Transepithelial corneal collagen cross-linking in ultrathin keratoconic corneas

RiTe conjugate mediated corneal collagen crosslinking, a novel therapeutic intervention for keratoconus - in vitro and in vivo study

Corneal collagen crosslinking (CXL) is an effective method to halt the disease progression of keratoconus, a progressive corneal dystrophy leading to cone shaped cornea. Despite the efficacy of standard protocol, the concerning step of this procedure is epithelial debridement performed to facilitate the entry of riboflavin drug. Riboflavin, a key molecule in CXL protocol, is a sparsely permeable hydrophilic drug in corneal tissues. The present study has employed cell penetrating peptide (CPP), Tat2, to enhance the penetration of riboflavin molecule, and thereby improve currently followed CXL protocol. This study demonstrates approximately two-fold enhanced uptake of CPP riboflavin conjugate, Tat2riboflavin-5'Phosphate (RiTe conjugate), both in vitro and in vivo. Two different CXL protocols (Epi ON and Epi OFF) have been introduced and implemented in rabbit corneas using RiTe conjugate in the present study. The standard and RiTe conjugate mediated CXL procedures exhibited an equivalent extent of crosslinking in both the methods. Reduced keratocyte loss and no endothelial damage in RiTe conjugate mediated CXL further ascertains the safety of the proposed CXL protocols. Therefore, RiTe conjugate mediated CXL protocols present as potential alternatives to the standard keratoconus treatment in providing equally effective, less invasive and patient compliant treatment modality.

Evaluation of the results of contact lens assisted corneal cross-linking treatment in keratoconus patients with thin corneas

PURPOSE

We aimed to compare the efficacy and safety of accelerated contact lens-assisted cross-linking (CA-CXL) with Lotrafilcon B and Comfilcon A lenses in keratoconus (KC) patients with thin corneas.

STUDY DESIGN

Retrospective, single-center study.

MATERIALS AND METHODS

We retrospectively included 51 eyes of 39 KC patients with corneal thickness <400µm after epithelial scraping (Epi-off), who underwent accelerated CA-CXL treatment with Lotrafilcon B (n=20) and Comfilcon A (n=31). Uncorrected and corrected distance visual acuity (UDVA and CDVA), manifest refraction values, corneal topographic data and endothelial cell density were recorded at preoperative and postoperative 1st, 3rd and 6th month controls.

RESULTS

CDVA in the Comfilcon A group was higher than CDVA before surgery at 6 months postoperatively (p<0.001). When the two lenses were compared, CDVA was found to be significantly higher in the Lotrafilcon B group in the preoperative, postoperative 1st month and 3rd month values, but there was no significant difference between the postoperative 6th month values (p=0.028, p=0.018, p=0.044, p=0.181, respectively). The maximum keratometry (Kmax) value at the 6th month after surgery in the Comfilcon A group was significantly lower than in the Lotrafilcon B group (p=0,009). There was no significant difference between the endothelial cell density values between the groups (p=0.623, p=0.609, p=0.794, p=0.458, respectively). There was no significant difference between the progression, regression, and stability rates of the two groups (p=0.714).

CONCLUSIONS

Accelerated CA-CXL with Lotrafilcon B and Comfilcon A silicone hydrogel lenses is a safe and effective method to stop progression in patients with thin corneas.

Long-term outcomes of tailored stromal expansion with refractive lenticule for crosslinking thin corneas

PURPOSE

To assess the long-term safety and stability of visual outcomes following the modified technique of collagen crosslinking (CXL) using refractive lenticule in eyes with thin corneas (<400 µm) and progressive keratoconus.

SETTING

A tertiary eye care hospital in India.

DESIGN

Prospective, interventional case series.

METHODS

Eyes with progressive keratoconus and thin corneas (<400 µm) underwent CXL with intraoperative stromal augmentation using a refractive lenticule obtained from small-incision lenticule extraction (SMILE). Preoperative and postoperative evaluation (3 months, and then yearly thereafter) included corneal tomography (Oculus Pentacam), uncorrected and corrected distance visual acuity (UDVA and CDVA, respectively), manifest refraction, and endothelial cell count (specular microscopy), and adverse events, if any, were noted. The patients were followed up for a period of 5 years.

RESULTS

Seven eyes were included in the analysis. Mean corneal flattening of -4.29 D was noted from preoperative maximum keratometry (P = 0.018). An improvement in UDVA and CDVA of 0.38 logarithm of minimum angle of resolution (logMAR) and 0.36 logMAR, respectively, was noted at 5 years postoperative visit. Four eyes demonstrated a gain of two lines in CDVA. Mean spherical equivalent improved from -6.85 D preoperatively to -6.05 D at 5 years postoperatively. Clear demarcation line was noted between 230 to 270 µm on anterior segment optical coherence topography. No significant endothelial cell loss was noted postoperatively.

CONCLUSION

Long-term outcomes demonstrated safety and disease stability following lenticule-assisted CXL.

Customised Peripheral Corneal Cross-linking (P-CXL) for Ultra-thin Corneas with Stage III and IV Keratoconus

PURPOSE

This study aims to determine whether customised peripheral corneal cross-linking (P-CXL) can halt keratoconus progression in ultrathin corneas with stage 3 and 4 keratoconus, with thinnest pachymetry well below 400 μm and therefore excluded from most treatment protocols.

METHODS

This retrospective study included 21 eyes with progressive keratoconus and thinnest pachymetry ranging from 97 to 399 μm (mean 315 μm), who underwent P-CXL between 2007 and 2020. The procedure involved preoperative NSAIDs therapy, tomography-guided customized epithelial debridement, the use of both hypo-osmolar and iso-osmolar riboflavin solutions, and 9.0 mW/cm2 UV-A irradiation for 10 minutes. The outcome measures were best spectacle-corrected visual acuity (BSCVA), mean keratometry, maximum keratometry, and thinnest pachymetry.

RESULTS

After a minimum follow-up period of 12 months, P-CXL stabilized or improved mean keratometry and maximum keratometry in 85.7% of eyes (Kavg from 57.48 ± 9.38 to 56.43 ± 8.96 D, p < 0.001; Kmax from 72.77 ± 12.74 to 70.00 ± 11.50 D, p < 0.001), BSCVA in 90.5% of eyes (from 4.48 ± 2.85 to 5.72 ± 3.34 decimals, p < 0.001), and thinnest pachymetry in 81% of eyes (from 315.81 ± 90.05 to 342.33 ± 74.22 μm, p = 0.08). No adverse events and no loss of endothelial cell density occurred.

CONCLUSIONS

Customised peripheral corneal cross-linking (P-CXL) treated very severe keratoconus with a success rate of 85.7% and improved visual acuity and tomographic indicators in most cases. While a longer follow-up and a larger sample would help to support such conclusions to a greater extent, these results allow to broaden the treatment spectrum for patients with stage 3 and 4 keratoconus and contact lens tolerance.

Recent advances in medicinal compounds related to corneal crosslinking

Corneal crosslinking (CXL) is the recognized technique to strengthen corneal collagen fibers through photodynamic reaction, aiming to halt progressive and irregular changes in corneal shape. CXL has greatly changed the treatment for keratoconus (KCN) since it was introduced in the late 1990's. Numerous improvements of CXL have been made during its developing course of more than 20 years. CXL involves quite a lot of materials, including crosslinking agents, enhancers, and supplements. A general summary of existing common crosslinking agents, enhancers, and supplements helps give a more comprehensive picture of CXL. Either innovative use of existing materials or research and development of new materials will further improve the safety, effectiveness, stability, and general applicability of CXL, and finally benefit the patients.

Efficacy, Safety, and Outcomes following Accelerated and Iontophoresis Corneal Crosslinking in Progressive Keratoconus

PURPOSE

To investigate the outcomes of accelerated (A-CXL) and iontophoresis (I-CXL) corneal crosslinking in a large retrospective cohort with progressive keratoconus.

METHODS

This retrospective observational cohort study included consecutive patients treated by A-CXL (9 mW/5.4 J/cm²) or I-CXL with a minimal follow-up of 12 months. Visual acuity, manifest refraction, topography, specular microscopy, and corneal optical coherence tomography (OCT) were evaluated at baseline and at the last visit. Progression was defined as an increase in the maximum topographic keratometry (Kmax) of 1D.

RESULTS

302 eyes of 241 patients with a mean age of 25.2 ± 7.5 years were included from 2012 to 2019: 231 and 71 eyes in the A-CXL and I-CXL groups, respectively. The mean follow-up was 27.2 ± 13.2 months (maximum: 85.7 months). Preoperatively, the mean Kmax was 51.8 ± 4.0D, with no differences between groups. Mean topographic measurements and spherical equivalent remained stable during the follow-up. At the last visit, CXL failure was reported in 60 eyes (19.9%): 40 (14.7%) versus 20 (28.2%) in A-CXL versus I-CXL, respectively, p = 0.005. The likelihood of progression after CXL was significantly higher following I-CXL: RR = 1.62, CI95 = [1.02 to 2.59], p = 0.04. Demarcation line presence at 1 month was positively correlated with higher efficacy of CXL, p = 0.03. No endothelial damage was reported, especially in 51 thin corneas (range = 342-399 µm).

