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Corneal collagen cross-linking for keratoconus

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Dr Elsie Chan
FRANZCO

 

Corneal collagen cross-linking (CXL) was introduced in 1998 as a treatment that could potentially halt the progression of keratoconus.1 Since the first clinical study of CXL was published in 2003,2 CXL has been rapidly incorporated into clinical practice. A growing number of modifications to the treatment protocol are now being explored in an effort to increase the efficacy and safety of the treatment.

Background

CXL is based on the theory that the decreased biomechanical strength in keratoconus is related to a reduction in cross-links within collagen fibres.3 The treatment utilises a photosensitiser, riboflavin (vitamin B2) which is exposed to ultraviolet A (UVA) irradiation to produce an oxygen-dependent chemical reaction, leading to cross-linking. Riboflavin also absorbs UVA to limit the depth of its effect.

The procedure is performed under topical anaesthesia, followed by a nine-millimetre epithelial debridement. This is necessary as riboflavin does not penetrate through an intact epithelial layer. Riboflavin drops are then instilled for 30 minutes followed by UVA irradiation (3mW/cm2) for a further 30 minutes, during which the riboflavin drops are continued.4

Pre-clinical studies have shown that CXL increases the corneal collagen diameter by 12.5 per cent, increases its stiffness by over 300 per cent and increases its stability to enzymatic digestion.5-7 The treatment depth has also been demonstrated to be limited to the anterior 300-350 µm, with endothelial cell damage occurring when the corneal thickness falls below 400 µm.4,8 (Figure 1)

OL 034 -Figure -1
Figure 1. Intra-operative photograph demonstrating the ultraviolet light source irradiating the cornea, which has been soaked with riboflavin

Clinical results

In 2003, Wollensak and colleagues published the first clinical results of CXL for progressive keratoconus.2 They reported that progression stopped in all treated eyes with a mean improvement of -2.01 D in the maximum keratometry value (Kmax) on corneal topography after an average follow-up period of 23 months. Since then, numerous case series have been published supporting these results, with reported improvements in Kmax ranging from -0.49 D to -2.66 D.9,10 Changes in uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) have been more variable, with several studies reporting a modest or no change in UCVA,11 whereas Caporossi and colleagues reported an improvement in UCVA by 2.85 Snellen lines and BSCVA by 2.03 lines.12

One of the only randomised, controlled trials comparing eyes treated with CXL and control eyes has been conducted in Melbourne at the Centre for Eye Research Australia and the Royal Victorian Eye and Ear Hospital. In this trial, we found that Kmax flattened by a mean of -1.03 D after three years in treated eyes, whereas eyes in the control group steepened by +1.75 D. A similar trend was seen in UCVA.13 (Figure 2)

OL 034 -Figure -2
Figure 2. Bar graph showing the mean change in maximum simulated keratometry value (Kmax) between baseline and 3, 6, 12, 24 and 36 months after treatment for the control and treatment groups. In the control group, there was a significant increase in Kmax compared with continued flattening observed in the treatment group. The columns represent the mean change in Kmax from baseline (DKmax) in dioptres (D) and the error bars represent the standard error. CXL = corneal collagen cross-linking.13


Studies focusing on the paediatric population have reported more variable results compared to the adult age group, with changes in Kmax ranging from an improvement of -1.27 D after two years14 to 55 per cent of eyes progressing by three years.15 There may be a less sustained effect due to a more rapidly progressing disease in this younger age group.

Overall, most studies report stabilisation in Kmax in over 80 per cent of treated eyes. Longer term studies suggest stability of the treatment until at least four to six years.16

Complications

Complications following CXL have been reported. While a temporary stromal haze is seen in almost 90 per cent of treated eyes (Figure 3), up to 8.6 per cent of cases have been reported to permanently affect visual acuity.17 Other complications include treatment failure, sterile infiltrates (up to 7.6 per cent of eyes), scarring,18 infectious keratitis19 and irreversible corneal oedema.20 Rare complications include corneal melting and perforation.21

OL 034 -Figure -3
Figure 3. Slitlamp photograph of post-operative haze one month following treatment

