Authors: Lupe Ivette Villegas LopezDavid BronteJames Germann & Susana Marcos
Visual Optics and Biophotonics Lab, Institute of Optics, CSIC, Madrid, Madrid, Spain
Date: June 2020
Source: ARVO 2020 Abstracts - video presentation
The sclera is at risk of deformation due to increasing axial length progression. The suggestion is that by strengthening the sclera using cross-linking treatment, this could help resist the traction caused by axial elongation and therefore reduce myopia progression. Cross-linking treatment using riboflavin drops and UV light is already being used for the treatment of corneal ectasia, such as keratoconus, to help strengthen the bonds between the collagen fibrils and resist IOP forces.
This research aimed to discover if the same treatment could be used on the sclera to similar effect using Rose Bengal and green light (RGX).
There are a few positives that we can gather from this research:
Scleral cross-linking could be an option for future myopia intervention. The results show that it may be possible to target certain areas of the sclera for treatment to help limit axial length increase.
Riboflavin is known to be cytotoxic and Rose Bengal has been shown to be less so – this is encouraging news as less cellular irritation can mean better healing.
However, the investigation was carried out in-vitro with the strips of tissue samples removed from enucleated porcine globes and although the results are very promising and informative, further research will need to show us how this will translate to a viable in-vivo human treatment!
Limitations and future research
- Surgical risks.
- Elsheikh and Philips (1) discussed cross-linking using riboflavin and UVA light in their paper in 2013 and raised concerns of the exposure of the scleral tissue during the procedure and the cytotoxic risk to the retina. Although Lopez et al were using Rose Bengal and green light instead, the process of any operation and risk to the retina is likely to be similar regardless of the substances used, even when Rose Bengal carries less of a toxicity issue.
- Another option?
- Elsheikh and Philips also suggested from their research that chemical cross-linking might be an alternative approach using glyceraldehyde that had shown to give a 150% stiffness increase in the sclera with no toxicity risk.
- Would other timings make a difference?
- The ARVO abstract video was a summary of the experimentation and therefore didn’t explain the reasons for choosing the immersion time for the strips in the Rose Bengal, or why a given time of green light exposure was chosen.
- Are these the optimum immersion and light exposure times for maximum effect, or could different timings give better results?
- Is one area enough?
- The research showed conclusively that the RGX treatment was most effective in the posterior temporal regions. Would this be enough on its own to resist deformation?
- When could this be done and who for?
- Further information would be needed as to the best time to undertake this procedure, and who it would be for.
- Presumably, it would be more effective if carried out before any rapid changes in myopic refraction have taken place (i.e, a child who has been diagnosed with myopia and who is identified as being in a higher risk group for progression).
- It may be too little too late if they are left to progress before considering it. However, we would have to be very sure of any growth and progression information we had before suggesting any operation which carries risks.
- Like any surgery, the benefits would have to outweigh the risks.
Those of us involved in everyday myopia management will need to continue to use whatever options we have at our disposal for now while this remains a ‘Tomorrow’s World’ possibility.
The sclera is a tough, collagenous structure responsible for support and protection of the globe, as well as anchorage for the extra-ocular muscles. Despite its strength, increasing axial length impacts its integrity and it stretches with the mechanical pull of the globe. Myopia is thought to force a decrease in the diameter of the collagen fibrils therefore weakening the general structure and the higher the myopia progression, the worse the impact.
After the epithelium was removed from post-mortem porcine eyes, the areas being treated were immersed in 0.1% Rose Bengal for 120 seconds before being irradiated with green light for 200 seconds, then immersed again for 30 seconds before radiated again for a further 200 seconds.
Strips of tissue were removed and measured on a uniaxial stretcher to check for changes in Young’s modulus at 8%, 10% and 12% strains. The anterior and posterior areas, with both nasal and temporal regions, were compared to each other aswell as the control samples.
An interesting finding was that the untreated anterior nasal sclera already showed greater resistance to strain than the posterior nasal sclera regions.
The RGX treatment proved to be 40% more effective across all strains on the posterior temporal region and the results comparing the treated anterior nasal tissue samples to the posterior nasal tissue showed that there was actually a 35% drop in the stiffness of the tissue, rather than an increase. The reason for this were postulated to be due to either optic nerve head traction, ex-vivo dehydration or swelling of the tissues.
This has interesting implications for future myopia management as a way of strengthening the sclera against axial elongation, especially if it can be adapted for use in living in situ tissue.
It could maybe be used as an initial treatment for those candidates most at risk of fast progression before they go on to also utilise other myopia management methods we currently have and therefore give a doubly effective approach.
Title: Scleral Cross-Linking Using Rose Bengal Green Light
Purpose: Scleral cross-linking has been proposed as a potential treatment for halting the progression of myopia. In this study, we test whether the Rose Bengal-Green Light collagen cross-linking (RGX) treatment that has been previously tested in the cornea (Cherfan et al.,IOVS 2013) can be translated to the sclera.
Methods: The outer tissue was removed from the sclera of ten porcine ocular globes (24-48h post-mortem).After removing the epithelium, globes were treated with RGX on the nasal (NR) and temporal (TR) regions, where one side was treated and the other kept as control. RGX treated regions were partially immersed in 0.1%(w/v) Rose-Bengal solution for 120s, irradiated by green light (532nm 0.5mW/cm2) for 200s, immersed again for 30s, and irradiated for an additional 200s. Anterior and posterior scleral strips (20×4 mm approx) were collected from the NR and TR, both from RGX and untreated areas of each eye. Strips were mounted in an uniaxial stretcher (CellScale, Canada) to determine the Young’s modulus (YM) at 8%, 10% and 12% strain. Sample cross-sections were measured with a custom spectral OCT system. T-test was used to test regional and RGX-induced differences in scleral biomechanics.
Results: All stress-strain curves of scleral tissue showed an exponential behavior. In untreated regions, anterior was stiffer than posterior sclera (7.35±3.31 vs 3.29±1.62 MPa at 8% and 16.11±8.00 vs 7.54±3.79 MPa at 10% strains).In the anterior sclera, the mean YM for the untreated regions was 7.40±4.01 (NR) and 7.29±2.42MPa (TR), and for the RGX regions, the average YM was 4.83±3.73 (NR) and 5.84±2.33MPa (TR), at 8% strain. In the posterior sclera, the average YM for untreated regions was 4.04±1.72 (NR) and 2.55±1.08MPa (TR), and for the RGX regions YM was 2.65±1.41 and 3.57±1.98MPa. At all strains, RGX produced a highest increase in stiffness in the posterior temporal sclera by 40.07%, compared with untreated sclera in the same region (T-Test,P<0.001 two-tailed), although it decreased in the posterior nasal sclera by 34.81% (T-Test,P<0.001 two-tailed).
Conclusions: RGX treatment significantly stiffened the posterior temporal sclera of the porcine ocular globe, making this zone a prime candidate for myopia treatment. Interestingly, other regions saw a decreases in stiffness after RGX, suggesting that a separate process may compete to RGX stiffening, which may include optic nerve head traction or dehydration or swelling in samples ex vivo.
Ailsa Lane is a contact lens optician based in Kent, England. She is currently completing her Advanced Diploma In Contact Lens Practice with Honours, which has ignited her interest and skills in understanding scientific research and finding its translations to clinical practice.