CONCLUSIONS

A-CXL seems more effective than I-CXL in stabilizing keratoconus; this is to be taken into account when a therapeutic indication is posed according to the aggressiveness of the keratoconus.

Norepinephrine as an Enhancer Promoting Corneal Penetration of Riboflavin for Transepithelial Corneal Crosslinking

Purpose

Previously, we found norepinephrine (NE) could affect the corneal epithelial integrity, herein we investigated the feasibility and safety of NE serving as a chemical enhancer to promote corneal penetration of riboflavin during transepithelial corneal crosslinking (CXL).

Methods

The dosage of NE that could promote riboflavin diffusion through the healthy epithelial barrier without inducing epithelial damage in C57BL/6 mice was determined. The safety of NE treatment was confirmed by morphological and histological examinations of the whole cornea. The efficacy of NE in promoting riboflavin penetration was verified by slit lamp and scanning electron microscope (SEM), and corneal biomechanical measurement after CXL. To better fit the clinical scenario, increased NE dosage and shortened riboflavin infiltration time were further evaluated.

Results

The lowest dosage of NE (1 mg/mL) that facilitated transepithelial riboflavin permeation was 2 µL. No visible corneal structure alteration was observed after NE treatment. SEM indicated dissociation of intercellular junctions among corneal epithelial cells. Homogenous distribution of riboflavin throughout corneal stroma was observed. NE-treated corneas reached comparable biomechanical properties after CXL, including stress-relaxation curve and elastic modulus, with corneas treated with the commercially available transepithelial drug Peschke TE. To better fit the clinical scenario, increasing NE up to 5.5 µL helped riboflavin infiltrate the corneal stroma within 30 minutes. After CXL with 9 mW/cm2 ultraviolet-A (UVA) for 2.5 minutes, the cornea showed significantly enhanced corneal biomechanical properties with undisturbed corneal endothelium.

Conclusions

NE serves as an effective enhancer in increasing riboflavin diffusion with limited impairment on corneal epithelium and has great potential for clinical application.

Translation Relevance

NE serves as an effective enhancer for riboflavin penetration and clinical transepithelial CXL.

Comparison of safety and efficacy of silicone hydrogel contact Lens-assisted CXL and accelerated CXL in keratoconus patients with thin corneas

PURPOSE

To evaluate the efficacy and safety of silicone hydrogel contact lens-assisted corneal cross-linking (CL-CXL) and compare 12-month visual and topographic outcomes with accelerated CXL using hypo-osmolar riboflavin (A-CXL) in keratoconus patients with thin corneas (below 400 μm with epithelium).

METHODS

This retrospective study included 27 eyes of 27 keratoconus patients who underwent CL-CXL (n = 14) or A-CXL (n = 13). Uncorrected and corrected distance visual acuity (UDVA and CDVA) and data obtained from corneal topography were analyzed at baseline and again at 6- and 12-month follow-ups. Corneal demarcation line depth (DLD) was measured at one month, and changes in the corneal endothelial cell density (ECD) at 12 months were also assessed.

RESULTS

Mean UDVA improved significantly in both groups at 12 months (all p < 0.05). Maximum keratometry (K-max) decreased by 1.04 ± 1.90 D in the CL-CXL group and by 0.87 ± 1.89 D in the A-CXL group at 12 months, which was not statistically significant (all p>0.05). Total corneal higher-order aberrations (HOAs) analysis showed a significant improvement in only the CL-CXL group at 12 months (p = 0.041). Average DLD was 227.18 ± 65.60 μm in the CL-CXL group and 245.30 ± 66.84 μm in the A-CXL group (p = 0.275). No significant change in ECD was found in either group (all p>0.05). Mean changes in UDVA, CDVA, K-max, K-mean, HOAs, and ECD were not statistically significant between the groups (all p>0.05).

CONCLUSIONS

Silicone hydrogel CL-assisted CXL seems as effective as A-CXL in halting keratoconus progression in thin corneas with no side effects during the one-year follow-up period.

Comparison of efficacy and safety of accelerated trans-epithelial crosslinking for keratoconus patients with corneas thicker and thinner than 380µm

PURPOSE

Accelerated trans-epithelial cross-linking (ATE-CXL), a therapy to halt keratoconus progression, has the merit of widening the indications for thinner corneas (<380 μm). Since a hypotonic solution affects the swollen cornea, corneas of <380 μm thickness at preoperative measurement can be an indication for ATE-CXL. The aim of this retrospective study was to compare the efficacy and safety of ATE-CXL for keratoconus between corneas with thicknesses <380 μm and ≥380 μm.

MATERIALS AND METHODS

Thirty-four eyes of 27 patients who underwent ATE-CXL (30 mW/cm²; 3 minutes) with completion of a 24-month follow-up, were enrolled and divided into two groups: Group 1, thinnest corneal thickness (TCT), <380 μm (n = 10) and Group 2, TCT, ≥380 μm (n = 24). A hypotonic solution was administered to Group 1 until the corneal thickness increased by >380 μm before UV-A irradiation. We measured uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), maximum and average keratometric values (Kmax and AveK), central corneal thickness (CCT), TCT by anterior segment optical coherence tomography, and corneal endothelial cell density (ECD) using specular microscopy. The changes from baseline to 24 months postoperatively between the two groups were compared accordingly.

RESULTS

The changes in Kmax and AveK from baseline to 24 months in Group 1 (ΔKmax: -7.8±7.7 D, ΔAveK: -4.3±6.1 D) showed significant decreases compared to those in Group 2 (ΔKmax: 0.2±3.0 D, ΔAveK: 0.6±2.7 D) (p = 0.004 and p = 0.001), and there were no significant changes from baseline to 24 months postoperatively in UCVA, BCVA, CCT, TCT, and ECD in both groups.

CONCLUSION

ATE-CXL is effective and safe for keratoconic corneas in both groups. The effect of reducing keratometric values was greater in the group with thinner corneas.

Long-term visual, refractive, tomographic and aberrometric outcomes of corneal collagen crosslinking (CXL) with or without hypoosmolar riboflavin solution in the treatment of progressive keratoconus patients with thin corneas

PURPOSE

To evaluate the long-term visual, refractive, tomographic, and aberrometric outcomes of corneal collagen crosslinking (CXL) with or without hypoosmolar riboflavin solution in the treatment of progressive keratoconus patients with thin corneas.

METHODS

Charts of consecutive progressive keratoconus patients with thinnest corneal thickness less than 470 µm who underwent corneal collagen CXL with or without hypoosmolar riboflavin solution and using a standardized protocol for treatment and examinations were analyzed retrospectively. The indication for hypoosmolar riboflavin use was a central corneal thickness less than 400 µm as measured by ultrasound pachymetry after epithelial debridement and before exposure to ultraviolet A (UVA) light. Uncorrected distance visual acuity (UDVA), best spectacle-corrected distance visual acuity (CDVA), manifest refraction, slit lamp biomicroscopy, corneal tomography, corneal aberrometry, and endothelial cell counts were evaluated at baseline and yearly at all postoperative follow-up examinations until month 36. The outcomes of corneal CXL procedure performed using hypoosmolar riboflavin were compared to those performed using the standard procedure.

RESULTS

Twenty-three eyes (19 patients) were treated using hypoosmolar riboflavin application, and 30 eyes (28 patients) were treated using the standard procedure. Compared to baseline, the mean UDVA, CDVA, and keratometric readings improved statistically significantly in both groups at postoperative year 3, without any statistically significant between-group differences. Progression was not observed in any patient eye in either group. No significant endothelial cell loss and no sight threating complication were observed in any patient eye.

CONCLUSION

At 3 years follow-up, the safety and efficacy of CXL using hypoosmolar riboflavin solution seems to be similar to that of standard CXL in progressive keratoconic eyes with thin corneas. The visual, refractive, keratometric, tomographic and aberrometric outcomes of the two procedures were comparable, as well.

Outcomes of customized topographic guided epithelial debridement for corneal collagen cross-linking

PURPOSE

To evaluate the outcomes of corneal collagen cross-linking performed with customized epithelial debridement technique in progressive keratoconic corneas.

MATERIALS AND METHODS

Forty eyes of 40 patients were included in the study. We performed an ophthalmologic examination and recorded the uncorrected visual acuity, best corrected visual acuity (BCVA), central corneal thickness at the thinnest point (t-CCT), flat meridian keratometry (K1), steep meridian keratometry (K2), endothelial cell density (ECC), hexagonal cells (HEX), and coefficient of variation of cell areas (CV) measured preoperatively and at the 1st, 3rd, 6th and 12th months postoperatively.

