LASIK + CXL?

Recent research suggests that routinely adding corneal crosslinking (CXL) to LASIK surgery may offer benefits, including improving the refractive outcome’s predictability and stability, and potentially reducing the risk of ectasia. However, the practice may also pose risks that have not been fully evaluated, possibly including more infections, corneal haze and complicating future procedures, debaters told Refractive Surgery Day at the 2014 American Academy of Ophthalmology annual meeting in Chicago, USA.

Arguing the pro side, Vance Thompson MD asserted that adding CXL addresses a known LASIK drawback – that it undermines the cornea’s structural integrity.

“We know that LASIK can weaken the cornea. We know that crosslinking stiffens the cornea. What we need to know is whether combining the two is safe, provides predictable refractive outcomes, and improves corneal stability. Recent research suggests the answer to all three questions is yes," said Dr Thompson, Sioux Falls, South Dakota, USA.

ENERGY DOSAGE

Dr Thompson, a consultant for Avedro, noted that CXL has been shown to stiffen the cornea, and the effect is dose-dependent. The CXL used in conjunction with LASIK uses only 2.7J/cm2 of energy, while CXL for traditional keratoconus treatment uses 5.4J/cm2, double the energy dosage.

CXL with LASIK is also safe, Dr Thompson said. Several studies have found that it does not increase adverse events when used with refractive surgery (Kanellopoulos AJ et al. Clin Ophthalmol. 2012; 6: 1125-30. Tomita M et al. J Cataract Refract Surg. 2014; 40 (6): 981-90).

As for predictability, finite element analysis at the Cleveland Clinic Cole Eye Centre found that CXL + LASIK that increased anterior residual stromal stiffness 50 per cent added less than 0.25 dioptre of hyperopic effect (Roy SI et al. ARVO 2014), Dr Thompson said.

A study by Minoru Tomita MD and colleagues found adding CXL did not reduce LASIK refractive predictability at one year (JCRS 2014 40 (6)). In fact, in a study of highly myopic patients conducted by Jerry Tan MD, LASIK Xtra significantly improved refractive predictability relative to LASIK alone.

LASIK Xtra has also been shown to reduce refractive drift, and is especially effective in stabilising hyperopic LASIK treatment (Kanellopoulos AJ. Clin Opthalmol. 2012).

“LASIK Xtra deserves more study and I am very excited about it. If I was undergoing LASIK this is how I would like to have it performed,” Dr Thompson said.

Taking the opposing side, George Kymionis MD, PhD, University of Crete, Greece, noted that his own research indicates that combining CXL and laser corneal surgery is effective for treating progressive keratoconus. But that doesn’t mean it is useful or even safe for routine use in refractive cases, he said.

LONG-TERM EFFECT

CXL, with the dosage used for the treatment of keratoconus, can have a continuous, long-term effect, with progressive corneal flattening of as much as 15 dioptres.

“For keratoconus patients this is a positive side effect, but for a refractive patient it would be a huge problem,”
he said.

Many other risks of adding CXL to LASIK have not been studied, Dr Kymionis added. These include the possibility of higher infection rates due to longer surgical exposure, and increased corneal scarring, infiltrates and diffuse lamellar keratitis.

CXL also kills keratocytes, raising questions about how repopulation of the stroma after surgery may affect refractive outcomes. Retreatment is also a question; CXL could interfere with raising the flap. Also, it is unknown whether crosslinked tissue ablates at the same rate as untreated stroma, Dr Kymionis said.

Exposing the crystalline lens to UV radiation could complicate future intraocular lens (IOL) power calculations, Dr Kymionis added. It may also kill corneal and conjunctival stem cells, and may damage endothelial cells. CXL also decreases corneal permeability for fluoroquinolones and voriconazole.

Also, prophylactic CXL has not been shown to prevent iatrogenic ectasia, Dr Kymionis noted. With an incidence of about one in 5,000 cases, it would take a study of thousands of patients over many years to confirm efficacy. For “high risk” corneas, instead consider photorefractive keratectomy (PRK), phakic IOLs or simply aborting the refractive procedure, Dr Kymionis advised.

Vance Thompson:
vance.thompson@vancethompsonvision.com

George Kymionis: kymionis@med.uoc.gr