Advancements in the management of Keratoconus

Anes Harid

Introduction

Keratoconus is a progressive corneal disorder characterised by thinning and conical shape of the cornea, posing a significant impact to ophthalmology physicians and patients’ quality of life. Patients are affected from a young age with various habitual, genetic, and environmental factors including eye rubbing, previous family history, as well as conditions such as Down Syndrome. Visual acuity is impacted through severe astigmatism, corneal scarring, and even corneal perforation with a prevalence of up to 5% of the population, especially in regions such as the Middle East (1).

Corneal collagen cross-linking (CXL) techniques were developed as an intervention for keratoconus to halt the progression and enhance corneal biostability. The therapeutic effect of CXL has been demonstrated by the results reported from human studies and was eventually approved by the Food and Drug Administration. However, despite this, the cytotoxic effects of ultraviolet A (UVA) used in CXL and the painful effects of CXL on patients have warranted the exploration of new management strategies for keratoconus (2). This review aims to evaluate research advancements in new corneal cross-linking techniques to manage keratoconus compared to traditional methods of CXL with UVA/riboflavin in terms of halting the progression of keratoconus and patients’ quality of life.

CXL

CXL was first described in 1997 by Spoerl et al in Dresden as a method of enhancing the biomechanical strength of the cornea to prevent the progression of keratoconus (3). The basic principles of CXL involve the formation of chemical bonds between the corneal collagen fibrils, through the formation of free radicals leading to photo-oxidative cross-linking that is stimulated by UVA/riboflavin. The absorption peak of light energy from riboflavin is noted to be around 370nm, making UVA ideal for CXL, which is performed by placing riboflavin on the corneal after removal of the corneal epithelium. This method became the mainstay of treatment after Wollensak et al demonstrated the prevention of keratoconus progression with CXL through a reduction in keratometry steepness and measuring astigmatism outcomes (4). Despite this, the hypothesis remains yet to be definitive due to the biochemistry of how UV/riboflavin affects the cornea is not fully well understood (5).

CXL is also indicated in conditions such as Pellucid Marginal Degeneration (PMD) and post-refractive surgery ectasia (6). However, it also has its limitations such as an associated risk of infections. CXL is contraindicated in patients with a corneal thickness under 400 microns, previous corneal infections, and/or any severe corneal scarring or opacification (7). In addition to the cytotoxic effects of UVA (corneal haze and therefore, reduced visual acuity), patients report having periods of significant ocular pain following CXL after the anaesthetic effects wear off due to the removal of the corneal epithelium (2). While there are strategies of that preserve the corneal epithelium, studies have demonstrated lower efficacy of CXL using this method (8). In addition to this, a systematic review in 2015 revealed limited evidence for the use of CXL in managing keratoconus due to the lack of randomised control trials that were properly conducted (9).

Recent Advancements in Corneal Cross-Linking

One method is exploring the use of what is known as Chemical cross-linking. This is a transepithelial method that does not require the activation complex of UVA/Riboflavin and is therefore a less cytotoxic hypothesis. The structure of one of these chemical cross-linkers contains a dicarboxylic acid spacer molecule that will bind to the amine functional groups of the proteins in the corneal stromal layer, thereby increasing stiffness within the corneal tissue (10). The benefits of this method include maintaining the corneal epithelium as it is not removed as well as its simple delivery method through eye drops. Therefore, this is a potentially safer method that minimises ocular pain and cytotoxic effects that the UVA/Riboflavin method would pose to patients. This method also does not require specialised equipment such as that used in traditional CXL which highlights the financial benefits from the efficiency of performing this procedure in local community centres rather than the need to travel to specialized eyecare clinics for treatment.

This has been performed on in-vivo and ex-vivo eyes, both showing clinically relevant corneal stiffening in ex-vivo pig and human tissue and in-vivo rabbit corneas. There was no toxicity demonstrated histologically, compared to UV/riboflavin, and the chemical cross-linking was performed at a stable pH of around 7-8 which was well-tolerated. These results seem promising and can be potentially applied to other conditions affecting corneal shape and curvature such as PMD as well as inflammatory causes such as keratitis. However, this remains an area for further research and experimentation for the long-term impacts.