RESULTS

The mean UCVA and BCVA were increased (p < 0.05). The mean flattest and steepest K readings were decreased (p < 0.05). The mean t-CCT decreased in the first months after treatment and increased after 6 months. The mean t-CCT was thicker at 12 months compared to pretreatment status (p < 0.05). The mean ECC and CV were not significantly different between follow-up intervals (p > 0.05). The mean HEX was statistically increased (p > 0.05).

CONCLUSIONS

Corneal collagen cross-linking performed with customized epithelial debridement technique is a successful alternative method for stopping the progression of keratoconus after 12 months of treatment.

Efficacy and Safety of Contact Lens-Assisted Corneal Crosslinking in the Treatment of Keratoconus With Thin Corneas

OBJECTIVE

To evaluate the safety and efficacy of contact lens-assisted crosslinking (CA-CXL) in progressive keratoconus with thin corneas (350-400 µm).

METHODS

Forty eyes (30 patients) underwent epithelium-off CA-CXL with iso-osmolar riboflavin and ultraviolet-A irradiation for 30 min (fluence, 3 mW/cm2). A non-ultra-violet-absorbing soft contact lens (sCL) soaked in riboflavin was applied over the cornea during irradiation. Demarcation line (DL) depth was measured 1 month postoperatively. Endothelial cell count (ECC) was measured preoperatively and 3 months postoperatively. Distance uncorrected visual acuity (UDVA) and best spectacle-corrected visual acuity (BDVA), and maximum keratometric values (K max) were evaluated preoperatively and 9 months postoperatively.

RESULTS

The sCL added a mean thickness of 100.05±1.23 µm. The mean ECC was 2,982±165 cell/mm2 preoperatively and 2,955±125 cell/mm2 postoperatively (endothelial cell loss, 0.9%, P=0.21). The mean DL depth was 204.8±20.2 µm. There was significant improvement in UDVA (P<0.001) and BDVA (P=0.011) with a stable K max (P=0.06).

CONCLUSION

Contact lens-assisted crosslinking was safe and effective, with endothelial loss less than 1% after 3 months and stable corneal tomography over 9 months, with improved visual acuity. Demarcation line depth was found to be shallower than conventional CXL. Therefore, the long-term efficacy of CA-CXL requires further study.

CLINICAL TRIAL REGISTRY NUMBER

NCT04504578 Clinical trials.gov.

Comparison of Contact Lens Assisted and Transepithelial Corneal Cross Linking with Standard 'Epithelium Off' Cross Linking for Progressive Keratoconus: 24 Month Clinical Results

PURPOSE

To compare outcomes of contact lens assisted corneal cross linking (CACXL) and transepithelial corneal cross linking (TECXL) with standard 'epithelium off'' ('epi -off') cross linking (CXL) for progressive keratoconus.

SETTING

Advanced Eye Centre, Post Graduate Institute of Medical Education and Research, Chandigarh , India.

DESIGN

Retrospective, comparative study.

METHODS

Patients with progressive keratoconus undergoing CXL with a minimum follow up of 24 months were included. CACXL and TECXL was performed in patients with 'epithelium on' minimal pachymetry between 350 µm to 450 µm. Main outcome measures included change in maximum keratometry (Kmax), corrected distance visual acuity (CDVA), and efficacy in halting progression (increase in Kmax ≥ 1 diopter [D]).

RESULTS

Standard 'epi -off' CXL, CACXL and TECXL was performed in 34,14 and 10 eyes respectively. Baseline Kmax and CDVA were comparable for all groupsKmax reduced significantly by -2.83 ± 3.35 D, -3.18 ± 2.74D and -2.02 ± 1.66 D in standard 'epi-off' CXL (p<0.01), CACXL (p=0.001) and TECXL (p=0.004) groups respectively; the reduction was comparable for all groups (p=0.63). CDVA improved by -0.14 ± 0.24 , -0.04 ± 0.19 and -0.12 ± 0.17 logMAR units in the standard 'epi -off' CXL (p =0.006), CACXL (p=0.42) and TECXL (p=0.05) groups respectively; the reduction was comparable for all groups (p=0.46). Progression was documented in 2 eyes (6%) of the standard 'epi -off' CXL group and 0% eyes of the CACXL and TECXL groups (p=0.61).

CONCLUSIONS

CACXL and TECXL were comparable to standard 'epi -off' CXL for progressive keratoconus.

Crosslinking und Keratokonus

Ein Keratokonus führt zu einer progressiven Vorwölbung und Verdünnung der Hornhaut. Um dies aufzuhalten, kann ein Crosslinking durchgeführt werden. Dabei ist eine Behandlung nach dem „Dresdener Protokoll“ eine effektive und sichere Behandlungsmöglichkeit, aber auch zahlreiche neue Anwendungsprotokolle (akzeleriertes Crosslinking, transepitheliales Crosslinking) und sogar weitere Indikationen (refraktive Eingriffe, infektiöse Keratitis) wurden in den letzten Jahren veröffentlicht.

[Crosslinking and Keratoconus]

Keratoconus leads to a progressive protrusion and thinning of the cornea. In order to stop this, corneal crosslinking can be performed if the progression of the disease is proven. Crosslinking according to the "Dresden protocol" includes abrasion of the corneal epithelium, application of riboflavin eye drops and irradiation with UV-A light of an intensity of 3 mW/cm² for 30 minutes. The efficacy has been shown in several prospective randomized studies. One of the more recent developments is accelerated crosslinking, which allows a shorter irradiation time. On the other hand, the possibility of transepithelial crosslinking was presented, which does not require an abrasion of the cornea. This should reduce the occurrence of postoperative pain. The range of indications has also been expanded. Corneal crosslinking is used for post-LASIK keratectasia as well. It is also being considered for use in infectious keratitis. Topographically controlled crosslinking can likewise be used to try to positively influence the refractive power of the cornea. The risks of crosslinking include the occurrence of pain, haze or scarring, endothelial cell damage and, rarely, the occurrence of keratitis.

Transepithelial versus epithelium-off corneal crosslinking for progressive keratoconus

BACKGROUND

Keratoconus is the most common corneal dystrophy. It can cause loss of uncorrected and best-corrected visual acuity through ectasia (thinning) of the central or paracentral cornea, irregular corneal scarring, or corneal perforation. Disease onset usually occurs in the second to fourth decade of life, periods of peak educational attainment or career development. The condition is lifelong and sight-threatening. Corneal collagen crosslinking (CXL) using ultraviolet A (UVA) light applied to the cornea is the only treatment that has been shown to slow progression of disease. The original, more widely known technique involves application of UVA light to de-epithelialized cornea, to which a photosensitizer (riboflavin) is added topically throughout the irradiation process. Transepithelial CXL is a recently advocated alternative to the standard CXL procedure, in that the epithelium is kept intact during CXL. Retention of the epithelium offers the putative advantages of faster healing, less patient discomfort, faster visual rehabilitation, and less risk of corneal haze.

OBJECTIVES

To assess the short- and long-term effectiveness and safety of transepithelial CXL compared with epithelium-off CXL for progressive keratoconus.

SEARCH METHODS

To identify potentially eligible studies, we searched the Cochrane Central Register of Controlled Trials (CENTRAL) (which contains the Cochrane Eyes and Vision Trials Register) (2020, Issue 1); Ovid MEDLINE; Embase.com; PubMed; Latin American and Caribbean Health Sciences Literature database (LILACS); ClinicalTrials.gov; and World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP). We did not impose any date or language restrictions. We last searched the electronic databases on 15 January 2020.

SELECTION CRITERIA

We included randomized controlled trials (RCTs) in which transepithelial CXL had been compared with epithelium-off CXL in participants with progressive keratoconus.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methodology.

