Corneal cross linking with the use of photosynthesizing riboflavin and UVA has demonstrated satisfactory results in stabilizing keratoconic and keratectatic corneas. A major limitation of the technique is that its application is contraindicated in corneas with stromal pachymetry less than 400 micrometers. This is due to the fact that an irradiance of 0.37 mW/cm2
has been found to be cytotoxic for the endothelial cell layer. Since the absorption coefficient for the human corneas is 70 cm-1
and the intended surface irradiance is 3.0 mW/cm2
, the 0.37 mW/cm2
irradiance is reached at 300 microns depth. In a 400 microns thick cornea saturated with riboflavin, the irradiance at the endothelial level is 0.18 mW/cm2
, which is a factor of 2 smaller than the damage threshold. Therefore, the 400 microns limit is considered the safe limit to protect the endothelium and intraocular structures from the adverse effects of UV irradiation and it has been established as a clinical standard [6
]. Unfortunately, the very patients who are in need of CXL, those with ectasia, are the same ones who have thin corneas often below the threshold of that considered to be safe for CXL treatment.
In order to avoid this limitation, we performed CXL treatment in two patients (thinnest pachymetry of 380 and 375, respectively) after pachymetric-guided corneal epithelial removal, leaving a small localized inferior area of corneal epithelium corresponding to the thinnest area over the apex of the cone. The procedure was performed uneventfully while no adverse effects were found during the nine month follow up period.
In a recent publication, Hafezi et al [5
] report a case of localized endothelial damage with transient stromal edema (0.75 mm × 0.75 mm) in the central deep cornea exactly below the corneal apex, corresponding to the area of the greatest steepness and corneal thinning. The stromal edema slowly resolved during the following 6 weeks. The authors suggest increasing stromal thickness to 400 microns by avoiding dehydrating the cornea using an alternative riboflavin solution without dextran after epithelium debridement. In the technique described in the current manuscript, we not only prevent local stromal dehydration by maintaining the epithelium and by using BSS, but we also, by the preservation of the corneal epithelium, may blocked the excessive UV irradiation in this vulnerable area, increasing the safety of the procedure.
During the CXL procedure, the epithelium is removed and riboflavin solution is placed onto the cornea until it penetrates into the anterior chamber and then UV irradiation is applied. BSS was used in the current study to maintain corneal stromal hydration (the current protocol suggests the use of hypoosolar riboflavin) in between application of riboflavin solution taking care not to over-dilute the riboflavin solution which is necessary for cross linking and protection of the intraocular structures (eg, lens). Since the cone is denuded of epithelium, it is vulnerable to excessive dehydration during the lengthy CXL treatment (total of 45 minutes). The selective preservation of epithelium in our technique may help to minimize the risk of excess dehydration and inadvertent local damage to the endothelium in the thinnest areas of the cornea.