Keratoconus / Corneal Cross-Linking
What is Keratoconus?
The cornea is the window or windscreen at the front of the eye. Light travels through the cornea past the lens to the retina and then the brain to form a visual image. The normal corneal surface is smooth and aspheric i.e. round in the centre, then flattening towards its outer edges. Light rays passing through it moves in an undistorted manner to the retina to project a clear image to the brain.
In patients with keratoconus the cornea is cone shaped (hence the name keratoconus, derived from the greek word for cornea (‘kerato’) and cone shaped (‘conus’). In patients with keratoconus the cornea is not only cone shaped, the surface is also irregular or warped resulting in a distorted image being projected onto the brain.
Keratoconus: Symptoms and Signs
Keratoconus can be difficult to detect, because it usually develops slowly. However, in some cases, keratoconus may proceed rapidly. As the cornea becomes more irregular in shape, it causes progressive nearsightedness and irregular astigmatism to develop, creating additional problems with distorted and blurred vision. Glare and light sensitivity also may occur. Often, keratoconic patients experience changes in their eyeglass prescription every time they visit their eye care practitioner.
It’s not unusual to have a delayed diagnosis of keratoconus, if the practitioner is unfamiliar with the early-stage symptoms of the disease. Many patients are initially unaware they have keratoconus and see their eye doctor because of increasing spectacle blur or progressive changes in their prescription. Keratoconus is most often diagnosed by a cornea specialist who may see typical findings when examining the patient at the slit-lamp. In early forms of the disease, there may be no obvious finding on slit-lamp evaluation and the diagnosis is made only by computerized topography (Pentacam or Orbscan topography). This test maps out the cornea on a contour map like an ordnance survey map. Hotter colours on the map correlate with steeper parts of the cornea. The typical finding on a cornea with keratoconus is a steep inferior cone-shaped portion.
Keratoconus may occur in one eye only initially but most commonly affects both eyes with one eye being more severely affected than the other. Both males and females are equally affected.
What Causes Keratoconus?
The exact cause of keratoconus is unknown. Eye-rubbing and Down’s syndrome are both associated with an increased risk. Although evidence supports a role for genetic factors this does not mean if you have a child with keratoconus they will necessarily develop keratoconus since only 15% of keratoconus patients have a family history of keratoconus. It can occur in one or both eyes and often begins during a person’s teens or early 20s and progresses to the mid-thirties at which time progression slows and often stops. Between age 15 and 35, it can arrest or may progress at any time. Unfortunately, there is no way to predict how fast it will progress or if it will progress at all. In general young patients with advanced disease at diagnosis are more likely to progress to the point where they may ultimately require some form of surgical intervention.
Contact Lenses or Spectacles
In the mildest form of keratoconus, eyeglasses or soft contact lenses may help. But as the disease progresses and the cornea becomes thinner and increasingly more irregular in shape, glasses and soft contacts no longer provide adequate vision correction. Rigid gas-permeable (RGP or GP) contact lenses are usually the preferred treatment. Their rigid lens material enables GP lenses to vault over the cornea, replacing its irregular shape with a smooth, uniform refracting surface to improve vision. Hence the vast majority of patients are treated with rigid contact lenses.
RGP contact lenses can be less comfortable to wear than soft contacts. Also, fitting contact lenses on a keratoconic cornea is challenging and time-consuming. You can expect frequent return visits to your optician to fine-tune the fit and the prescription, especially if the keratoconus continues to progress.
Corneal Cross-Linking with ultraviolet light
This is a new type of treatment that aims to strengthen the cornea (see below) and stop any further progression of the condition. This is only indicated in patients that show progression on corneal topography (computerized test).
In advanced cases in which RGP contact lens wear is not possible the final option is to proceed with a corneal graft. This can be either a full thickness (penetrating keratoplasty) or partial thickness (lamellar keratoplasty).
What is corneal collagen cross-linking?
Corneal collagen cross-linking (CXL) is a new, noninvasive treatment for keratoconus. CXL involves applying a photosensitizing solution to the cornea and exposing it to a low dose of ultraviolet light. The photosensitizer reacts with the ultraviolet light to create new collagen bonds (cross-links) throughout the cornea.
In corneas affected by keratoconus there are too few collagen bonds to maintain structural integrity. The reduced number of collagen bonds and weakened configuration result in corneal bulging, steepening, and irregularity all of which significantly affect vision.
By creating new collagen bonds, CXL strengthens and adds resilience to corneas weakened by keratoconus. If performed early enough, CXL can counteract its effects and allow good vision to be preserved. In advanced cases, CXL can postpone the need for invasive corneal transplants and prevent vision from getting worse.