MAIN RESULTS

We included 13 studies with 723 eyes of 578 participants enrolled; 13 to 119 participants were enrolled per study. Seven studies were conducted in Europe, three in the Middle East, and one each in India, Russia, and Turkey. Seven studies were parallel-group RCTs, one study was an RCT with a paired-eyes design, and five studies were RCTs in which both eyes of some or all participants were assigned to the same intervention. Eleven studies compared transepithelial CXL with epithelium-off CXL in participants with progressive keratoconus. There was no evidence of an important difference between intervention groups in maximum keratometry (denoted 'maximum K' or 'Kmax'; also known as steepest keratometry measurement) at 12 months or later (mean difference (MD) 0.99 diopters (D), 95% CI -0.11 to 2.09; 5 studies; 177 eyes; I² = 41%; very low certainty evidence). Few studies described other outcomes of interest. The evidence is very uncertain that epithelium-off CXL may have a small (data from two studies were not pooled due to considerable heterogeneity (I² = 92%)) or no effect on stabilization of progressive keratoconus compared with transepithelial CXL; comparison of the estimated proportions of eyes with decreases or increases of 2 or more diopters in maximum K at 12 months from one study with 61 eyes was RR 0.32 (95% CI 0.09 to 1.12) and RR (non-event) 0.86 (95% CI 0.74 to 1.00), respectively (very low certainty). We did not estimate an overall effect on corrected-distance visual acuity (CDVA) because substantial heterogeneity was detected (I² = 70%). No study evaluated CDVA gain or loss of 10 or more letters on a logarithm of the minimum angle of resolution (logMAR) chart. Transepithelial CXL may result in little to no difference in CDVA at 12 months or beyond. Four studies reported that either no adverse events or no serious adverse events had been observed. Another study noted no change in endothelial cell count after either procedure. Moderate certainty evidence from 4 studies (221 eyes) found that epithelium-off CXL resulted in a slight increase in corneal haze or scarring when compared to transepithelial CXL (RR (non-event) 1.07, 95% CI 1.01 to 1.14). Three studies, one of which had three arms, compared outcomes among participants assigned to transepithelial CXL using iontophoresis versus those assigned to epithelium-off CXL. No conclusive evidence was found for either keratometry or visual acuity outcomes at 12 months or later after surgery. Low certainty evidence suggests that transepithelial CXL using iontophoresis results in no difference in logMAR CDVA (MD 0.00 letter, 95% CI -0.04 to 0.04; 2 studies; 51 eyes). Only one study examined gain or loss of 10 or more logMAR letters. In terms of adverse events, one case of subepithelial infiltrate was reported after transepithelial CXL with iontophoresis, whereas two cases of faint corneal scars and four cases of permanent haze were observed after epithelium-off CXL. Vogt's striae were found in one eye after each intervention. The certainty of the evidence was low or very low for the outcomes in this comparison due to imprecision of estimates for all outcomes and risk of bias in the studies from which data have been reported.

AUTHORS' CONCLUSIONS

Because of lack of precision, frequent indeterminate risk of bias due to inadequate reporting, and inconsistency in outcomes measured and reported among studies in this systematic review, it remains unknown whether transepithelial CXL, or any other approach, may confer an advantage over epithelium-off CXL for patients with progressive keratoconus with respect to further progression of keratoconus, visual acuity outcomes, and patient-reported outcomes (PROs). Arrest of the progression of keratoconus should be the primary outcome of interest in future trials of CXL, particularly when comparing the effectiveness of different approaches to CXL. Furthermore, methods of assessing and defining progressive keratoconus should be standardized. Trials with longer follow-up are required in order to assure that outcomes are measured after corneal wound-healing and stabilization of keratoconus. In addition, perioperative, intraoperative, and postoperative care should be standardized to permit meaningful comparisons of CXL methods. Methods to increase penetration of riboflavin through intact epithelium as well as delivery of increased dose of UVA may be needed to improve outcomes. PROs should be measured and reported. The visual significance of adverse outcomes, such as corneal haze, should be assessed and correlated with other outcomes, including PROs.

Compaction of very thin corneas from ultraviolet A riboflavin-vitamin E transepithelial cross-linking

The purpose of the study was to determine the decrease in pachymetry of very thin corneas with advanced keratoconus due to corneal compaction from the ultraviolet-A (UV-A) irradiation phase of transepithelial (epi-on) cross-linking. Twenty removed corneal buttons were obtained from patients who underwent penetrating keratoplasty for advanced keratoconus. Removed corneal buttons selected from among the post-surgical specimens for this study had intact epithelium, no scarring or surgical cautery, endothelial cell density >2500 cells/mm², and average pachymetry over the measured points of below 400 μm. Corneas were mounted in a Franz chamber. Each epithelial surface was soaked in isotonic riboflavin and D-alpha-tocopheryl polyethylene glycol 1000 succinate (Ribocross® IROMED Group, Italy) for 15 min. Pachymetry was measured at three points over both the shielded and unshielded corneal halves for each corneal button. Surfaces were then washed in saline to remove the Ribocross®. Shields from UV-A irradiation over half of each cornea were then fixed to stand 5 mm above the test corneas. UV-A irradiation using the custom fast cross-linking (CF-CXL) protocol was then performed for the typical 10 ± 1.5 min, for a total energy of 1.08 ± 0.6 J/cm² after which pachymetry was re-measured. The average percent change in pachymetry was -0.43% ± 0.38% (maximum -1.06%) in the shielded half. Pachymetry change was -6.2% ± 2.2% (maximum 12%) in the cross-linked halves. In conclusion, we estimate that the change in corneal thickness from corneal compaction due to the cross-linking reaction itself was -5.8% ± 2.2%. Scanning electron microscopy of cross-linked corneal segments showed stromal fiber contraction.

Keratoconus: cross-linking the window of the eye

Keratoconus is a condition in which the cornea progressively thins and weakens, leading to severe, irregular astigmatism and a significant reduction in quality of life. Although the precise cause of keratoconus is still not known, biochemical and structural studies indicate that overactive enzymes within the cornea break down the constituent proteins (collagen and proteoglycans) and cause the tissue to weaken. As the disease develops, collagen fibres slip past each other and are redistributed across the cornea, causing it to change shape. In recent years, it was discovered that the photochemical induction of cross-links within the corneal extracellular matrix, through the use of riboflavin and ultraviolet (UVA) light, could increase the strength and enzymatic resistance of the tissue and thereby halt keratoconus progression. Worldwide acceptance and use of riboflavin/UVA corneal cross-linking therapy for halting keratoconus progression has increased rapidly, in accordance with the growing body of evidence supporting its long-term effectiveness. This review focusses on the inception of riboflavin/UVA corneal cross-linking therapy for keratoconus, its clinical effectiveness and the latest scientific advances aimed at reducing patient treatment time, improving patient comfort and increasing patient eligibility for treatment.

Plain language summary

Review of current treatments using cross-linking to halt the progress of keratoconus Keratoconus is a disease in which the curved cornea, the transparent window at the front of the eye, weakens, bulges forward into a cone-shape and becomes thinner. This change of curvature means that light is not focussed onto the retina correctly and vision is progressively impaired. Traditionally, the effects of early keratoconus were alleviated by using glasses, specialist contact lenses, rings inserted into the cornea and in severe cases, by performing a corneal transplant. However, it was discovered that by inducing chemical bonds called cross-links within the cornea, the tissue could be strengthened and further thinning and shape changes prevented. The standard cross-linking procedure takes over an hour to perform and involves the removal of the cells at the front of the cornea, followed by the application of Vitamin B2 eye drops and low energy ultraviolet light (UVA) to create new cross-links within the tissue. Clinical trials have shown this standard procedure to be safe and effective at halting keratoconus progression. However, there are many treatment modifications currently under investigation that aim to reduce patient treatment time and increase comfort, such as accelerated cross-linking procedures and protocols that do not require removal of the surface cells. This review describes the different techniques being developed to carry out corneal cross-linking efficiently and painlessly, to halt keratoconus progression and avoid the need for expensive surgery.

Epi-off-lenticule-on corneal collagen cross-linking in thin keratoconic corneas

Purpose

To evaluate the safety and efficacy of corneal collagen cross-linking (CXL) performed on overlaying a corneal lenticule to thin recipient corneas of progressive keratoconus (KC) patients.

Methods

In this study were enrolled eyes of patients affected by progressive KC with a minimum corneal thickness less than 400 µm, after overlaying a lenticule of human corneal stroma prepared with the femtosecond laser. The lenticules used were 100 µm thick and of 8.5 mm diameter in all the cases. Both the host cornea and the lenticules were subjected to epithelial debridement. CXL was carried out according to the standard protocol. Visual acuity, refraction, slit-lamp examination, endothelial cell density, pachymetry and keratometry, anterior segment tomography (AS-OCT) and confocal microscopy were evaluated preoperatively and at 1, 3, 6 and 12 months postoperatively.

Results

CXL was performed in 10 eyes of 8 patients (main age 23), corneal thickness range 379–414 µm, mean 387.6 µm. One patient was lost at follow-up. In all other cases, visual acuity and the endothelial cell density remained stable over a 12-month follow-up. Preoperative mean K1 and mean K2 were 46.91 ± 1.9 and 50.75 ± 2.93, respectively, and at 12 months mean K1 was 47.36 ± 2.66 and mean K2 50.53 ± 3.35. The AS-OCT clearly showed a demarcation line in all patients at 1, 3 (mean depth 283 µm and 267 µm, respectively) and in some cases at 6 months. Reduced keratocyte density and stromal oedema were observed immediately up to 1 month after treatment, while a slight subepithelial haze was present at 1-month and completely disappeared by 6 months.

Conclusion

This new technique seems to offer a therapeutic opportunity for young patients suffering from progressive KC with very thin corneas, in which the standard treatment is not indicate, and delay or avoid the need for a corneal transplant.

