Corneal Cross-linking (CXL):
For Keratoconus Treatment And
LASIK-Related Corneal Ectasia
Corneal cross-linking (CXL) is an in-office eye procedure that strengthens the cornea if it's been weakened by keratoconus, other corneal disease, or (rarely) a complication of LASIK surgery. Alternative and brand names for the procedure include corneal crosslinking, corneal collagen crosslinking, C3-R, CCL and KXL.
The minimally invasive CXL procedure involves applying liquid riboflavin (vitamin B2) to the surface of the eye, followed by treatment with a controlled application of ultraviolet light, to eliminate corneal ectasia.
The two basic types of corneal cross-linking are:
- Epithelium-off CXL. In this type of cross-linking procedure, the thin outer layer (epithelium) of the cornea is removed to allow the liquid riboflavin to more easily penetrate the corneal tissue.
- Epithelium-on CXL. In this procedure (also called transepithelial CXL), the protective corneal epithelium is left intact, requiring a longer riboflavin "loading" time.
Corneal crosslinking also can be combined with other procedures for keratoconus treatment. For example, combining CXL with implanting tiny arc-shaped corneal inserts called Intacs has been shown to help reshape and stabilize the cornea in more advanced cases of keratoconus.
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Current Status of CXL in the United States
In April 2016, the pharmaceutical and medical device company Avedro announced it received FDA approval for the company's KXL System that provides corneal collagen cross-linking for the treatment of progressive keratoconus. The approval includes Avedro's Photrexa Viscous and Photrexa, which are riboflavin solutions used with the KXL System during the procedure.
The aim of corneal crosslinking is to strengthen the cornea by increasing the number of "anchors" that bond collagen fibers together. (Diagram: Boxer Wachler Vision Institute)
This is the first corneal collagen cross-linking system to gain the required regulatory approval to be performed by eye doctors in the United States for this sight-threatening condition.
Approval of Avedro's cross-linking system was based on the results of clinical trials that showed the Photrexa/KXL System halted and produced a mild reversal of the bulging of the cornea in patients with progressive keratoconus three to 12 months after the procedure. Patients who received a sham treatment for comparison purposes had a worsening of their keratoconus over the same period.
Prior to receiving FDA approval of the Photrexa/KXL System, Avedro announced in June 2015 that approximately 200,000 patient eyes outside the United States have been treated with the corneal cross-linking procedure.
Though Avedro's Photrexa/KXL System currently is the only FDA-approved cross-linking procedure for the treatment of progressive keratoconus in the U.S., if you have this condition or corneal ectasia following LASIK surgery, you also may be able to be treated with other crosslinking procedures that are not yet FDA approved.
This is possible due to something called "off-label use" a term used to describe when a drug or medical device is used for a purpose for which it does not yet have regulatory approval. Off-label use of drugs and medical devices is legal in the United States, provided certain conditions are met and the patient is fully informed and agrees to the treatment.
Before agreeing to undergo an off-label procedure for the treatment of keratoconus or corneal ectasia, be sure to consult with your eye doctor so you fully understand the approval status, legality, risks, contraindications and possible side effects of the procedure.
Another option is to participate in a clinical trial designed to evaluate the safety and effectiveness of a new procedure for the treatment of keratoconus or corneal ectasia that is not yet FDA approved. If you are interested in participating in a study of this sort, visit our access page for the clinical trials operated by the National Institutes of Health, and type "corneal cross-linking" in the search box on the page to see if any trials are recruiting patients.
Who Are the Best Candidates for Corneal Crosslinking?
Corneal crosslinking is most effective if it can be performed before the cornea has become too irregular in shape or there is significant vision loss from keratoconus or other causes of corneal ectasia. If applied early, CXL typically will stabilize or even improve the shape of the cornea, resulting in better visual acuity and an improved ability to wear contact lenses.
Other potential applications of CXL include the treatment of corneal ulcers that are unresponsive to treatment with topical antibiotics. Studies also have found that corneal cross-linking can be effective for eradicating a variety of corneal infections.
SEE ALSO: Ask the Eye Doctor About Keratoconus >
Some surgeons have also reported reasonable results in improving the stability of the cornea in people who have undergone radial keratotomy, an incisional refractive procedure from the 1980s and 1990s. Crosslinking appears to work best for those who are experiencing daily fluctuations in their vision.
People who are considering vision correction procedures such as LASIK also might eventually be pre-treated with corneal crosslinking to strengthen the eye's surface before undergoing an excimer laser ablation that reshapes the cornea.
