Corneal cross-linking (Corneal Cross Linking)

Corneal cross-linking

Corneal cornea cross-linking has revolutionized our ability to treat keratoconus and ectasia of any cause and to prevent the progression of this common disease. Keratoconus is still the second most common indication for corneal transplantation in the United States. Cross-linking can be defined as the creation of bonds that connect one polymer chain to another. This is in fact a physiological process that occurs with age through enzymatic processes such as transglutaminase and lysyl oxidase. For several decades, photochemical cross-linking has been used in various surgical fields in order to shape the biochemical characteristics of connective tissues. Cross-linking was used in the past to stabilize tissues for restoration and to strengthen the enamel of teeth.

Corneal cross-linking with riboflavin and ultraviolet light

The idea of using the cross-linking technique in the cornea in order to treat ectasia on a background of keratoconus was developed by the University of Dresden by Seiler and Spoerl in 1998, and was subsequently reported in the literature in 2003 (they in fact first described the "Dresden protocol"). Corneal cross-linking involves the use of a riboflavin solution, which is essentially vitamin B2, combined with exposure to Ultraviolet A light. The activated riboflavin causes the collagen fibers in the tissue to form strong chemical bonds with adjacent fibrils. In the cornea, as in the skin, collagen cross-linking can occur naturally, due to oxidative deamination of the end chain of the collagen molecule. In addition, other pathways can lead to collagen cross-linking.

Corneal cross-linking by the release of free radicals

Most technologies that perform cross-linking of collagen fibers in the cornea do so by bringing about the release of free radicals. Riboflavin serves as a source of free radicals after it is activated by the ultraviolet light (they are released from its ring structure). It is this interaction of free radicals created by the combination of riboflavin and exposure to ultraviolet light that enables cross-linking of collagen and increases the stiffness of the cornea. In the presence of riboflavin, about 95% of the ultraviolet light is absorbed in the front 300 microns of the corneal stroma. Therefore, a minimum corneal thickness of 400 microns (after removal of the corneal epithelium) is recommended in order to avoid damage to the corneal endothelium by the ultraviolet light irradiation. Thin corneas can be temporarily thickened with hypotonic riboflavin before the ultraviolet light treatment.

The effect of cross-linking on the cornea

Although flattening of the cornea may occur after the treatment, the most important effect of corneal cross-linking is stabilization of the corneal curvature and prevention of further steepening and protrusion of the cornea in patients with ectatic diseases of the cornea such as keratoconus (and post-LASIK ectasia). There is no significant change in the refractive index of the cornea or in corneal transparency. The main clinical application of corneal cross-linking is to prevent the progression of keratoconus disease and ectasia after glasses-removal surgery.

The Dresden protocol

In the Dresden protocol, the riboflavin solution is continuously instilled onto the cornea after removal of the epithelium for 30 minutes (in most studies). Afterwards, the riboflavin is activated by Ultraviolet A illumination that is irradiated onto the cornea for 30 minutes, during which riboflavin continues to be instilled. It was demonstrated that after this procedure corneas become 300% stiffer and resistant to enzymatic digestion.

Studies also showed that the treated corneas contained higher molecular weight polymers of collagen due to cross-linking of the fibrils. Safety studies showed that the endothelium was not damaged by the treatment if the ultraviolet radiation level was kept below a certain threshold and the corneal thickness was greater than 400 microns. This type of cross-linking is called epithelium off or epi-off. Today, work is being done on riboflavin solutions and cross-linking methods that avoid the need to remove the epithelium.

Cross-linking quickly became a first-line treatment for treating keratoconus around the world and was approved by the FDA in April 2016 for treating progressive keratoconus and ectasia. Studies in humans began in 2003 in Dresden and the initial results were very promising. The initial study included 16 patients with advanced keratoconus, and after treatment there was a complete halt of the disease. In addition, flattening of the cornea was demonstrated in 70% and an improvement in visual acuity in 65%. No complications were reported in this initial study.

In a clinical study in the United States, all patients with keratoconus or ectasia after LASIK underwent treatment according to the Dresden protocol. In this study there were two control groups: the first sham, meaning they did not receive any treatment, and the second the other eye of the patients who did receive treatment. All patients were followed for one year. Treated eyes demonstrated temporary steepening with a decrease in corrected visual acuity and subsequently flattening of the cornea that reached a peak around 1-3 months after the corneal cross-linking. Beyond that, temporary compression of the cornea (a decrease in thickness) and an improvement in corrected visual acuity were also observed. Overall, it seems that there is stabilization in most treated eyes and some of the eyes (between 0-20%) will need repeat treatment. There are also rare reports of vision loss (2 lines or more) in some of the studies.

