Riboflavin and ultraviolet-A (UVA)–induced crosslinking of corneal collagen (CXL) is a method to increase the biomechanical1–4 and biochemical5,6 stability of the cornea by inducing additional crosslinks within or between collagen fibers using UVA light and the photomediator riboflavin. The therapeutic potential of CXL to treat progressive keratoconus was demonstrated in a clinical pilot study,3 as well as in controlled prospective studies and in case reports and case series of patients with iatrogenic keratectasia after refractive laser surgery.7–10 Before the first application in humans in 1999,3 the parameters of the method (riboflavin concentration, intensity and wavelength of UVA light, and duration of treatment) were established in a series of time- and dose-response assays in animal models over several years.2-4,11–14 To protect the corneal endothelium and deeper ocular structures, the currently used treatment parameters are set so the anterior 250 to 350 μm of corneal stroma are treated.15 Accordingly, the current inclusion criteria require a minimum stromal thickness (without the corneal epithelium) of 400 μm, including a safety margin.
In many cases of advanced progressive keratectasia, patients achieve a satisfactory visual acuity with contact lenses, and a low minimum stromal thickness is the only parameter prohibiting safe CXL. We present a modified technique of CXL using hypoosmolar riboflavin solution to induce stromal swelling and increase the stromal thickness before CXL in cases with preoperatively thin corneas.
Isoosmolar riboflavin 0.1% solution is generated by diluting vitamin B2-riboflavin-5-phosphate 0.5% (G. Streuli & Co. AG) with dextran T500 20% (Roth AG) (402.7 mOsmol/L). The solution is protected from light and used within 2 hours. Hypoosmolar riboflavin 0.1% solution is generated by diluting vitamin B2-riboflavin-5-phosphate 0.5% with physiological salt solution (sodium chloride 0.9% solution, B. Braun Medical AG) (310 mOsmol/L). Hypoosmolar riboflavin solution does not contain dextran T500. It is protected from light and used within 2 hours.
After 9.0 mm of the corneal epithelium is abraded, isoosmolar riboflavin 0.1% solution with dextran T500 is applied to the cornea every 3 minutes for 30 minutes. After 30 minutes, ultrasound pachymetry (5 repetitive measurements) is performed on the deepithelialized cornea at approximately the thinnest point. Hypoosmolar riboflavin is applied every 20 seconds for 5 more minutes, and the corneal thickness is checked by ultrasound pachymetry again. Hypoosmolar riboflavin solution is again administered until the minimal corneal thickness reaches 400 μm. Successful penetration of riboflavin through the cornea is ensured by visualization of riboflavin in the anterior chamber by slitlamp biomicroscopy (using blue light).
The eye is then irradiated for 30 minutes with UVA at a working distance of 5 cm with an irradiance of 3 mW/cm2, corresponding to a surface dose of 5.4 J/cm2 (UV-X, Peschke Meditrade). During treatment, isotonic riboflavin 0.1% solution and a topical anesthetic agent (oxybuprocaine 0.4%) are administered every 5 minutes to saturate the cornea with riboflavin.
After the treatment, antibiotic ointment (ofloxacine 0.3%) and a bandage contact lens soaked in a preservative-free antibiotic agent (ofloxacine 0.3%) are applied until the corneal epithelium has healed completely. Fluorometholone eyedrops are then applied twice daily for 2 weeks.
Clinical Study and Results
Since 2006, the technique has been used in 20 patients with progressive keratoconus and iatrogenic keratectasia after refractive laser surgery. The mean patient age of the 13 men and 7 women was 29.5 years (range 19 to 42 years). In all patients, the central stromal thickness after abrasion of the epithelium was at least 320 μm measured by ultrasound pachymetry. Progression of the keratectasia was verified by corneal topographies over a minimum of 6 months. Only eyes with distinct keratoconus (maximum K-reading >58 diopters [D]) and a minimum stromal thickness of 320 μm to 400 μm after removal of the epithelium were included in the treatment group. Before the study, patients were informed that if stromal thickness were less than 320 μm after the epithelium was removed, CXL would not be performed. In this circumstance, the epithelium would be allowed to heal as described below. However, in all patients in the study, the remaining stromal thickness after removal of the epithelium and before swelling with hypoosmolar riboflavin solution measured at least 320 μm.
The patients were examined preoperatively, early postoperatively (1 to 3 days until healing of the epithelium), and 1, 3, and 6 months after the treatment. Preoperatively and at every follow-up except early postoperatively, the examination included corneal topography (Keratograph C, Oculus), Scheimpflug imaging (Pentacam 70700, Oculus), and slitlamp examination of the anterior and posterior segments of the eyes.