Variations in CXL treatment protocol

  • Trans-epithelial treatment

There has been much interest in the establishment of a technique to enable CXL to be performed without epithelial debridement. Trans-epithelial or ‘epithelium-on’ CXL has the advantage of minimising post-operative pain and reducing the risk of infectious keratitis. It may also be useful in the treatment of thin corneae and the paediatric age group. One of the more popular means to enhance penetration of riboflavin across the epithelium has been to use additional agents such as EDTA and benzalkonium chloride in combination with riboflavin. Despite the popularity of trans-epithelial CXL, clinical results have been variable, ranging from flattening of Kmax by -2.97 D22 to progression by +0.48 D,23 suggesting that this technique may be less effective than conventional treatment.

  • Accelerated CXL

Treatment protocols using decreased irradiation times are gaining popularity. While the conventional protocol achieves a total UVA exposure of 5.4J/cm2 with 30 minutes of UVA at 3mW/cm2, a similar exposure is possible by increasing the irradiance and decreasing the treatment time according to the Bunson-Roscoe law of reciprocity. While devices offering UVA at higher irradiances are commercially available and being used widely, there remains limited evidence for accelerated CXL in the peer-reviewed literature. There may also be a limit to the minimum treatment time, as oxygen depletion may occur with higher oxygen usage rates at high irradiances. Hammer and colleagues demonstrated that the in vitro stiffening effect of CXL using UVA at 18mW/cm2 for five minutes was unchanged compared to control eyes.24 A clinical study comparing accelerated (30mW/cm2 for three minutes) and conventional CXL found an improvement in Kmax in the conventional group and Kmean (mean keratometry value) in both groups compared to baseline after 12 months.25

  • Treatment of thin corneae

Treatment of corneae less than 400 µm has been shown to lead to endothelial cell damage.26 Modifications in the treatment protocol are therefore necessary before treatment can be considered these eyes. One option is the use of hypotonic riboflavin to swell the cornea, although there are limited published results using this technique. Two studies suggest stabilisation of keratoconus 12 months following treatment without any damage to the endothelial cells.27,28 Our own unpublished data also support the safety and efficacy of using hypotonic riboflavin.

  • CXL with refractive surgery

As CXL gives only modest improvements in visual acuity, there has been much interest in combining CXL with refractive procedures such as intra-corneal ring segments, PRK and phakic intraocular lenses. Results suggest that refractive procedures combined with CXL can lead to significant improvements in visual acuity for at least 12 months.29-31 Some authors also advocate using CXL to stabilise the cornea during refractive procedures for patients at risk of developing ectasia,32 although there remains a lack of clinical data to support the efficacy of CXL for this indication.

Other indications

The efficacy of CXL for keratoconus has led to its use in other corneal ectasias including post-LASIK ectasia and pellucid marginal degeneration.

Conclusion

The current recommendations for CXL are in eyes with documented progression of keratoconus where the corneal thickness is above 400 µm at the time of treatment. While results show a modest improvement in corneal topography and visual acuity measurements, stability can be achieved in over 80 per cent of treated eyes with only a small risk of treatment-related complications. Further clinical studies with extended follow-up are needed to establish the safety and efficacy of alternative treatment protocols.

 

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  17. Raiskup F, Hoyer A, Spoerl E. Permanent corneal haze after riboflavin-UVA-induced cross-linking in keratoconus. J Refract Surg 2009; 25: S824-828.
  18. Koller T, Mrochen M, Seiler T. Complication and failure rates after corneal crosslinking. J Cataract Refract Surg 2009; 35: 1358-1362.
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  21. Labiris G, Kaloghianni E, Koukoula S, Zissimopoulos A, Kozobolis VP. Corneal melting after collagen cross-linking for keratoconus: a case report. J Med Case Rep 2011; 5: 152.
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  24. Hammer A, Richoz O, Arba Mosquera S, Tabibian D, Hoogewoud F, Hafezi F. Corneal biomechanical properties at different corneal cross-linking (CXL) irradiances. Invest Ophthalmol Vis Sci 2014; 55: 2881-2884.
  25. Tomita M, Mita M, Huseynova T. Accelerated versus conventional corneal collagen crosslinking. J Cataract Refract Surg 2014; 40: 1013-1020.
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