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Exploring promising treatment modifications for cross-linking thin corneas
Corneal collagen cross-linking, or CXL, was first introduced in 1998 and has now become a valuable treatment to prevent progression of keratoconus and keratectasia. CXL is applicable in corneas that are thicker than 400 µm after epithelial debridement. If the cornea is thinner than 400 µm, there may be corneal endothelium damage from the UVA radiation. In cases of adequate corneal thickness, the clinical outcomes of CXL are promising. However, patients with keratoconus and keratectasia often present with corneal thickness less than 400 µm. New treatment protocols have been tried with good, promising results, and the cover story in this issue discusses in detail the various treatment modifications for eyes with thin corneas.
In early studies, transepithelial CXL and pachymetry-guided epithelial debridement were tried. Transepithelial CXL was performed without removing the epithelial layer. This was made possible by adding ethylenediaminetetraacetic acid sodium salt to riboflavin. Such a mixture could break intercellular bonds and facilitate penetration of riboflavin through an intact epithelium. The results, however, were controversial, and most surgeons believe that the presence of the epithelium can impair the effects of CXL. Pachymetric-guided CXL also failed to show the desired effect.
Another described approach was the use of hypo-osmolar riboflavin solution. This method used a hypo-osmolar riboflavin solution to swell the cornea to meet the prerequisite criterion of a corneal thickness greater than 400 µm. Promising results were reported in corneas no less than 325 µm without the epithelium and prior to swell. In thinner corneas, distinct interindividual variations were observed in the treatment responses. Although endothelial toxicity from the hypo-osmolar solution was a concern, this procedure has found favor among cornea specialists.
Contact lens-assisted CXL, a technique that increases corneal thickness artificially, was recently published. A total thickness greater than 400 µm was achieved with a soft contact lens. The technique was described to be effective and safe for thin corneas. The barrier effect of the lens, which may cause an energy loss, remains a concern, and long-term follow-up is needed to study the efficacy of this promising approach.
Epithelial-island CXL was modified from transepithelial CXL. This method retains a small epithelial island on the thinnest part of the cornea to increase the thickness and avoid damage while the surrounding epithelium was removed. By doing this, it allowed a better effect in the paracentral area and the thinnest point was protected. Long-term clinical outcomes are required to better assess the safety and efficacy of this approach.
Modifications in treatment parameters, such as decreasing the UVA exposure and/or increasing the UVA fluence, are other options that have been suggested.
While there are many new and exciting modalities that build on existing CXL approaches to try to treat thin corneas, many require further follow-up to establish safety and efficacy. It is also important, therefore, to try to detect corneal ectasia at an early stage, when standard protocols with established outcomes can be used.
Disclosure: The authors report no relevant financial disclosures.