EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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30 EW CORNEA July 2012 SimLC for the treatment of keratoconus by Arthur Cummings, M.D. T he U.S. Food and Drug Administration trial on corneal crosslinking (CXL) for keratoconus is coming to an end, but for those of us working outside the U.S., there is little doubt of the value of CXL for keratoconus. In our experience at the Wellington Eye Clinic, Dublin, Ireland, CXL stops the progression of keratoconus in the great majority of cases and slows down the progres- sion of keratoconus in all the others. In a percentage of patients— somewhere between 30-70%—there is also a further improvement in the corneal shape over the ensuing 6 months to 3 years (personal commu- nication with colleagues). At the Wellington Eye Clinic, CXL is deemed successful if the corneal shape stays stable over the course of the following year. But while we have succeeded medically, we may have failed to improve our patients' quality of vision or eased their con- tact lens (CL) rehabilitation. As clinicians we have several adjuvant treatments in our armamentarium that we can use in conjunction with CXL to improve the corneal shape before applying the crosslinking. These include the use of intracorneal rings such as Intacs (Addition Tech- nology, Des Plaines, Ill.), the use of topography-guided laser followed by crosslinking, and the use of thermal procedures such as Keraflex (Avedro, Waltham, Mass.) followed by crosslinking. John Kanellopoulos, M.D., introduced the Athens protocol (a procedure in which the surgeon per- forms corneal crosslinking and to- pography-guided surface ablation concurrently rather than sequen- tially).1-4 We introduced the term SimLC—simultaneous laser crosslinking—4 years ago at the Wellington Eye Clinic for a particu- lar application of the Athens proto- col: one where a topography-guided procedure was done by surface ablation at the largest optical zone possible that did not ablate excessive amounts of tissue especially over the cone. Refractive error is not a con- sideration, as the treatment's sole objective is to first improve and then stabilize the new corneal shape. Further visual rehabilitation will be through CLs, spectacles, or phakic IOLs. which leads to a redistribution of corneal tension in the fibers and the improvement in the shape. So the cornea is initially weakened by 5- 10% by the laser, but then strength- ened by a factor of 300-500% by the crosslinking procedure. After the combined procedure, the patient's cornea is much stronger and more stable with a better shape that pro- vides better visual quality. At the Wellington Eye Clinic we do the topography-guided ablation with the Wavelight Allegretto 400 Eye-Q laser or the Wavelight EX500 excimer laser (Alcon, Fort Worth, Texas). Each laser uses either the placido disc Topolyser Vario or the Scheimpflug-based Oculyzer for the topography guidance. Because the EX500 also features a cyclorotational tracker, we believe it can offer better outcomes in topography-guided keratoconus cases than other lasers might, so we have begun to make the EX500 our laser of choice. For every keratoconus patient, we consider one of three procedures— Keraflex, Intacs, or SimLC—to improve the cornea before the patient undergoes CXL. We will crosslink only (without Figure 1. SimLC performed September 2008. More than 2.5 years later there was 7.9 D of flattening over the cone and 4 D of steepening over the superior flat area Source: Arthur Cummings, M.D. The concern with laser treat- ment for keratoconus is that the laser removes tissue and further weakens an already weakened cornea. Recent studies presented by Prof. John Marshall have determined the treatment only affects the sur- face, and therefore the weakening effect is limited to less than 10%.5,6 Further, the ablation profile in SimLC provides more treatment in the superior thick cornea and less treatment over the thinned cone, an adjunct procedure) when the uncorrected vision is approximately 20/40 or better. If the uncorrected vision is poor (around 20/80 or worse), and specifically when there is anisometropia between the two eyes, we treat the more myopic eye with either Intacs or Keraflex; corneal thickness and other factors will determine which treatment is preferred. However, if the patient is myopic with an uncorrected vision no better than 20/80 with sufficient corneal tissue, then SimLC becomes a very attractive option. In these patients, a minimum residual corneal thickness after SimLC would be about 340-350 microns, as this can be swollen to 380-400 microns with hypotonic riboflavin before CXL commences. Over the past 4 years, we've per- formed 25 of these SimLC cases; two are illustrated in the figures. The first case was one of the first we per- formed approximately 4 years ago and shows progressive improve- ment, validating the hypothesis that the crosslinking stabilizes the cornea even after a laser procedure. The second case illustrates a successful refractive outcome, even though we had entered plano at the time of the topography-guided PRK. Because SimLC moves the cone centrally, it increases the myopia slightly, but substantially reduces the astigmatism to a more normal range and symmetry. For surgeons, the key is ensuring patients under- stand their uncorrected vision may deteriorate but that the corrected vi- sion will improve (see Figure 1). At our clinic, during the first post-op visit we use an oblique slit on the slit lamp to assess the depth of crosslinking following SimLC. We record this as a percentage of the corneal pachymetry. So, for exam- ple, if 50% of a 450-micron cornea has been crosslinked, I'll note it's about 225 microns. We do this be- cause 6 months later the demarca- tion line is no longer visible, and it is far more difficult to know how much crosslinking was achieved. We've found this to be extremely helpful, as 2-3 years post-SimLC we know the cornea is stable and there are no further refractive or topo- graphic changes, but the residual refractive error can be very small. Having that percentage available allows us to perform small surface treatments, presuming it would in- volve less than 25% of the initial crosslinking amount. (So in that example of 225 microns being crosslinked, I would be happy to do an ablation of up to 56 microns). In refractive terms, 56-micron depth is sufficient to correct as much as –3.50 D and up to 3.50 D of astigmatism. SimLC is our procedure of choice when the corneal thickness is at least 450 microns at its thinnest point. In my hands, SimLC also works well when the uncorrected vision is worse than 20/80 and when the steepest point on the cornea is no more than 54-56 D (see Figure 2). SimLC allows surgeons to provide a big improvement in the corneal shape without removing too much tissue, thus giving the patient distinctly improved best corrected acuity and quality of vision and an improved CL fit. Post-SimLC, most patients can achieve good corrected vision with a soft CL. In our clinic the majority of cases of keratoconus are treated only with crosslinking, but about 30% continued on page 31