Indications for corneal cross-linking

Although the leading indication for corneal cross-linking is to prevent the deterioration of keratoconus, it is also used in the treatment of Pellucid Marginal Degeneration and iatrogenic ectasia (secondary to LASIK, PRK and radial keratotomy surgeries). Cross-linking is also used in combination with additional treatments such as intracorneal ring segments and limited topography-guided photoablation. Since the cross-linking process leads to compression of the corneal stroma and reduces the space for accumulation of fluid in the cornea, it is used for palliative treatment in patients with pseudophakic bullous keratopathy. Furthermore, after initial laboratory studies indicated antimicrobial properties, there are reports of the use of corneal cross-linking in order to treat bacterial keratitis, fungal, protozoal and atypical keratitis.

The main purpose of corneal cross-linking is the stabilization of the progression of the ectasia. And the most common indication for corneal cross-linking is keratoconus. Additional possible indications include Pellucid Marginal Degeneration and ectasia after glasses-removal surgeries.

Selecting a patient for cross-linking

The main purpose of cross-linking is to halt the deterioration of ectasia in the cornea. Although there are no accepted universal criteria, there are several parameters that serve as a measure of progression over a period of 12 months:

• An increase of 1 diopter in the steepest simulated keratometry as measured by tomography
• An increase in astigmatism as detected by subjective refraction of 1 diopter
• A decrease of one line or more in uncorrected or corrected visual acuity

Contraindications to corneal cross-linking

There are several contraindications to cross-linking. A corneal thickness of less than 400 microns is a contraindication to the standard treatment, and this is because at such a thickness there is a concern of irreversible damage to the corneal endothelium from the ultraviolet light. There are solutions for performing corneal cross-linking in thin corneas, such as corneal cross-linking with the help of a contact lens and the use of hypotonic riboflavin, which allow corneal cross-linking down to a minimum thickness of 300 microns.

Additional contraindications are:

• Keratometry above 58 diopters, since there is a lower chance of halting progression and a higher chance of haze after the procedure
• Age above 35-40 years, since the disease is usually stable at this age and there is a higher chance of vision loss as a result of the procedure
• Corrected visual acuity above 20/25, since there is an increased chance of loss of visual acuity
• Significant scarring of the cornea, since the patient will probably need a corneal transplant anyway to improve vision
• A background of herpetic infection in the cornea, due to the concern of reactivation of the virus following treatment and of corneal melting
• It is preferable not to perform this treatment in patients with a history of poor healing of the corneal epithelium, in patients with ocular surface diseases and autoimmune diseases.

Surgical techniques

There are different techniques for performing corneal cross-linking. There is the original Dresden protocol, transepithelial corneal cross-linking, accelerated cross-linking and combined methods. The main purpose of the first stage is to allow sufficient diffusion of riboflavin into the cornea. In the first studies, the riboflavin did not penetrate the intact epithelium of the cornea, and therefore the Dresden protocol requires prior removal of the epithelium in order to allow penetration of riboflavin into the corneal stroma. However, new riboflavin solutions have been demonstrated that do manage to penetrate the intact epithelium, which may allow faster healing, faster improvement in visual acuity, less pain and fewer complications. There are studies examining these solutions and the use of the iontophoresis method by means of an electrical current that allows riboflavin to penetrate through the epithelium of the cornea. After the riboflavin penetrates the cornea, the second stage is cross-linking of the collagen fibers by exposing the cornea to Ultraviolet A light. The Dresden protocol requires exposure for 30 minutes at an intensity of 3 mW/cm2. New accelerated protocols try to shorten the exposure duration by increasing the intensity of the radiation.

Transepithelial corneal cross-linking

Pain after the procedure, delays in epithelial healing, infection, haze (opacity) and corneal melting are disadvantages of epithelium removal that led to searches for an alternative through corneal cross-linking across an intact epithelium. There are studies that examined this possibility with promising reports. However, other studies demonstrated that there may be reduced efficacy in this method compared to the standard method due to low penetration of the riboflavin. It is possible that various additional chemical preparations will allow higher penetration of the riboflavin through the corneal epithelium without removing it. The use of iontophoresis and partial disruption of the corneal epithelium also increase the penetration of the riboflavin through the corneal epithelium. No matter which method is used, diffusion of riboflavin into the corneal stroma is a critical stage in the process.

Accelerated cross-linking

There are new ultraviolet lamps that allow shortening of the ultraviolet light exposure duration. There are systems that allow built-in treatments of 10 and 5 minutes (by using an intensity of 10 mW/cm2 and 18 mW/cm2 respectively). Other systems allow the doctor to choose the duration and accordingly determine the intensity. The Kanellopoulos group found that exposure for 15 minutes of 7 mW/cm2 was effective and safe and led to results similar to those of Dresden.