How is the treatment done?
CXL is a straightforward procedure performed in an operating theatre under topical anaesthesia. During treatment, the upper-most layer of the cornea (called the epithelium) is gently rubbed off to allow for absorption of the photosensitizing agent. Next, photosensitising drops are applied every 2 minutes for 10 minutes. The cornea is exposed to a controlled dose of ultraviolet light for the next 8 minutes of the treatment. Once the procedure is complete, the eye is covered with a bandage contact lens to help the epithelium grow back (usually within 2 to 3 days) and facilitate a quick visual recovery. Overall, the procedure takes about 25 minutes.
Can ultraviolet light harm my eyes?
Ultraviolet light used for the CXL treatment was specifically selected to be safe on the cornea, lens and retina while still being sufficient to induce collagen cross-linking. Laboratory and clinical studies have demonstrated that the amount of light that reaches the deeper structures in the eye is not strong enough to cause damage. Furthermore, the ultraviolet light is measured and calibrated prior to each treatment to ensure that safe exposure levels are never exceeded.
When will I notice changes?
Within one month, vision returns to pre-surgery levels. The most dramatic improvements tend to occur during the first three months and then, gradually over a period of up to one year, the cornea continues to stiffen and stabilize resulting in further improvements. In most cases, the keratoconus stops getting worse and in some cases vision may improve.
How long is follow-up?
You will be seen shortly (3-7 days) after the procedure to remove the bandage contact lens and assess your recovery. Regular follow-up visits will be at one, three, six and twelve months. After that, you will be seen annually to monitor your results.
When can I wear contact lenses again?
In most cases, you may return to wearing contact lenses after one month. However, your lenses may need to be changed occasionally until your cornea fully stabilizes.
What makes corneal collagen cross-linking unique?
CXL is a unique treatment because it is noninvasive and targets the cause of the problem, which is the weakened cornea in keratoconus. CXL significantly increases the number of collagen bonds between the corneal layers, in effect returning the cornea to a more stable state. Studies have shown an increased rigidity of up to 300% in corneas treated with CXL. In contrast to other procedures like intrastromal ring segments or laser surgery, only CXL offers the potential to target the underlying cause and thus offer long-term benefits for those suffer from keratoconus.
What are the possible risks of cross-linking?
- The surface of the cornea (epithelium) is removed prior to the treatment. Until the epithelium heals over, the eye will often be very sore. A bandage contact lens will be placed onto the cornea after the procedure to make the eye more comfortable, and antibiotic and steroid eye drops will need to be applied regularly. You may also need to take some pain-killers for a few days. Some patients find the eye sensitive to light for some weeks after the treatment. In some patients, the wound healing may lead to haze or scarring on the corneal surface, which could adversely impact on the quality of vision. Some mild haze is very common in the first few weeks following treatment. The degree of corneal haze peaks between 1 and 3 months after surgery and diminishes to baseline levels at 1 year ( J Cataract & Refractive Surgery, December 2010).
- An infection of the cornea may occur in around 1 in 300 patients.
Antibiotic drops are prescribed after surgery to reduce this risk.
- Ultraviolet light is potentially damaging to cells. Studies have shown that the cells within the cornea called keratocytes are mostly destroyed by the cross-linking treatment in the central area of the cornea, but are later replaced by healthy cells within a few months. Theoretically the inner lining cells of the cornea (endothelium) could also be damaged if your cornea were too thin. For this reason people with particularly thin corneas are excluded from treatment. Studies performed so far have not demonstrated any damage to the endothelial cells. Ultraviolet light may also potentially damage retinal cells, but virtually all the ultra-violet light is absorbed by the riboflavin in the cornea, so the UV light level reaching the retina should be well within recommended safe limits.
- Your keratoconus might progress in the future. In this case cross-linking treatment could potentially be repeated, or a conventional corneal graft operation could be performed
What is the Cost?
Blackrock Clinic charges €1600 for the CXL procedure.
Up until recently most health insurers did not cover the cost of this procedure. Irish Life Healthcare recently announced that they will now cover some of the cost (procedure code 2801). You are advised to contact Irish Life to confirm that you are covered prior to having the treatment. Laya Healthcare agreed to return a portion of the cost of the procedure to the patient after surgery. This must be arranged between each patient and Laya Healthcare directly prior to treatment. A Pre-operative approval letter is required from your insurance company if they agree to pay the surgeons fee (€875).