Corneal collagen cross-linking in bullous keratopathy.

To the Editor: In their article, which was published in the September 2008 issue of the Journal of Refractive Surgery, Krueger et al1 detailed their use of an adapted version of Wollensak et al’s2 corneal collagen cross-linking (CXL) therapy to treat a patient with bullous keratopathy. Although the patient’s symptoms appeared much improved at 6-month follow-up, the safety of such a treatment, in which ribofl avin is injected into superfi cial and deep stromal layers of the cornea and subsequently exposed to high-level ultraviolet A (UVA) irradiance, is a cause for concern. In standard corneal collagen CXL therapy, UVA of irradiance 3 W/cm2 is applied to the de-epithelialized surface of a ribofl avin-soaked cornea for 30 minutes.2 This recommended UVA irradiance level is based on the results of multiple scientifi c experiments aimed at optimizing the safety and effi cacy of the procedure.3-6 Experimental data have shown that increasing the irradiance level above 3 mW/cm2 results in endothelial cell death3,4 and an increased loss of keratocytes throughout the cornea.5 However, the toxicity effect of two doses of 15 mW/cm2 UVA irradiance each lasting 7 minutes (one dose for each stromal pocket), as used by Krueger et al1 to treat a patient with bullous keratopathy, is currently unknown. Furthermore, as the rate of ribofl avin photodegradation has not yet been investigated, it is not clear whether a single intrastromal injection of ribofl avin prior to a 7-minute exposure to high-level UVA is suffi cient to prevent damage to the deeper ocular structures. Based on current scientifi c research, it seems likely that the treatment described in this article would cause signifi cant damage both to keratocytes and endothelial cells. In our opinion, it is dangerous to perform such a procedure until its safety has been established, and this is especially true in cases of bullous keratopathy in which the endothelial layer is already compromised. Sally Hayes, PhD Christina S. Kamma-Lorger, PhD Cardiff, United Kingdom