CROSSLINKING AND PRK

Combining corneal crosslinking (CXL) and surface ablation stabilises ectasia and improves vision by reducing irregular astigmatism resulting from keratoconus. But for early keratoconus should crosslinking be done first and surface ablation after the cornea stabilises, or should the two be combined in a single procedure? There are advantages and disadvantages to each approach, according to presenters at the annual meeting of the American Academy of Ophthalmology.

The chief advantage of combining crosslinking and surface ablation is that it results in significantly better corrected vision in eyes with highly aberrated corneas, said John A Kanellopoulos MD of Athens, Greece, and New York University, US. 'Topo-guided intervention is not a refractive procedure, it is a therapeutic procedure that attempts to improve best corrected vision in these eyes.' Other advantages of same-day treatment include less pain and rehabilitation time for patients and reduced risk from eliminating a second surgical episode. The biggest disadvantage is that the uncorrected refractive outcome is unpredictable – though the cornea and the refraction remain stable for years, he noted. Dr Kanellopoulos reviewed a study he published in 2009 comparing visual outcomes of keratoconus treated sequentially, with photorefractive keratectomy (PRK) performed six months to a year after crosslinking, with simultaneous treatment. 

Mean best corrected visual acuity in the simultaneous group was 20/30, compared with 20/40, for the sequential
group (p<0.001) (J Refract Surg 2009; 25(9): S812-818). The simultaneously treated group showed less PRK-related scarring. In eyes that were so severely abberated that they were candidates for penetrating keratoplasty, more than half had improved best corrected vision and 98 per cent achieved 20/40 or better. No eyes lost vision and 75 per cent gained at least one line, with 25 per cent gaining two or more lines.

There is a dramatic difference in the variability of elevation and distribution of astigmatism, Dr Kanellopoulos said. This results in greatly improved corrected vision, which is a better way to evaluate outcomes in highly aberrated eyes than uncorrected visual acuity or keratometric change. The 'Athens protocol' involves a partial topography-guided PRK combining myopic and hyperopic treatment zones that flatten the apex of the cone while steepening the central cornea. No more than 40 microns of tissue are removed, but this does help normalise the shape of the cornea, Dr Kanellopoulos said. This is followed by application of mitomycin C 0.02 mg/ml for 30 seconds, immediately followed by corneal crosslinking. He believes that regularising the cornea improves its biomechanical properties, resulting in a more even distribution of intraocular pressure across its surface, which may contribute to a more even-thickness healing.

The figure above depicts published data on UV absorption by normal Bowman's and elucidates to the potential advantage of having it removed prior to CXL. Additionally left is the cornea of the same patient that has had CXL alone and right the cornea OCT of the other cornea of the same patient that has undergone the Athens Protocol for KCN. It is obvious that the right picture shows deeper and broader hyper-reflectivity suggesting more efficient and broader CXL. Renato Ambrosio of Rio, Brazil has confirmed using the CORVIS system that CXL with the AP appears to provide better biomechanical response of corneas than CXL alone (personal communication), Dr Kanellopoulos said. But since both crosslinking and PRK flatten the cornea, and the flattening continues for months after surgery, combining the two makes for an unpredictable refractive result, Dr Kanellopoulos said. However, it does result in a more-regular corneal topography, which makes best spectacle corrected outcomes much more predictable than with sequential approaches.

'It is normal for refractive surgeons to fear a PRK intervention in an ectatic cornea and prefer to do so maybe at a later time. Our experience has proven to us that the contrary is a better option: Combined same day approach. We have published this work in 2009 and it includes a very large cohort of patients in either group, with a significant follow-up time of several years,' he said. 'The advantages for the population that we treat (South Eastern Europe) is based on the poor tolerance to RGP and other ‘specialty' contact lenses. I always like to make the point that the ‘PRK' treatment applied in our protocol is not a refractive treatment. It is based on the Wavelight topography-guided platform and aims to normalise the irregular cornea and improve CDVA and UDVA,' Dr Kanellopoulos said. Osama Ibrahim MD, professor of ophthalmology and president of Alexandria University, Egypt, presented a different view. 'It is mandatory to differentiate between frank keratoconus and topographically – suspicious keratoconus,' he said. 'For frank keratoconus, concurrent corneal crosslinking and PRK are not justified. Not only as it removes tissues from an already very thin cornea which may aggravate the condition, but also due to the lack of refractive predictability. In frank keratoconus, only corneal crosslinking either alone or combined with intra-corneal ring segments is indicated,' he said.

Dr Ibrahim prefers surface ablation only for patients with suspicious or forme fruste keratoconus, which he characterised as topographical aberrations with normal thickness and no posterior elevation on Scheimpflug imaging . 'In such cases, we use wavefrontoptimised or topography-guided ablation profile and we aim at full correction. We have done more than 100 eyes with excellent refractive results and they did not show any progression into frank keratoconus. We believe surface ablation per se is a crosslinking procedure that helps to stabilise the condition.' Therefore, for frank keratoconus with thinning of the cornea and a progressive course, Dr Ibrahim prefers crosslinking first. In 3,000 eyes he has followed for four years, all were stabilised after crosslinking. Moreover, corneal flattening occurs in 20 to 25 per cent, and it is significant, averaging about 4.5 D and ranging from 0.5 to 12.0 D. Nonetheless, patient satisfaction with crosslinking is higher than can be explained by objective findings, he said. 'We believe there is not only flattening but also improvement in higher order aberrations.'

For patients who receive corneal crosslinking only, Dr Ibrahim waits six to 12 months for the cornea to stabilise and then if the condition necessitates does surface ablation to improve quality of uncorrected and best spectacle-corrected visual acuity. He may also add intracorneal rings for only selected cases with residual high irregular astigmatism and ametropia.

The advantages of waiting are more predictable refractive outcomes and better quality of vision, Dr Ibrahim said. Disadvantages include the pain, cost and additional rehabilitation from doing a second surgical procedure. But he believes the advantages of sequential treatment outweigh the risks, especially that the risks of simultaneous treatments has also to be considered. 'If you do crosslinking and PRK simultaneously there are advantages in ease, cost and some pain. But there are many disadvantages, including adding a variable to a procedure that is unpredictable already. Crosslinking causes flattening. If you combine it with another flattening procedure like PRK, you will get a decrease in accuracy and predictability of both, and a higher incidence of overcorrection. So I think the simultaneous approach is not justified, and I strongly recommend staging the two procedures if needed,' Dr Ibrahim said.