thickness. It's composed of water and tightly bound collagen fibers, which give the cornea its form, strength, and elasticity. The cornea's ability to transfer light is dependent on the orderly arrangement of these filaments.
Bowman's membrane. One of two corneal membranes, it's a transparent film composed of irregularly arranged collagen fibers that protect the next layer, the stroma.
Endothelium. The layer at the very back of the cornea, it's responsible for keeping the cornea clear by ensuring a perfect balance of fluid coming in and going out. If it doesn't work correctly, the stroma can swell with fluid, which can lead to loss of vision and even blisters forming on the surface of the cornea.
Keratoconus and Post-LASIK Corneal Ectasia
As mentioned, keratoconus and post-LASIK corneal ectasia are two corneal diseases whose progression is being successfully halted with the newly approved FDA procedure, corneal cross-linking procedure.
What Is Keratoconus?
Keratoconus is a progressive disease in which the cornea loses its normal, dome-like shape. The resulting bulge in the cornea leads to distorted vision that worsens over time and can lead to significant vision loss.
Pronounced "kera-toe-coe-nus," this condition occurs because the critical support "beams and anchors" within the cornea weaken, and the cornea is no longer strong enough to maintain its domelike shape. The cornea can become even more cone-shaped and the tissues become thinned. Derived from the Greek terms, conus, for cone and kerato for cornea, keratoconus literally means cone-shaped cornea.
Although the exact number of people who have keratoconus is not known, it is estimated that 1 in 2,000 individuals have this eye disorder.
There is no way to prevent keratoconus. The goal of treatment is to stop the progression of the disease.
When keratoconus starts to develop, patients often don't notice a major vision change. However, at some point the following symptoms may affect your vision:
The illustration on the left shows a normal cornea. The illustration on the right shows keratoconus — notice how a weakened cornea allows it to bulge.
Increased sensitivity to light
Progressively poor vision not easily corrected with eyeglasses
If you have such symptoms, you may think that you simply need new eyeglasses or contact lenses. Over time, however, your eyesight becomes blurry enough that you feel the need to contact your eye-care professional. As your vision worsens, the fuzziness and other distortions such as halos and glare at night are more noticeable. A new eyeglass prescription will not improve your vision.
Causes of Keratoconus
The exact cause of keratoconus is unknown, even though several theories exist. It may even be a combination of factors working together that cause a breakdown of the corneal tissue, resulting in the cornea becoming cone-shaped. Researchers are focused on the following factors as possible causes.
Genetics. Family genes have been mentioned as a factor in keratoconus, even though the majority of patients don't share their condition with a blood relative. The latest available information, according to the National Keratoconus Foundation (NKCF), suggests a less-than-one-in-ten chance of several family members being diagnosed with keratoconus. Although bloodlines are likely not a primary cause, being related to a family member with the condition may place you at a higher risk for the disease. A family history of keratoconus may also be a factor in predicting how the condition might progress.
Eye rubbing and allergies. Individuals who frequently rub their eyes are at higher risk than others to damage their corneas and develop keratoconus. If they've been prone to hay fever and other allergies that cause itchy eyes, a common theory is that the two factors — eye rubbing and allergies — together may cause keratoconus. However, the actual connection between these two factors remains unclear. Still, eye rubbing, especially with allergies, can aggravate keratoconus.
Inability to self-repair. Studies have shown that individuals with keratoconus have corneas that are unable to repair damage caused by the routine chemical reactions that cells undergo to work correctly. Healthy corneas have a well-functioning defense mechanism that neutralizes the harmful by-products of metabolism before they damage the eye's collagen. But because people with keratoconus lack that ability, the chemical changes in their tissue causes structural damage that weakens and thins the cornea, causing it to bulge.
Research continues into other factors that may lead to keratoconus. These include an imbalance of chemical reactions within the cornea that may weaken the tissue and make it susceptible to damage.
When Does Keratoconus Develop?
Commonly, blurred vision begins in one's late teens or early twenties. The average age of onset is fifteen. The disease progresses until about age forty when it stabilizes because of normal corneal stiffening. Even though keratoconus involves one eye initially, it can eventually affect both eyes. The progression can differ in each eye. Although the disease normally takes years to advance, it can progress quickly. The rule of thumb is that keratoconus usually doesn't get worse after age thirty-nine. Still, there are cases in which the condition has worsened after this age.
What Is Post-LASIK Corneal Ectasia?
Post-LASIK corneal ectasia is a rare but serious complication that may occur after LASIK vision-correction surgery. It can occur in patients who were considered good candidates for a LASIK procedure. Post-LASIK ectasia is sometimes referred to as "keratoconus after refractive surgery." It occurs when the cornea becomes weakened and thin, leading to a bulging cornea. Most patients who undergo LASIK surgery can expect a quick recovery from the procedure, which is considered safe and effective. However, side effects can occur. If not treated, corneal extasia can lead to permanent vision loss. Studies suggest that corneal ectasia occurs in about one in every 2,000 LASIK procedures.
Symptoms of Post-LASIK Ectasia
As with keratoconus, the symptoms of post-LASIK ectasia are blurriness and distorted vision, commonly due to increasing nearsightedness and astigmatism. Refractive surgeons are especially concerned if their patients report vision loss, glare, halos, and/or "ghosting" around images. Onset of the ectasia can vary widely from a month after LASIK surgery to several years later. So even if your LASIK surgery is successful initially, it's important to keep up with regular eye exams — your eye specialist needs to monitor you for complications, especially corneal ectasia.
