Corneal collagen cross-linking

Corneal Collagen Cross-Linking (CXL) is a medical procedure aimed at treating keratoconus and other conditions that cause corneal weakening. The technique strengthens the cornea by increasing the number of natural "anchors" within the corneal stroma, these anchors are known as cross-links. Cross-linking involves the use of ultraviolet (UV) light and a photosensitizer, typically riboflavin (vitamin B2), to induce the formation of these additional cross-links.

Procedure

The standard protocol, known as the Dresden protocol, involves the removal of the corneal epithelium (epi-off CXL) to allow for better penetration of riboflavin into the stroma. After de-epithelialization, riboflavin drops are applied to the cornea for approximately 30 minutes. The cornea is then exposed to UV-A light for about 30 minutes. The combination of riboflavin and UV-A light increases the formation of covalent bonds between collagen molecules, leading to increased corneal stiffness.

An alternative method, known as transepithelial or epi-on CXL, does not involve the removal of the epithelium. This method aims to reduce post-operative pain and risk of infection, although it is debated whether it is as effective as the epi-off technique due to the barrier the epithelium poses to riboflavin penetration.

Indications

Corneal collagen cross-linking is primarily indicated for the treatment of keratoconus, a progressive thinning and bulging of the cornea. It is also used in cases of corneal ectasia post-refractive surgery and pellucid marginal degeneration. The goal is to halt the progression of corneal deformation by strengthening the corneal structure.

Risks and Complications

While CXL is generally considered safe, it is not without risks. Potential complications include infection, delayed epithelial healing, corneal haze, and, rarely, corneal scarring. The risk of complications is higher in the epi-off technique due to the removal of the protective epithelial layer.

Effectiveness

Studies have shown that corneal collagen cross-linking can effectively halt the progression of keratoconus and corneal ectasia in a significant number of patients. Some patients may experience an improvement in corneal shape and visual acuity, although the primary goal is to prevent further deterioration.

Future Directions

Research is ongoing to refine the procedure, reduce risks, and improve outcomes. This includes the development of new riboflavin formulations, alternative UV light sources, and techniques that can penetrate the epithelium more effectively.