Correlation between corneal demarcation line depth in epithelium-off and trans-epithelium accelerated corneal cross linking and keratoconus progression

AIM

To compare corneal demarcation line (DL) depth in both accelerated epithelium-off and trans-epithelium cross linking (CXL) using anterior segment optical coherence tomography (AS-OCT) and its relation to maximum keratometry (Kmax) progression in both techniques.

METHODS

A prospective comparative interventional study where patients with mild to moderate keratoconus (KC) were classified into two groups: accelerated epithelium-off and trans-epithelium CXL based on corneal pachymetry. Assessment of corneal DL depth was carried out after 3mo by AS-OCT. Kmax readings were evaluated after one year follow up using the Scheimpflug imaging system.

RESULTS

Study included 74 eyes of 44 patients. Group A underwent epithelium-off CXL (41 eyes), while Group B underwent trans-epithelium CXL (33 eyes). At 3mo follow up, mean corneal DL depth in Group A was 219.9±58.4 µm while in Group B was 127.2±7.8 µm (P<0.05). The mean Kmax changed from 51.9±3.9 to 51.3±4.2 diopters in Group A and from 53.1±4.1 to 53.6±5 diopters in Group B with insignificant difference in Kmax changes in either group (P>0.05). In addition, no significant change in corneal pachymetry was found in both groups (mean change at 1y: 6.4±4.7 and -10.1±2.3 µm in Groups A and B respectively).

CONCLUSION

Despite a significantly deeper corneal DL depth created by accelerated epithelium-off CXL technique compared to accelerated trans-epithelium CXL, there is no significant impact on keratoconus progression.

[Ultraviolet crosslinking in the treatment of keratoconus in patients with thin corneas]

Corneal collagen cross-linking (CXL) is a procedure that aims to halt the progression of corneal ectasia in keratoconic eyes. It is achieved by inducing cross-links in the corneal stroma and extracellular matrix by exposing it to ultraviolet-A (370 nm) irradiation while it is filled with photosensitizer (riboflavin). According to the conventional protocol, the recommended de-epithelialized corneal thickness should be higher than 400 μm in order to avoid radiation damage to the corneal endothelium. However, in progressive keratoconus, corneal thickness is often close to or lower than this threshold of 400 μm, which limits the application of cross-linking for these patients. The present article reviews the different protocols of cross-linking for thin corneas, their advantages and disadvantages. At present, clinical research on modified cross-linking protocols is still limited due to the methodology and a low number of patients involved. Thus, comparative randomized controlled studies with long-term follow-up are necessary to confirm the safety and effectiveness of several CXL protocols and identify the most efficient one.

Effect of corneal cross-linking on endothelial cell density and morphology in the peripheral cornea

BACKGROUND

To compare the endothelial cell density and morphology in the peripheral cornea before and after corneal cross-linking (CXL).

METHODS

This study evaluated thirty-one eyes of 31 patients who were treated with standard CXL for progressive keratoconus. Preoperatively and 6 months postoperatively, we compared the corneal endothelial cell density (ECD), the coefficient of variation in cell size (CV), and the percentage of hexagonal cells (HEX), in the peripheral regions of the cornea, using a non-contact specular microscope (EM-3000, Tomey).

RESULTS

All keratoconic eyes in this series were measurable in the peripheral regions. No significant differences were found in the peripheral ECD preoperatively and 6 months postoperatively at each point (Wilcoxon signed-rank test, superior, p = 0.16, nasal superior, p = 0.12, temporal superior, p = 0.17, inferior, p = 0.37, nasal inferior, p = 0.28, temporal inferior, p = 0.17). The mean percentage of the ECD loss was 1.3, 1.3, 1.0, 1.4, 0.7, and 1.4%, respectively. No significant differences in the peripheral CV or HEX were found preoperatively and 6 months postoperatively at each point.

CONCLUSIONS

Standard CXL does not cause significant changes in endothelial cell density, polymegethism, or polymorphism, in the peripheral regions of the cornea. It is suggested that CXL is a minimally invasive surgical approach for progressive keratoconus, even in terms of peripheral endothelial cells.

TRIAL REGISTRATION

University Hospital Medical Information Network Clinical Trial Registry (000031162).

Optimization of Oxygen Dynamics, UV-A Delivery, and Drug Formulation for Accelerated Epi-On Corneal Crosslinking

Purpose: Corneal collagen crosslinking (CXL) through an intact epithelium (epi-on) at high irradiance could potentially improve patient comfort, visual recovery, and clinical workflow compared to conventional epi-off CXL. However, intact epithelium limits stromal delivery of the oxygen, photosensitizer, and ultraviolet-A (UV-A) radiation needed to drive CXL. This ex vivo study evaluated three different epi-on CXL protocols compared to positive and negative controls, specifically focusing on the impact of supplemental oxygen. Endpoints included stromal oxygen levels, stiffness of crosslinked tissue, and acute flattening of whole eyes.Materials & Methods: Ex vivo porcine eyes were held in a custom environmental chamber. Intrastromal oxygen levels were continuously measured before, during, and after UV illumination by a fiberoptic probe inserted into a laser-cut flap. Accelerated, high irradiance, epi-on CXL protocols using riboflavin formulated with benzalkonium chloride (BAC) were studied, with and without supplemental oxygen. These were compared to an alternate, low irradiance, epi-on protocol using riboflavin formulated with sodium iodide. Both negative (no CXL) and positive (epi-off modified Dresden protocol) controls were performed. Post-CXL elastic modulus was measured using extensiometry and anterior tangential curvature was measured using a Scheimpflug tomographer.Results: Protocols including supplemental oxygen resulted in an approximately 5-fold increase in stromal oxygen levels prior to CXL. During epi-on, high-irradiance UV-A delivery under hyperoxic conditions, an aerobic state was maintained. Conversely, under normoxic conditions, stromal oxygen rapidly depleted to 0-5% for all other protocols. The combination of supplemental oxygen, BAC formulation, and high-irradiance UV-A resulted in the largest biomechanical changes and most pronounced flattening effects of the three epi-on protocols.Conclusions: Ex vivo analysis of stromal oxygen levels, corneal stiffness, and acute anterior curvature change indicates that simultaneous optimization of the oxygen environment, riboflavin formulation, and UV-A protocol can significantly increase the effects of corneal collagen crosslinking.

Effect of Riboflavin Solution With Hydroxypropyl Methylcellulose and Eyelid Speculum on Pachymetry Changes During Accelerated Collagen Crosslinking

PURPOSE

To assess corneal thickness changes with isotonic riboflavin (RF) solution with hydroxylpropyl methylcellulose in patients undergoing accelerated corneal collagen crosslinking (CXL) with and without an eyelid speculum.

METHODS

Fifty-two eyes of 48 patients with progressive keratoconus were enrolled in this study. The patients in this study were divided into 2 groups: in group 1 an eyelid speculum was removed during 20-minute RF (0.1%) + hydroxylpropyl methylcellulose (Mediocross M; Avedro Inc, Waltham, MA) instillation, and in group 2 the eyelid speculum was retained in place during the entire CXL procedure. All patients underwent accelerated CXL using continuous ultraviolet-A (UVA) light exposure at 9 mW/cm for 10 minutes; total energy dose was 5.4 J/cm. Intraoperative ultrasound pachymetry measurements were obtained before and after epithelial removal, after RF loading, and after UVA light exposure at 5 and 10 minutes.

RESULTS

The preoperative pachymetric measurements decreased in both groups after the removal of epithelium [group 1 (n = 26): -25 μm, group 2 (n = 26): -31 μm, P = 0.234]. Although the thinnest pachymetry significantly increased after soaking in both group 1 (52.26 μm) and group 2 (27.88 μm, P < 0.001), closure of the eyelids during RF instillation further increased the pachymetry readings (P < 0.0001). The corneal thickness remained stable in both groups during UVA irradiation at 5 and 10 minutes (P > 0.05).

CONCLUSIONS

Closure of the eyelids further induces corneal swelling that may offer an advantage to improve safety of the procedure particularly in thin corneas.

Current concepts in crosslinking thin corneas

Corneal cross-linking (CXL), introduced by Wollensak et al. in 2003, is a minimally invasive procedure to halt the progression of keratoconus. Conventional CXL is recommended in eyes with corneal thickness of at least 400 microns after de-epithelialization to prevent endothelial toxicity. However, most of the keratoconic corneas requiring CXL may not fulfill this preoperative inclusion criterion. Moderate-to-advanced cases are often found to have a pachymetry less than this threshold. There are various modifications to the conventional method to circumvent this issue of CXL thin corneas while avoiding the possible complications. This review is an update on the modifications of conventional CXL for thin corneas.