Causes of Post-LASIK Ectasia
Post-LASIK corneal ectasia develops much the same way as keratoconus. It involves the same tissue breakdown that cause the cornea to weaken and bulge, distorting vision. There are several factors that may contribute to that process.
Removal of too much corneal tissue. Removing too much corneal tissue during a LASIK procedure can diminish the strength and integrity of the cornea, causing it to push outward. The critical amount that must be left after the surgeon creates a flap and reshapes the middle layer of tissue, the stroma, is referred to as the residual stromal or corneal bed. If this layer is not thick enough, corneal ectasia can result. Patients with severe nearsightedness are at particular risk because the surgeon is already removing more tissue to correct vision compared to what they remove in patients who are farsighted or have astigmatism.
On the other hand, a LASIK procedure may have been expertly performed, but the thinning of the cornea still occurs.
Not being a good candidate for LASIK. The risk for corneal ectasia also increases if your eye's overall structure and condition made you a poor candidate for LASIK initially. If your central cornea was too thin for a LASIK procedure, losing more tissue diminishes your remaining cornea even further.
Abnormally shaped cornea. Another risk for corneal ectasia is an abnormal corneal shape. Even if your corneal thickness is adequate, if you had an unusually shaped cornea prior to LASIK, you have a greater risk for problems afterward.
Age. Studies show that age may be a risk factor. Younger LASIK patients may be at greater risk for corneal ectasia than older individuals.CHAPTER 2
Getting a Diagnosis of Keratoconus or Post-LASIK Ectasia
Your ophthalmologist will perform a variety of tests to determine whether you have keratoconus or post-LASIK corneal ectasia. He or she will also perform other tests to check your eyes for other potential problems.
You may find it helpful to have a basic understanding of the types of tests that may be performed on your eyes during your visit to your eye-care specialist.
Your Medical History
First, your ophthalmologist will want to learn about your overall health and medical history, especially whether you have any previous or existing eye conditions. Also, you will need to report any underlying health conditions along with medications you're currently taking.
Your family eye history will be particularly important, especially if there's a strong pattern of progressive keratoconus. Knowing that you have blood relatives with the disease may prompt your eye doctor to recommend corneal cross-linking sooner rather than later because a family history of the disease may suggest your condition may worsen without treatment.
In addition to evaluating your pupil function, your cornea specialist will also check your peripheral (side) vision and eye movement. You're probably familiar with the Snellen eye chart, which measures the clarity of your vision at a distance. With this test, you'll be asked to tell the examiner which letters you can see clearly among the progressively larger letters.
Overall Assessment of the Eyeball
With the help of a slit lamp, a high-powered microscope with a light on it, your ophthalmologist will evaluate the front and back of your eyes for overall health and to determine if you have keratoconus or corneal ectasia and what its status is. The slit lamp is not only helpful in diagnosing your condition, but also in determining your readiness for corneal cross-linking. As you rest your forehead on the slit lamp's support, your physician examines the structure of your eyes, including the cornea. After dilating your eye, your doctor will also examine the retina and the optic nerve at the back of the eye.
During the slit lamp examination, changes to your cornea may not be evident if you have early keratoconus; however, as the disease advances your doctor can identify iron deposits in the cornea that form a yellowto-brownish ring. An eye exam may also show thin, vertical, white stress lines in the tissue at the back of the cornea. Other indicators of keratoconus include corneal thinning and scarring.
The slit lamp also aids your physician in evaluating other eye characteristics, such as your intraocular or internal eye pressure, which can play a role in underlying eye disease. To perform this test, a technician inserts a numbing eyedrop and then uses a cobalt blue light and a small probe to touch the eye's surface.
A painless procedure, corneal topography is used to map the curvature of the cornea. It produces a computerized picture of the cornea's shape, steepness, and thickness — all factors involved in diagnosing disease and screening for possible treatment with corneal cross-linking. In just minutes, corneal topography produces a colorful three-dimensional picture, capturing fine details and irregularities that can be missed by other conventional testing.
During the corneal topography procedure, you're seated in front of a large lighted bowl-like instrument with your forehead pressed against a headband. A pattern of illuminated rings is projected onto the cornea. As the rings reflect back, a computer analyzes data points to build a precision map. Vivid colors identify elevations; red identifies steeper areas while blue shows flatter surfaces. The resulting printout reveals the cornea's steepness and amount of astigmatism.
Testing Cornea Thickness
Pachymetry is a simple, painless test for measuring the thickness of your cornea. It's used to detect, assess, and track various eye diseases, and it is especially helpful in determining whether you are a good candidate for cross-linking. To perform this test, a probe that emits sound waves is placed gently on the corneal surface. These waves travel through the eye and send data back to the computer; the data is translated into images and measurements of the cornea's thickness.(Continues…)
Excerpted from "Corneal Cross-Linking"
Copyright © 2018 Lawrence M. Hopp, M.D..
Excerpted by permission of Addicus Books, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
1 Corneal Cross-Linking: An Overview,
2 Getting a Diagnosis of Keratoconus or Post-LASIK Ectasia,
3 Understanding Corneal Cross-Linking,
4 Preparing for Your Corneal Cross-Linking Procedure,
5 Undergoing Corneal Cross-Linking,
6 Recovery from Corneal Cross-Linking,
About the Author,