Another method of chemical cross-linking being experimented is using “Genipin”, a herbal medicine agent extracted from the fruit of Gardenia Jasmioides. Its use in herbal medicine includes xerograft scaffolds for heart valves (11). Wenjing Song et al utilised corneal thickness measurements as well as confocal microscopy to investigate corneal stiffness 24 hours after induction of Genipin onto 5 rabbits’ corneas and compare this to rabbit corneas that received the traditional UVA/Riboflavin CXL method. After surgery, there was an increase in corneal thickness in both groups, with a statistically significant increase after receiving traditional CXL. Despite this confocal microscopy demonstrated an increase in endothelial cell density for corneas that received the Genipin while the opposite was observed in corneas receiving CXL.

These results demonstrate that Genipin, although may not be as effective as CXL, can still result in increased corneal stiffness with the additional benefit of maintaining endothelial cell density which is crucial as these do not regenerate in-vivo in humans (12). Like the previous example, these studies are yet to prove the long-term results to confirm if there is any toxicity from Genipin and, therefore, conclude its safety and therapeutic index in clinical practice.

Future Directions and Implications

There remains a scarce input of research into enzymatic cross-linking. Variations in the Lysyl Oxidase (LOX) gene express specific enzyme proteins that determine corneal stiffness based on the density of different enzymatic variations. One study analysing variations in corneal LOX enzymatic protein density identified that patients with keratoconus have displayed lower levels of Lysinonorleucine (LNL) and Dihydroxylysinonorleucine (DHLNL) (14). These may indicate a defect in the collagen structure or elastic network in the cornea. Despite these differences, the total cross-link density was similar between the keratoconus and control samples (13). They did not detect other specific types of cross-links and the study did not find a clear correlation between cross-link levels and the stability of collagen fibrils. Additionally, an elaboration on the rationale of the use of C8 high-performance liquid chromatography for cross-link analysis and differential scanning calorimetry for determining thermal denaturation would be beneficial. While this is an appropriate method, it does not overlap with the dependent variables of the previous studies mentioned and, therefore, results can be more challenging to compare.

Human studies of chemical cross-linking, using IVMED-80, copper sulphate used as a twice-daily eye drop acting as a cofactor for LOX, have demonstrated increased LOX activity and corneal stiffness in human keratoconus corneas ex-vivo (15). However, it would be beneficial to have more information about the characteristics of the samples and any potential limitations in terms of demographics or sample size. A larger, more diverse sample may strengthen the generalizability of the findings.

Patients with keratoconus can be a specific patient group to target for new treatment methods. Other causes of corneal ectasia include Keratoglobus, PMD, and Terrien Marginal Degeneration, however, these conditions are rare. LASIK procedures are a post-operative cause of corneal ectasia, which is contraindicated in patients with keratoconus (16). A potential area for research is the ability to minimise corneal ectasia induced by more common surgical procedures such as Laser-assisted in situ keratomileusis (LASIK). This can be done using chemical cross-linking to target a wider variety of patients, which can preserve corneal thickness and prevent ectasia when used before the procedure. This remains a hypothesis, and further studies are required to demonstrate the effectiveness for this patient population.

Conclusion

In conclusion, the review of new experimental approaches for treating keratoconus, as compared to traditional cross-linking methods such as chemical cross-linking and LOX-enzymatic cross-linking, highlights promising advancements in the field. While traditional methods of CXL have demonstrated efficacy in strengthening corneal tissue and slowing the progression of keratoconus, recent experiments introduce innovative strategies that aim to enhance clinical outcomes. Further analysis is required to determine the long-term safety of chemical cross-linking and a better understanding of LOX enzymatic cross-linking for potentially newer management strategies.

References

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