Combined techniques

In some cases, patients do not achieve a sufficient improvement in visual acuity to allow basic functioning after corneal cross-linking. Therefore, ophthalmologists have tried to combine corneal cross-linking with various refractive surgeries. Implantation of intracorneal ring segments and afterwards performing corneal cross-linking has been proven effective. The considered use of topography-based transepithelial PRK and afterwards performing corneal cross-linking has been proven to improve vision and stabilize keratoconus. It seems that performing PRK and afterwards corneal cross-linking (and not the reverse) leads to better results, as is performed in the Athens protocol. Combining corneal cross-linking with implantation of a toric phakic lens allows safe and effective correction of myopic astigmatism in eyes suffering from mild to moderate keratoconus. Combining corneal cross-linking with topography-based PRK and implantation of a toric phakic lens can improve the patient's visual acuity with more consistent results than corneal cross-linking combined with PRK alone.

Complications of corneal cross-linking

Corneal cross-linking has become the accepted treatment for preventing the progression of keratoconus, with a growing sense of confidence that it is a fairly safe treatment. However, it is worth remembering that although the complications are quite rare, there are several complications that may occur during or as a result of the procedure. Follow-up of over 10 years in patients who underwent this treatment demonstrates efficacy and safety of the treatment in preventing corneal transplantation.

The reported complications of cross-linking can be divided into one of two groups: the first is treatment failure when the disease does not stabilize despite the treatment, and the second is complications resulting from infection, inflammatory processes and suboptimal healing that lead to loss of corrected visual acuity. Treatment failure is defined as progression of the disease with an increase in maximum keratometry above one diopter compared to before the surgery. Reports indicate that treatment failure may occur in 7-10% of patients in the first year. Cases were also reported in which patients were stable for several years after the treatment and subsequently the disease nevertheless progressed, which indicates that changes in the structure of the corneal stroma can nevertheless cause loss of the therapeutic effect in a small proportion of patients.

Koller et al evaluated 117 eyes with keratoconus that underwent standard cross-linking and reported treatment failure in about 8% of the eyes and loss of corrected visual acuity of two lines or more in 3% of the eyes. There is great variety in the various reports regarding the type of complications and the frequency of complications (depending on the study group, the group of subjects and the method used to perform cross-linking). Some of the more common complications after cross-linking treatment include sterile infiltrates in about 8% of patients and central stromal scars in about 3% of patients. There are several reports of stromal haze after corneal cross-linking, probably secondary to temporary formation of fibroblasts. This phenomenon is especially prominent about a month after the surgery, stabilizes around three months and fades up to 12 months after the treatment. Permanent corneal haze was reported in some of the series in up to 8% of the eyes, and when it appears prolonged treatment with topical steroids is recommended.

Rarer complications include infectious keratitis from bacteria, parasites (Acanthamoeba), viruses (herpes) and fungi that were reported in some of the series. In addition, iritis (anterior uveitis), increased intraocular pressure (probably secondary to steroids) and even corneal melting that requires corneal transplantation were reported.

Additional worrying complications that have been reported in the past (extremely rarely) include persistent corneal edema, probably secondary to damage to endothelial cells, and epithelial growths of the corneal conjunctiva (conjunctival intraepithelial neoplasia).

Future directions

It has now been over a decade since corneal cross-linking was first reported, and it is still in a process of evolution and progress. Recently, there are studies examining the possibility of performing ultrafast flash-linking, and in addition, the use of other photomediators such as rose bengal instead of riboflavin in order to further improve the ability to strengthen the biochemical structure of the cornea and reduce the secondary damage to other structures in the cornea.

Conclusions

Ectasia of the cornea is a condition that severely impairs vision with a very serious effect on quality of life. It seems that early treatment of the disease with cross-linking is effective in preventing deterioration of the disease and preserving visual acuity, and it may delay and even prevent the need for corneal transplantation surgery down the road. Corneal cross-linking has led to a real revolution in our ability to treat keratoconus in recent years, in that this simple procedure stabilizes the disease and in certain cases even causes an improvement in some of the measures (visual acuity, topography, refraction, etc.). This innovative treatment has great potential, and there are those who believe that this treatment will reduce the amount of corneal transplants by 50% in the coming years in the United States.

The efficacy of standard cross-linking (the Dresden protocol) as a method for halting an ectatic process in the cornea is supported by laboratory and clinical studies from the last 15 years. It seems that overall this is a relatively safe procedure with a low percentage of complications (1-3%), most of them secondary to epithelial peeling and transient. Although the reported complications are rare and mostly reversible, we encourage practitioners to adhere to accepted protocols that reduce the chance of complications among patients. As always, it is preferable that an ophthalmologist experienced in treating keratoconus patients and in performing corneal cross-linking discuss with the patient the risks involved in the procedure and the benefits of performing the procedure.