Potentiality of microemulsion systems in treatment of ophthalmic disorders: Keratoconus and dry eye syndrome - In vivo study

Microemulsions are widely studied as potential ocular drug delivery vehicles. In the present study we show the versatility of possible use microemulsions as ocular delivery vehicle. The ME is loaded with a hydrophilic drug, riboflavin phosphate (RFP) and a lipophilic, docosahexaenoic acid in triglyceride form (TG-DHA), each separately. These drugs treat keratoconus and dry eye syndrome, respectively. The advantage of using ME loaded with RFP is in overcoming eye epithelium debridement during collagen cross-linking therapy for treatment of keratoconus. ME loaded with lipophilic TG-DHA provides convenient dosage in liquid aqueous form of administration of highly lipophilic TG-DHA, which is known as a protective molecule in dry eye syndrome. The capability of RFP-loaded MEs was demonstrated in terms of improvement of biomechanical strength of the rabbit cornea, as a result of successful penetration of RFP through the intact epithelium. TG-DHA-loaded microemulsion applied topically onto an eye with induced dry eye syndrome showed the significant relief of the dry eye condition.

Current perspectives on corneal collagen crosslinking (CXL)

Corneal collagen crosslinking has revolutionized the treatment of keratoconus and post-refractive corneal ectasia in the past decade. Corneal crosslinking with riboflavin and ultraviolet A is proposed to halt the progression of keratectasia. In the original "Conventional Dresden Protocol" (C-CXL), the epithelium is removed prior to the crosslinking process to facilitate better absorption of riboflavin into the corneal stroma. Studies analyzing its short- and long-term outcomes revealed that although there are inconsistencies as to the effectiveness of this technique, the advantages prevail over the disadvantages. Therefore, corneal crosslinking (CXL) is widely used in current practice to treat keratoconus. In an attempt to improve the visual and topographical outcomes of C-CXL and to minimize time-related discomfort and endothelial-related side effects, various modifications such as accelerated crosslinking and transepithelial crosslinking methods have been introduced. The comparison of outcomes of these modified techniques with C-CXL has also returned contradictory results. Hence, it is difficult to clearly identify an optimal procedure that can overcome issues associated with the CXL. This review provides an up-to-date analysis on clinical and laboratory findings of these popular crosslinking protocols used in the treatment of keratoconus. It is evident from this review that in general, these modified techniques have succeeded in minimizing the immediate complications of the C-CXL technique. However, there were contradictory viewpoints regarding their effectiveness when compared with the conventional technique. Therefore, these modified techniques need to be further investigated to arrive at an optimal treatment option for keratoconus.

[Riboflavin UVA crosslinking in progressive keratoconus]

In patients with keratoconus, a progressive, ectatic disease of the cornea, the shape of the cornea is continuously changing leading to a reduction in visual acuity by progressive myopia and more and more (irregular) astigmatism. The symptomatic treatment consists of the prescription of glasses or special gas-permeable rigid contact lenses. Corneal tomography is generally used for diagnosis. After initial diagnosis of keratoconus, regular tomographic follow-ups should be performed. If clinically significant progression is found and confirmed by repeated measurements, riboflavin UVA collagen crosslinking should be offered to the patients. The aim of riboflavin UVA collagen crosslinking is to halt the progression of the disease to avoid further complications. The therapeutic principle is a combined effect of the photosensitizer riboflavin and UVA light. This stiffening effect of the corneal tissue halts the progression of keratoconus. The efficacy of this treatment has been demonstrated in various randomized, controlled trials.

Recent advances in corneal collagen cross-linking

Corneal collagen cross-linking has become the preferred modality of treatment for corneal ectasia since its inception in late 1990s. Numerous studies have demonstrated the safety and efficacy of the conventional protocol. Our understanding of the cross-linking process is ever evolving, with its wide implications in the form of accelerated and pulsed protocols. Newer advancements in technology include various riboflavin formulations and the ability to deliver higher fluence protocols with customised irradiation patterns. A greater degree of customisation is likely the path forward, which will aim at achieving refractive improvements along with disease stability. The use of cross-linking for myopic correction is another avenue under exploration. Combination of half fluence cross-linking with refractive correction for high errors to prevent post LASIK regression is gaining interest. This review aims to highlight the various advancements in the cross-linking technology and its clinical applications.

Efficacy of Epithelium-Off and Epithelium-On Corneal Collagen Cross-Linking in Pediatric Keratoconus

OBJECTIVES

To evaluate the efficacy of riboflavin/ultraviolet A epithelium-off (epi-off) and epithelium-on (epi-on) corneal collagen cross-linking (CXL) in progressive pediatric keratoconus.

METHODS

Thirty-six eyes of 27 patients aged 18 years or younger (12-18 years) diagnosed with progressive keratoconus and treated with epi-off (n=18 eyes) or epi-on (n=18 eyes) CXL were included in this study. All patients were followed up for 24 months postoperatively.

RESULTS

At 24-month follow-up, the mean corrected distance visual acuity (CDVA) improved from 0.24±0.17 to 0.17±0.11 logMAR in the epi-off group (P=0.032). In the epi-on group, CDVA improved from 0.33±0.23 to 0.26±0.20 logMAR (P=0.012), but the improvement was not significantly different between groups (P>0.05). In the epi-off group, all mean K values improved: K1 (Δ=-0.63 D, P=0.024), K2 (Δ=-0.73 D, P=0.008), and Kmax (Δ=-1.4 D, P=0.035), whereas in the epi-on group, Kmax and K2 did not change significantly and K1 (Δ=0.68 D, P=0.029) significantly worsened. Postoperative mild corneal haze occurred in 5 eyes (28%) in the epi-off group. No postoperative complications were observed in the epi-on group. In 94.4% of the epi-off group, keratoconus regressed (44.4%) or stabilized (50.0%), whereas in the epi-on group, only 66.6% regressed (33.3%) or stabilized (33.3%), and the difference was significant (P=0.038).

CONCLUSIONS

The efficacy of the epi-on procedure in terms of its ability to halt keratoconus progression appears to be 0.70 of the efficacy of epi-off CXL.

Updates on Managements for Keratoconus

Purpose

Keratoconus is a progressive disease of the cornea which can lead to blindness as irregular astigmatism increases. Currently, a variety of methods are available for the treatment of keratoconus, and in certain cases, it may be difficult to choose the most appropriate option. This article reviews available treatment modalities for keratoconus to provide the practitioner with practical and useful information for selecting the most suitable option for each individual patient.

Methods

To review treatment methods for different stages of keratoconus, PubMed (United States National Library of Medicine) and Scopus (Elsevier BV) databases were searched using the keywords “keratoconus”, “contact lens”, “cross-linking”, “Intacs”, “keratoplasty”, “gene therapy”, and “irregular astigmatism”, and related articles were reviewed based on disease assessment parameters and treatment methods.

Results

Various methods are available for the treatment of keratoconus: eyeglasses and contact lenses in the early stages, cross-linking for stabilizing disease progression, intrastromal corneal ring segments (ICRS) for reducing refractive errors or flattening the cornea, and penetrating keratoplasty (PK) and deep anterior lamellar keratoplasty (DALK), conductive keratoplasty, gene therapy and more recently, bowman layer transplantation (BL transplantation) in advanced stages of the disease. To achieve optimum results, it is essential to choose the best option for each individual patient.

Conclusions

A commonality of the reviewed papers was the advancement of novel diagnostic and treatment methods in ophthalmology, which can delay the need for corneal grafting. A better understanding of keratoconus treatment options can help enhance visual rehabilitation and prevent blindness in keratoconus patients.

Standard, transepithelial and iontophoresis corneal cross-linking: clinical analysis of three surgical techniques

BACKGROUND

To evaluate the clinical results of standard, transepithelial (TE) and iontophoresis (I) corneal cross-linking (CXL), in patients with progressive keratoconus.

METHODS

Thirty eyes of 30 patients with progressive keratoconus treated by CXL (10 by standard-CXL, 10 by TE-CXL and 10 by I-TE-CXL) with 12 months of follow-up. Pre- and postoperative ophthalmologic testing were: uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), refractive examination (spherical error, spherical equivalent), corneal topography (corneal astigmatism, simulated maximum, minimum and average keratometry), aberrometry (coma and spherical aberration), pachymetry and endothelial cell density.

RESULTS

In all groups, UDVA and CDVA improved significantly after treatment. Furthermore, a significant improvement in spherical error, spherical equivalent, topographic and aberrometric outcomes was observed in 3 groups at 1 year posttreatment. No significant variations were recorded in corneal thickness and endothelial cellular density.

CONCLUSION

Our results showed efficacy, clinical and refractive stability after standard-CXL, TE-CXL and iontophoresis-CXL.

