CXL at the Slit Lamp: No Clinically Relevant Changes in Corneal Riboflavin Distribution During Upright UV Irradiation.

To the Editor: Photo-activated chromophore for keratitis crosslinking (PACK-CXL) 1-4 may become an attractive treatment for developing and emerging countries where access to ophthalmic care and medical equipment is limited.5 PACK-CXL treatment has the potential to be performed at the slit-lamp within 3 to 5 minutes.3 We have addressed a remaining concern: is the stromal riboflavin distribution affected by gravity if the ultraviolet irradiation takes place in the upright position? Freshly enucleated porcine eyes were incubated in 0.1% hypoosmolar riboflavin solution without dextran for 30 minutes and then divided into three groups: corneas fixed in a vertical position for 30 minutes (n = 4), corneas fixed in a vertical position for 60 minutes (n = 8), and corneas fixed in a horizontal position for 60 minutes (n = 8). Four corneas were immersed in 0.9% sodium chloride for 60 minutes and served as fluorescence controls. The fluorescence signal of each cornea was measured with a spectrophotometer. Statistical significance was determined with Student’s t tests and a confidence interval of 95%. After 30 minutes, there was no difference (P = .22) between superior and inferior riboflavin saturation, whereas a statistically significant (P = .002) gradient of 3.36% was found after 60 minutes (Figure 1). Gravitational influence on the riboflavin distribution was observed only after 60 minutes of vertical positioning. Given that a PACK-CXL treatment typically lasts for 3 to 30 minutes,2-4 this difference can be considered not clinically relevant. Limitations of this study include a small sample size and the use of porcine instead of human corneas.