Patients using rigid contact lenses were asked to remove them at least 3 weeks before the preoperative examination and before each follow-up examination.
Table 1 summarizes the preoperative corneal swelling behavior in the 20 patients. At 6 months, no signs of postoperative endothelial damage or changes in corneal clarity or any other side effects were seen. Scheimpflug analysis of the maximum K-readings showed no progression of the keratectasia (ΔKmax ≥ +1.0 D); stabilization of the keratectasia (+1.0 D ≥ ΔKmax+B6 ≤ −1.0 D) was observed in 12 patients and regression, in 8 patients (ΔKmax ≥ −1.0 D). The absolute increase in corneal thickness ranged from 36 μm to 110 μm; the thinnest cornea was 323 μm after the epithelium was removed and before the hypoosmolar riboflavin solution was instilled.
Figure 1 shows a thickness map generated by optical pachymetry (Pentacam image) to demonstrate the effect of hypoosmolar riboflavin solution on a moderately thinned keratoconus cornea. Figure 2 shows the intraoperative changes in corneal thickness in one case. Before corneal abrasion, the minimum corneal thickness measured by ultrasound pachymetry was 398 μm. After corneal abrasion, the minimum thickness was 323 μm, which increased slightly to 330 μm after the isotonic riboflavin solution was applied for 30 minutes. Following an additional application of hypoosmolar riboflavin solution for 10 minutes, the stromal thickness increased to 410 μm.
Control of corneal hydration is based on several factors such as active transendothelial transport and the epithelial and endothelial barrier. If one of these mechanisms is impaired, corneal swelling and edema occur.16–19 Under physiologic circumstances, the corneal stroma shows a swelling pressure of 50 to 60 mm Hg.16 Swelling of the corneal stroma can be achieved using a solution with a low colloid osmotic pressure. Such a hypoosmolar solution was used in this case series. The deepithelialized cornea can swell to double its normal thickness when irrigated with a hypoosmolar solution.19 This phenomenon is not due to an increase in the diameter of the collagen fibrils but rather to the hydrophilic capacities of the stromal proteoglycans, creating collagen-free “lakes.”17 This characteristic behavior is used to increase corneal thickness before the CXL procedure.
In Figure 1, a thickness map generated by optical pachymetry shows the effect of hypoosmolar riboflavin solution on a moderately thinned keratoconus cornea. However, in clinical practice, optical pachymetry does not provide reliable data in swollen corneas because of the increased light scattering and absorption.20 Therefore, in our cases, all measurements were carried out with an ultrasound pachymeter (SP 2000, Tomey GmbH) (Figure 2).
Clinically, the promptness of the stromal swelling response and the amount of swelling showed distinct interindividual variation (3 minutes to 20 minutes; 36 to 105 μm). In several cases, repeat application of the hypoosmolar solution followed by ultrasound pachymetry was needed to obtain a minimum stromal thickness of 400 μm. In this case series, we could not determine whether age, sex, or the underlying condition influenced the variation in promptness and/or the amount of stromal swelling.
Parameters other than the osmolarity of the riboflavin solution could be modified to obtain a more shallow depth of treatment; ie, the intensity of the UVA light, the duration of treatment, or the intensity of riboflavin concentration. However, changing these parameters would substantially modify the amount and distribution of induced radicals and, for safety reasons, would require new dose-response assays in the animal model or at least a computational model that fits experimental data.15 In contrast, the exclusive change in the osmolarity of the riboflavin solution, while maintaining the concentration at 0.1%, probably does not alter the final riboflavin concentration in the cornea.
Human keratoconus corneas show an altered ratio of dermatan sulfate/keratan sulfate proteoglycans,21 and little is known about the swelling properties of the keratoconus cornea. Although the results in our series are promising, it is not yet clear whether swollen corneas demonstrate the same behavior after CXL as nonswollen keratoconus corneas. Furthermore, clinical follow-up of more than 6 months is necessary to determine whether the high success rate in stabilization achieved in keratoconus patients with thicker corneas can be achieved in patients with swollen corneas. Finally, we cannot rule out subclinical effects on the corneal endothelium.
We perform corneal swelling before CXL in cases of progressive keratoconus and iatrogenic keratectasia when the minimum stromal thickness is less than 400 μm. Some might consider implantation of intrastromal rings as a therapeutic alternative to CXL in such cases. However, it has been demonstrated repeatedly that in advanced keratoconus, keratectasia progresses after intrastromal ring implantation.
In conclusion, preoperative swelling of the cornea safely broadens the spectrum of CXL indications to corneas that would otherwise not be eligible for treatment due to low minimum stromal thickness.
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