Outcomes of iontophoretic corneal collagen crosslinking in keratoconic eyes with very thin corneas

The purpose of this retrospective study was to report the results of iontophoretic corneal collagen crosslinking (I-CXL) with riboflavin and ultraviolet A irradiation in patients affected by keratoconus, each with thinnest pachymetry values of <400 μ (with epithelium) and not treatable using standard epithelium-off technique.Fifteen eyes of 15 patients affected by progressive keratoconus and with thinnest pachymetry values <400 μ underwent I-CXL. The uncorrected (UDVA) and corrected (CDVA) distance visual acuity, maximum and minimum keratometry (K max and K min) readings, corneal thickness at the thinnest point (CTTP), endothelial cell density (ECD), and intraocular pressure (IOP) were assessed before I-CXL, at 1, 3, 6, and 12 months postoperatively.The mean UDVA and CDVA significantly increased 12 months after I-CXL (P = .002 for both comparisons). The K max readings significantly decreased at 6 and 12 months postoperatively (P = .04 and P = .02, respectively). The mean CTTP improved at the end of the follow-up (P = .008). ECD was unchanged. No side effects or damage to the limbal region was observed during the follow-up period.I-CXL has been proved to be effective in halting keratoconus progression in eyes with very thin corneas, with no side effects during the follow-up period. This procedure could be used in patients with more advanced keratoconus.

Corneal Collagen Cross-Linking Complications

Corneal cross-linking was approved by United States Food and Drug Administration for the treatment of progressive keratoconus in April 2016. As this approach becomes more widely used for the treatment of keratoconus and post-laser in situ keratomileusis (LASIK) ectasia, the medical community is becoming more familiar with potential complications associated with this procedure. This article aims to review the reported complications of collagen cross-linking for the treatment of keratoconus and post-LASIK ectasia.

The comparative safety of genipin versus UVA-riboflavin crosslinking of rabbit corneas

PURPOSE

To investigate, after 24 h, the safety of genipin or ultraviolet A (UVA)-riboflavin crosslinking of keratocytes and endothelial cells.

METHODS

Fifteen New Zealand white rabbits were selected and divided into a PBS group (five rabbits), a 0.2% genipin crosslinking (GP-CXL) group (five rabbits), and a UVA-riboflavin crosslinking (UVA-CXL) group (five rabbits). In the GP-CXL and PBS groups, 0.2% genipin or PBS was applied to the corneal surface of the right eyes. In the UVA-CXL group, a clinical crosslinking procedure was used. Before and after surgery, the operated eyes of each group were characterized with confocal microscopy, and the corneal buttons were excised for endothelium staining and electron microscopy.

RESULTS

The corneal endothelial cell density of the GP-CXL, UVA-CLX, and PBS groups changed. There was a statistically significant difference in thickness and changes in corneal endothelial cell density between the UVA-CXL group and the PBS group (p<0.05), and between the UVA-CXL group and the GP-CXL group (p<0.05), but no statistically significant difference between the GP-CXL group and the PBS group. Confocal microscopy, transmission electron microscopy, and hematoxylin and eosin staining showed that there was keratocyte apoptosis in the anterior and middle stroma and endothelial cell damage in the UVA-CXL group. In the GP-CXL group, only active keratocytes were found and minimal endothelial cell damage.

CONCLUSIONS

Treatment of rabbit corneas with 0.2% genipin showed minimal toxicity toward keratocytes and endothelial cells. Genipin is safer than UVA-CXL for crosslinking of thin corneas.

TFOS DEWS II iatrogenic report

Dry eye can be caused by a variety of iatrogenic interventions. The increasing number of patients looking for eye care or cosmetic procedures involving the eyes, together with a better understanding of the pathophysiological mechanisms of dry eye disease (DED), have led to the need for a specific report about iatrogenic dry eye within the TFOS DEWS II. Topical medications can cause DED due to their allergic, toxic and immuno-inflammatory effects on the ocular surface. Preservatives, such as benzalkonium chloride, may further aggravate DED. A variety of systemic drugs can also induce DED secondary to multiple mechanisms. Moreover, the use of contact lens induces or is associated with DED. However, one of the most emblematic situations is DED caused by surgical procedures such as corneal refractive surgery as in laser-assisted in situ keratomileusis (LASIK) and keratoplasty due to mechanisms intrinsic to the procedure (i.e. corneal nerve cutting) or even by the use of postoperative topical drugs. Cataract surgery, lid surgeries, botulinum toxin application and cosmetic procedures are also considered risk factors to iatrogenic DED, which can cause patient dissatisfaction, visual disturbance and poor surgical outcomes. This report also presents future directions to address iatrogenic DED, including the need for more in-depth epidemiological studies about the risk factors, development of less toxic medications and preservatives, as well as new techniques for less invasive eye surgeries. Novel research into detection of early dry eye prior to surgeries, efforts to establish appropriate therapeutics and a greater attempt to regulate and oversee medications, preservatives and procedures should be considered.

Epithelium-on photorefractive intrastromal cross-linking (PiXL) for reduction of low myopia

Purpose

To report the 9–12-month outcomes of a novel procedure for reduction of low myopia through epithelium-on photorefractive intrastromal cross-linking (PiXL) with customized control of topographic distribution of ultraviolet (UV)-fluence.

Method

Myopic patients with normal (non-ectatic) corneas underwent the PiXL procedure for reduction of low myopia. PiXL treatments were delivered through selective application of UVA light based on the refractive error of each patient. Clinical evaluation included safety (corrected distance visual acuity, endothelial cell count, central corneal thickness, anterior ocular health) and efficacy (uncorrected distance visual acuity, manifest refraction, K-mean) examinations. In addition, a patient satisfaction survey was conducted at 9 months post-procedure to evaluate patients’ subjective experience with the procedure.

Results

Fourteen myopic eyes (mean manifest refraction spherical equivalent −1.62±0.6D; range −0.75 to −2.65D) of 8 subjects (mean age 30 years old; range 24–51 years old) were enrolled in the study. At 12 months post-procedure, a mean manifest refraction spherical equivalent reduction of 0.72±0.43D (P<0.001) was observed, with a corresponding gain in uncorrected visual acuity of 0.25 logMAR and mean K-mean flattening of 0.47±0.46D. All patients achieved best corrected visual acuity of 20/20 or better from 1 month onward. There were no cases of ocular infection or secondary changes to the crystalline lens and retina due to UV exposure, while transient corneal haze subsided gradually.

Conclusion

The epithelium-on PiXL procedure was safe and effective in reducing myopic refractive error in this study with up to 12 months follow-up. Early results of this novel application of collagen cross-linking are encouraging but longer-term data in larger studies are required.

Precis

This paper serves to introduce and report the early clinical results of epithelium-on PiXL, a novel application of cornea cross-linking, in reducing low myopia in Asian eyes, which are under-represented in studies of similar design.

Transepithelial corneal crosslinking in treatment of progressive keratoconus: 12 months' clinical results

OBJECTIVE

The purpose of this study was to evaluate the safety and efficacy of transepithelial corneal collagen cross linking (TE-CXL) with modified riboflavin and accelerated UVA irradiance in thin corneas with pachymetry less than 400 microns at thinnest point, untreatable by epithelium off corneal collagen cross linking (CXL) in adult Pakistani population with progressive keratoconus.

METHODS

This quasi experimental study included twenty six eyes of 26 patients with progressive keratoconus who underwent accelerated transepithelial CXL in Armed forced institute of ophthalmology with 12 months follow up. Modified riboflavin, ParaCel ((riboflavin 0.25%, Benzalkonium chloride, EDTA, Trometamol, hydroxypropyl methylcellulose) and vibeX Xtra (riboflavin 0.25%) (Avedro, USA)) were applied to cornea in two stages. Uncorrected and Corrected Distant Visual Acuities (UDVA, CDVA), spherical equivalent (SE), astigmatism, pachymetry at thinnest point (Pachy thin), apex keratometry (Kmax), simulated and steep keratometry (Sim K, steep K) were measured at baseline and at 3, 6 and 12 months post operatively. The cornea was then exposed to accelerated UVA irradiance of 9mW/cm² for 10 min (total dose 30 mW/cm²).

RESULTS

The mean age of the patient was 24.54±5.16 years. UDVA, CDVA, SE, astigmatism significantly improved at all postoperative test points (p=0.000, 0.004, 0.000, 0.004 respectively). Kmax and pachy thin were significantly reduced over baseline at 1 year (p=0.000, 0.004 respectively). Topographic indices Sim K and steep K did not show significant changes. No intra or post-operative complications were reported.

CONCLUSION

Transepithelial accelerated CXL with modified riboflavin is a safe and effective procedure which halt disease progression in thin corneas with progressive keratoconus.

