Current status of accelerated corneal cross-linking

Abstract

Corneal cross-linking with riboflavin is a technique to stabilize or reduce corneal ectasia, in diseases such as keratoconus and post-laser-assisted in situ keratomileusis (LASIK) ectasia. There is an interest by patient as well as clinicians to reduce the overall treatment time. Especially, the introduction of corneal cross-linking in combination with corneal laser surgery demands a shorter treatment time to assure a sufficient patient flow. The principles and techniques of accelerated corneal cross-linking is discussed.

Corneal cross-linking with riboflavin is a technique to stabilize or reduce corneal ectasia, in diseases such as keratoconus and post- laser-assisted in situ keratomileusis (LASIK) ectasia. Corneal cross-linking has been demonstrated to improve corneal strength by creating additional bonds within the corneal tissue in in vitro investigations and to stop the progression of the ectasia in clinical trials. The standard corneal cross-linking treatment uses a 30-min instillation of drops (0.1% riboflavin in 20% dextran), followed by 30 min of 365 nm ultraviolet-A (UV-A) illumination at 3 mW/cm² for 30 min (5.4 J/cm² dose). This procedure has been found to reduce corneal curvature greater than 1-1.5 D after treatment, with improvement persisting for 5 or more years.[1]

There is an interest by patient as well as clinicians to reduce the overall treatment time. Especially, the introduction of corneal cross-linking in combination with corneal laser surgery demands a shorter treatment time to assure a sufficient patient flow. Therefore, a reduction of the illumination time to a few minutes or even to several seconds has to provide sufficient efficacy.

There are attempts to shorten the illumination time by increasing the illumination intensity. Here, it is assumed that the Bunsen-Roscoe law of reciprocity is valid for the corneal cross-linking effect, having a constant radiant exposure of 5.4 J/cm². It has been shown in ex vivo experiments that the biomechanical stiffening effect of the corneal tissue is equivalent with 10 mW/cm² (illumination time 9 min) to the standard protocol.[2] However, more recently Wernli et al.,[3] determined the efficacy of corneal cross-linking for higher intensities, the change in corneal stiffness that is evoked by the cross-linking treatment of ex vivo porcine corneal tissue. As samples for the biomechanical measurements, one test strip was cut out of each cornea, which was randomly assigned to one of the groups with different treatment intensities. The intensities varied from 3-90 mW/cm², with a constant energy dose of 5.4 J/cm² and corresponding illumination times ranging from 30 to 1 min. To each of these groups, control eyes of the same date were tested, which were not irradiated with UV light, but otherwise underwent the same procedure including a soaking with riboflavin solution.

It is known from photography that the Bunsen-Roscoe law is only valid for a certain range and so far, it is not known how large this range is for corneal cross-linking. This means that the corneal cross-linking effect may be dependent on a threshold and the increase in illumination intensity (decrease in illumination time) is limited. As a fact, the data presented by Wernli et al.[3] shows the dependence of increase in corneal stiffness on illumination intensity while keeping a constant irradiation dose of 5.4 J/cm². An equivalent stiffness increase can be achieved up to an illumination intensity of approximately, 40-45 mW/cm² corresponding to illumination times of approximately 2 min. For higher intensities ranging from 50 to 90 mW/cm², no statistically significant stiffness increase could be achieved.

Conversely, Hafezi and coworkers (III Joint International Congress Refractive. On-line and SICSSO; Siena, Italy, 27-29 June 2013) reported on a more linear decrease of the corneal stiffening effect with increasing intensity leading to a reduced effect at about 18 mW/cm² and no significant stiffening increase at 30 mW/cm² and more. Such biomechanical measurements are in accordance with observations on collagen density using second harmonic imaging techniques.

The most interesting finding of this study is the failure of the Bunsen-Roscoe reciprocity law for short illumination time and high intensities. The Bunsen-Roscoe law describes the photoresponse of a material to a certain energy dose. It concludes that all photochemical reaction mechanisms depend only on the total absorbed energy and are statistically independent of the two factors that determine total absorbed energy-that is, radiant intensity or irradiance, and exposure time. A review of the validity of the Bunsen-Roscoe law in biology and medicine shows that approximately 95% and over 80%, respectively, of the evaluated reactions follow the law of reciprocity. The failure of the law observed in this study is probably due to the relative complex photochemistry that is not fully understood at this time.

On the basis of the known chemical reactions and diffusion rates of riboflavin and oxygen into the cornea, some authors[4] developed a theoretical model consistent with corneal oxygen consumption experimental results during UV-A irradiation under different conditions. Oxygen concentration in the cornea is modulated by UV-A irradiance and temperature and quickly decreased at the beginning of UV-A exposure. The time-dependence of both type-I and type-II photochemical mechanisms in corneal cross-linking with riboflavin are considered. Based on their chemical kinetics modeling approach, the authors suggested that the main photochemical kinetics mechanism is the direct interaction between riboflavin triplets and reactive groups of corneal proteins, which leads to the cross-linking of the proteins mainly through radical reactions (Type I) and not mainly driven Type II mechanism where the excited sensitizer reacts with oxygen to form singlet molecular oxygen.

Recently presented in vitro data presented by Hafezi and co-workers (III Joint International Congress Refractive On-line and SICSSO; Siena, Italy, 27-29 June 2013), however, are not in alignment with model findings of Kamaev et al.[4] Hafezi and co-workers reported about a complete failure of cross-linking, if pig corneas are placed in an atmosphere without oxygen in the air during the CXL treatment. An explanation for their findings might be that the higher intensities might be the quick consumption of oxygen within the cornea during the first seconds of the UV light illumination, but no effect from the Type I reactions. In case of higher intensity, the oxygen consumption rate is faster and new oxygen is not able to diffuse into the stroma during the treatment time as the diffusion of oxygen is a slow process and can take a couple of minutes. This, might also explain why higher intensity (>10 mW/cm²) demonstrate either a reduced or no stiffening effect.

The clinical benefits of increasing the intensity and reducing the treatment time are still unknown. Based on the in vitro results, one might expect a reduced efficacy when using short treatment times. Some authors report on the use of 40 mW/cm² for only 45 s (1.8 J/cm²) in combination with LASIK procedures to prevent patients from iatrogenic ectasia. However, such small energy doses where found to be insufficient to increase corneal stiffening in vitro.[5,6] As iatrogenic ectasias are known to be a seldom complications, a clinical trial to demonstrate efficacy of corneal cross-linking to prevent iatrogenic ectasia are somewhat not practical as they would require very large study cohorts. On the other hand, cross-linking might provide a better refractive predictability after a LASIK procedure. Kanellopolous has reported during the 8^th International Conference on Corneal Cross Linking in Geneva, Switzerland that the introduction of corneal cross linking has led to an improved refractive predictability after hyperopic LASIK. These findings are somewhat contrary to the reported influence of the corneal epithelium hyperplasia after hyperopic LASIK.[7] Until today we are still lacking a prospective clinical proof of concept for the use of corneal cross linking in combination with LASIK procedures.

In conclusion, there is good experimental evidence that higher intensities of more than 10 mW/cm² have a reduced biomechanical effect compared to the standard protocol (3 mW/cm² for 30 min). A reason for this might be the oxygen consumption and the failure of Type I photochemical reactions. As a clinical consequence, a reduction of the treatment time by simply increasing the intensity might not lead to the same efficacy of stopping the progression of corneal ectasia and might lead to higher failure rates. Application of corneal cross linking in corneal laser surgery can still be considered as experimental and is lacking clinical trials to demonstrate efficacy.