What To Expect During and After a Corneal Crosslinking Procedure
During preliminary examinations, your eye doctor will measure the thickness of your cornea and make sure you are a good candidate for the procedure. You also will need to have a routine eye exam to assess your visual acuity and general eye health. And your doctor will perform mapping of your cornea (called corneal topography) to determine the level of your eye condition.
Layers of the cornea. Corneal crosslinking strengthens bonds in the stroma of the cornea, which is the layer from which tissue is removed during LASIK surgery.
The crosslinking procedure takes 60 to 90 minutes in most cases. If you are receiving epithelial-off crosslinking, your surgeon will place you in a reclining position and remove the epithelial layer from your cornea. Riboflavin eye drops will then be placed.
If you are receiving epithelial-on crosslinking, you are placed in a reclining position, and riboflavin drops are started. After enough time has passed (depending on the technique), the surgeon will evaluate your eye to ensure that the cornea has sufficient riboflavin present.
The corneal thickness will be checked, and then the UV light will be applied for up to 30 minutes.
In some cases, a bandage contact lens will be placed. Topical antibiotic and anti-inflammatory drops will be prescribed.
With both epithelial-off and epithelial-on CXL, studies have found that 99 percent of patients will remain stable or achieve improvement in the corneal shape, which is highly significant for people with progressive keratoconus that otherwise might lead to severe vision loss.
And though corneal-crosslinking may cause some initial eye irritation, the surface of the eye and the tear film appear to fully recover within several weeks. In a recent study designed to investigate potential alterations of the eye's surface and tear film parameters three months after CXL in progressive keratoconus patients, no adverse effects on the ocular surface and tear function (which are important for visual quality) were found.
Epithelium-off vs. Epithelium-on CXL
One of the big questions is whether epithelium-on CXL (transepithelial CXL) can provide similar results to crosslinking procedures where the epithelium has been removed (epi-off CXL).
One recent study that compared the effectiveness of the two procedures for the treatment of 36 eyes of 27 patients ages 12 to 18 years who were diagnosed with progressive keratoconus found that at 24 months after treatment, 94.4 percent of eyes in the epi-off CXL group (18 eyes) either showed less keratoconus or had no worsening of the disease, whereas only 66.6 percent of eyes in the epi-on CXL group (18 eyes) improved or stabilized. The study authors concluded the epi-on CXL procedure was only 70 percent as effective as epi-off CXL in terms of its ability to stop the progression of keratoconus.
In Europe, a special riboflavin formulation (Ricrolin TE) has been developed that helps riboflavin penetrate the corneal epithelium more easily to facilitate epithelium-on CXL. And in the United States, other researchers have reported visual results from epi-on CXL that are similar to those of epi-off CXL.
Advantages of epithelium-on CXL include reduced risk of infection, less corneal haze and fewer delays in epithelial healing. And though some studies show outcomes with epithelial-off CXL tend to be significantly better in younger candidates than in people older than age 35, this difference does not appear to be an issue with transepithelial CXL. Epithelium-on CXL also allows for quicker visual recovery and less discomfort.
Cost of Corneal Cross-Linking
The cost of a corneal cross-linking procedure for keratoconus or corneal ectasia can range from $2,500 to $4,000 per eye.
Some of this cost may be covered by your health insurance policy. Ask your eye doctor or insurance company for assistance in determining what your total out-of-pocket expense for the procedure will be.
Avedro receives FDA approval for Photrexa Viscous, Photrexa and the KXL System for corneal cross-linking. Avedro. Press release issued April 2016 at www.avedro.com.
Efficacy of epithelium-off and epithelium-on corneal collagen cross-linking in pediatric keratoconus. Eye & Contact Lens. February 2016 [online ahead of print].
Avedro announces 500th cross-linking device sale outside the US. Press release issued by Avedro, Inc. in June 2015.
Multicenter evaluation of trans-epithelial versus epithelial-off corneal collagen crosslinking (CXL) for keratoconus and post-LASIK ectasia. Poster 363, American Academy of Ophthalmology Annual Meeting. October 2011.
Evaluation of epithelial-on corneal collagen crosslinking in patients ages 35 or older. E-Poster 101325, International Society of Refractive Surgery Annual Meeting. October 2011.
Collagen corneal cross linking. Presentation by Ramon Naranjo-Tackman, MD, at Cornea Day 2011 sponsored by the American Society of Cataract and Refractive Surgery. March 2011.
Corneal collagen crosslinking: promises and problems. British Journal of Ophthalmology. December 2010.
Marilyn Haddrill also contributed to this article.
[Page updated May 2016]