Clinical outcomes at one year following keratoconus treatment with accelerated transepithelial cross-linking

This study evaluated the clinical outcomes in keratoconus corneas following accelerated transepithelial corneal collagen cross-linking (CXL) (Avedro KXL^® system, Waltham, MA, USA) over one year of follow-up. The mean depth of the demarcation line measured by optical coherence tomography (OCT) was 205.19 µm. One month after surgery, a non-statistically significant change was noted in sphere (P=0.18) and in spherical equivalent (P=0.17), whereas a significant improvement was observed in corrected distance visual acuity (P=0.04). A significant change was observed in topographic astigmatism (P=0.03) and posterior corneal a sphericity (P=0.04). Accelerated transepithelial CXL may be a useful technique for the management of progressive keratoconus.

Corneal collagen crosslinking for corneal ectasias: a review

PURPOSE

To review the published literature on corneal collagen crosslinking (CXL).

METHODS

Importance has been placed on seminal publications, systemic reviews, meta-analyses, and randomized controlled clinical trials. Where such evidence was not available, cohort studies, case-controlled studies, and case series with follow-up greater than 12 months were examined.

RESULTS

Corneal collagen crosslinking with riboflavin and ultraviolet A (UVA) 370 nm radiation appears to be capable of arresting the progression of ectatic corneal disorders, with most studies reporting significant improvements in visual, keratometric, and topographic measurements. Its mode of action at the molecular level is undetermined. Follow-up is limited to 5-10 years but suggests sustained stability and enhancement in corneal shape with time. Nearly all published long-term data and comparative studies are with epithelium-off techniques. Epithelium-on investigations suggest some efficacy but less than with epithelium-off treatments and long-term data are unavailable. Accelerated techniques with higher UVA fluencies and shorter treatments times, delivering the same UVA energy dosage, are the subject of recent investigation, with some laboratory and clinical studies suggesting reduced efficacy compared to the standard 3 mW/cm2 for 30 minutes irradiation procedure. Combined methodologies of CXL with techniques such as photorefractive keratectomy and intrastromal rings show promise but long-term follow-up is indicated. Sight-threatening complications of CXL are rare.

CONCLUSIONS

Studies of epithelium-off CXL with irradiation at 3 mW/cm2 for 30 minutes support its efficacy. Refinement in techniques may allow for safer and more rapid procedures with less patient discomfort but require further investigation.

Efficacy and safety of transepithelial collagen cross linking for progressive keratoconus

Objective:

To evaluate the efficacy and safety of transepithelial (TE) collagen cross-linking (CXL) in patients with progressive keratoconus (KC).

Methods:

This Quasi Experimental Study was conducted at PNS Shifa Naval Hospital, Karachi from June 2015 to June 2016. Sixty eyes of 32 patients who underwent TE CXL for progressive KC from June 2015 to June 2016 were analysed to ascertain efficacy and safety of TE CXL procedure. Statistical analysis of the data was done using SPSS version 17.0.

Results:

Twenty eight (87.5%) patients underwent TE CXL bilaterally, while 4 (12.5%) underwent unilateral CXL. Mean change in astigmatism, Maximum simulated Keratometry value (Kmax), Spherical equivalent (SE) and Central Corneal Thickness (CCT) were -0.67±0.35D, 1.28±0.64D, -0.58±0.17D and 0.40±7.58µm respectively, from baseline. Mean gain in lines on Snellen’s visual acuity chart was 1.13±0.83 lines. Changes in astigmatism, Kmax and SE were statistically significant (p<0.001), while change in CCT was not statistically significant. The procedure had excellent safety profile, with no major complication till 6 months follow up period.

Conclusion:

TE CXL is a safe and effective procedure with statistically significant reduction in corneal astigmatism, Kmax and SE with reasonable gain in Snellen’s visual acuity.

Corneal stromal demarcation line after collagen cross-linking in corneal ectatic diseases: a review of the literature

Collagen cross-linking (CXL) is a relatively new conservative approach for progressive corneal ectasia, which is able to strengthen corneal tissue reforming new covalent bonds. Subjective and objective results following this method seem to be promising. In recent years, newer CXL protocols have been developed to perform more effective and less invasive procedures. The increasing diffusion of CXL in the corneal ectatic disease has increased the need to have actual indices regarding the efficacy of the treatment. Evaluation of demarcation line (DL), a transition zone between the cross-linked anterior corneal stroma and the untreated posterior corneal stroma, is considered a measurement of the depth of CXL treatment into the stroma. Some evidence in the literature emphasize that DL could be a measure of effectiveness of the CXL. On the contrary, some authors believe that the “the deeper, the better” principle is rather a simplistic approach for interpreting the clinical importance of the corneal stromal DL.

Efficacy of Corneal Collagen Cross-Linking for the Treatment of Keratoconus: A Systematic Review and Meta-Analysis

PURPOSE

To examine the efficacy of corneal collagen cross-linking (CXL) for the treatment of keratoconus (KCN).

METHODS

A systemic literature review and meta-analysis of ocular functional and structural parameters of patients with KCN undergoing cross-linking procedures were performed using PubMed and the web of science. A literature search was performed for relevant peer-reviewed publications on population-based studies. Data were analyzed with R software (Meta library), and heterogeneity was assessed with the Cochran Q and I. A random-effects model was used for high heterogeneity; otherwise a fixed model was used. Sensitivity analysis of particular tested groups was used to explain high heterogeneity. The main outcome measures extracted from the articles were corrected distance visual acuity, uncorrected distance visual acuity, and maximum K.

RESULTS

An improvement in visual acuity of 1 to 2 Snellen lines was found 3 months or more after undergoing CXL. Changes were more pronounced in uncorrected visual acuity. Some topography parameters were found to be improved (0.6-1 diopters) 12 to 24 months after CXL. The refractive cylinder improved by 0.4 to 0.7 diopters. Endothelial cell density decreased by 225 cells per square millimeter in the first 3 months and thereafter returned to normal. Corneal thickness was reduced by 10 to 20 μm in the year following CXL but not after 24 months. No changes in intraocular pressure were noted.

CONCLUSIONS

CXL is a safe and effective method for halting the deterioration of KCN, while slightly improving visual function.

Corneal Biomechanical and Anterior Chamber Parameters Variations after 1-year of Transepithelial Corneal Collagen Cross-linking in Eyes of Children with Keratoconus

Aim:

To assess the changes in corneal hysteresis (CH) and corneal resistance factor (CRF) 1-year following transepithelial corneal collagen cross-linking (CXL) treatment in eyes of children with keratoconus.

Methods:

This case series was conducted in 22 eyes of 22 children. Children aged < 18 years with progressive keratoconus were included. They were treated with transepithelial CXL. Corneal biomechanical and anterior chamber parameters such as CH, CRF, and peak 1 were noted using ocular response analyzer, Pentacam, intraocular lens master, and anterior segment optical coherence tomography before and 1, 3, 6, and 12 months after treatment.

Results:

Our series had 22 eyes of 22 children with a mean age 15.7 ± 2.1 years. The CH and CRF 1-year after treatment declined (difference of mean 0.1 mmHg (95% confidence interval [CI] 0.04–0.24), P = 0.2] and (difference of mean 0.1 mmHg [95% CI 0.04–0.30], P = 0.3), respectively. Peak 1 and peak 2 increased (difference of mean 0.1 mmHg [95% CI 0.006–0.008], P = 0.2) and (difference of mean 0.1 mmHg [95% CI 0.007–0.006], P = 0.3), respectively. The uncorrected distance visual acuity improved from 0.95 ± 0.34 logMAR to 0.68 ± 0.45 logMAR (P < 0.05). No statistically significant changes during 12 months follow-up were observed in axial length, corneal volume, anterior chamber volume, and anterior chamber depth (P > 0.05).

Conclusion:

Transepithelial CXL in keratoconus in pediatric age group seems to have good stability in corneal biomechanical parameters after 1-year. Further studies with a larger sample and better study design are recommended to confirm our findings.

Outcomes of corneal collagen crosslinking using a customized epithelial debridement technique in keratoconic eyes with thin corneas

The purpose of the study was to evaluate the outcomes and possible complications of CXL performed with customized epithelial debridement technique to keratoconic corneas with the thinnest pachymetry values less than 400 µm. Nineteen eyes of 19 patients were included. The uncorrected (UCVA) and best corrected visual acuity (BCVA), flattest and steepest keratometric (K) readings, central corneal thickness at the thinnest point (t-CCT), endothelial cell density (ECD) were assessed before and 12 months after CXL. The mean UCVA was increased (p = 0.001), while the mean BCVA did not show any difference (p > 0.05). The mean flattest and steepest K readings were decreased (p = 0.001). No change was observed in the mean t-CCT (p > 0.05). The mean ECD was decreased (p = 0.001). The mean pre-CXL and post-CXL percentages of polymegathism and pleomorphism did not show any significant difference (p > 0.05). CXL performed with customized epithelial debridement technique is successful in halting the progression of keratoconus in corneas thinner than 400 µm after 12 months of treatment. However, significant endothelial cell loss can occur after this procedure.