CROSS-LINKING

A new technique for facilitating the passage of riboflavin into the cornea through the epithelium in collagen cross-linking procedures appears to be effective in the treatment of keratoconus and causes less pain and allows faster recovery than the conventional epithelial debridement approach, Patrick Condon FRCS, Waterford, Ireland, told a conference of the Irish College of Ophthalmologists. The new approach to the application of riboflavin to the cornea is called the epithelial disruption technique. It involves first making numerous small perforations in the epithelium with a device designed by Sheraz Daya MD, FACP, FACS, FRCS(Ed). That allows the riboflavin to penetrate into the cornea at a very high dosage, Dr Condon said. After applying the riboflavin for 45 minutes, he and his associates check at the slit lamp to make sure that the penetration of the agent is complete, he said. They then expose the cornea to ultraviolet light with a variable aperture for up to 30 minutes and then place a bandage contact lens on the cornea which is removed after 48 hours, Dr Condon noted. “Riboflavin is a large molecule which doesn't penetrate the epithelium very well, so the traditional method of performing corneal collagen cross-linking has been to remove the epithelium completely.

To reduce the amount of pain that patients experience, Roberto Pinelli in Italy has devised a transepithelial approach using mixtures of different kinds of riboflavin to enhance the passage of riboflavin into the cornea. However, Dr Daya and I have been using an epithelial disruption technique for several years,†Dr Condon said. He presented a retrospective study with up to four years of follow-up involving 111 consecutive eyes of 72 keratoconus patients who underwent collagen cross-linking with the epithelial disruption technique. The patients included those treated by Dr Condon and his associates in Waterford and those treated by Dr Daya’s team at the Centre for Sight in London over a four-year period. He noted that there was a general improvement in uncorrected and best-corrected visual acuity and astigmatism, and there were no instances of persistent epithelial defects, infectious keratitis or sterile infiltrates. In addition, although patients reported pain, it was significantly less than that associated with removal of epithelium, Dr Condon said.

Furthermore, the mean spherical equivalent refraction improved from a preoperative value of around -4.0 D to -2.0 D at a year’s follow-up, indicating a flattening of the cornea postoperatively. Similarly, the mean refractive astigmatism improved from -4.0 D preoperatively to -3.0 D during the first postoperative year. He noted that a common side effect of the treatment is a line of anterior corneal haze, visible under the slit-lamp and by OCT imaging, which persists for about a month or two postoperatively. In this particular series it occurred in 61 per cent of eyes, he added. “In summary, we feel that the epithelial disruption technique is an effective technique for cross-linking, gives a faster recovery, less pain, less discomfort, and there is not much of a change in the mean K or steep K readings after two years. There is a tendency to improve in both the uncorrected and the best corrected visual acuity and we feel that further long-term analysis is required,†Dr Condon concluded.

DALK most widely applicable

Dr Guell noted that DALK is his preferred technique for the majority of cases of corneal refractive surgery complications that require keratoplasty. “Dissection at Descemet’s or 75 microns pre-Descemet’s is my procedure of choice when keratoplasty is indicated and the endothelium is healthy, because we preserve the endothelium and visual quality and acuity are the same as with penetrating keratoplasty [PKP]. The endothelial cell loss is also much less with DALK as is the number of rejection episodes,†he said. The main contraindications to DALK are eyes where the health of the endothelium is in doubt and eyes with very thin, previously perforated corneas. The most common complications after DALK are perioperative perforation – with reported incidences ranging from zero to 17 per cent – and the formation of a double anterior chamber between the Descemet’s membrane and the endothelium after the surgery.

“Sometimes you're not aware of the perforation until the part Descemet’s dissection is being performed, but we all need to be aware that in very thin postoperative corneas the rate of perforations is higher. The result is that sometimes we will have a double anterior chamber, although most of the cases are solved with time and/or the injection of air/gas in the anterior chamber. He added that injecting viscoelastic material directly rather than air beneath the stroma when performing the dissection with the “big-bubble†technique can reduce the chance of perforation and anterior chamber perforation because it is a slower and more easily controlled process. It also provides superior visualisation of the process, he said. Dr Guell recommended restricting the use of PKP to those refractive surgery patients whose complications damage the stroma and also damage or obscure the endothelium to a point where its status cannot be confirmed.

Finally, Dr Guell noted that endothelial complications of refractive surgery have become more common in recent years because of problems with some models of anterior chamber phakic IOLs (angle supported). In such cases he said that he prefers DMEK rather than DSAEK in order to provide optimum visual outcome. In patients older than 55 years, simultaneous refractive lens exchange can enhance patient satisfaction in such cases, he added.