LENS OR LASER?

Phakic posterior chamber IOL. Courtesy of Sonia Yoo MD

High myopia presents a variety of challenges in choosing among corneal and lenticular refractive surgical solutions. Beyond refractive error, patient age, corneal and retinal characteristics, cataract development, anterior segment anatomy and axial length are among factors to consider, according to an expert panel at the Ophthalmology Futures Forum Barcelona 2015, in Spain.

Wide biometric variability and prevalence of anatomic anomalies among patients with long eyes and high myopic refractive errors complicate lens power calculations, noted panel moderator Marie-José Tassignon MD, PhD, FEBO, Professor and Head of the Department of Ophthalmology at University Hospital Antwerp, Belgium. Similarly, lack of biomechanical data can make corneal ablation uncertain. New instruments that measure features such as corneal stiffness and capsular bag volume may help improve refractive outcomes, she added.

DEFINING HIGH MYOPIA

High myopia is generally defined as a refractive error of -6.0 dioptres or more, or an axial length of 26.0mm or more, noted Dr Tassignon. But the two measures do not always correlate well.

In a study at Dr Tassignon’s clinic, axial lengths in patients with zero refractive error ranged from 21.50mm to 25.50mm. While no emmetropic patients were seen with axial lengths of 26.0mm or more, about three per cent of patients with -2.0D to -3.0D had axial length of ≥26.0mm. Also, only about one-quarter of patients with -6.0D to -7.0D refractions had axial lengths ≥26.0mm, rising to nearly two-thirds at -8.0D and 86 per cent at -9.0D.

“When we speak of myopia, what is your definition? How do you treat it at the corneal level or in a lenticular plane?” Dr Tassignon asked.

The answer depends on the patient, said Michael Mrochen PhD, founder and CEO of IROC Science, Zurich, Switzerland.

Some patients may not be suitable for corneal surgery at all, while it might be appropriate for a younger patient who still has accommodation, while an older patient without accommodation might be better off with a lens implant. “A treatment paradigm based only on the patient biometrics in my opinion is not really adequate, and might be misleading,” he said.

At the Singapore National Eye Centre, most refractive procedures are LASIK or small incision lenticule extraction (SMILE), though she does not do refractive procedures herself, said Soon Phaik Chee FRCOphth. Long axial lengths are very common in Singapore, which is the most myopic city in the world, and therefore are not usually considered a contraindication for surgery, she added.

“Whether a patient has a lens replacement or corneal refractive procedure does not depend on axial length, but on the absolute sphero-cylinder equivalent. My colleagues correct up to -15.0D provided the cornea is thick enough,” she said.

However, very high corneal corrections can produce poor visual outcomes, noted Abhay Vasavada FRCS, founder of Raghudeep Eye Hospital, Ahmedabad, India.

“Any time you have to do more than 100 micron removal the aberration profile is very high… in reality we don’t end up doing more than -8.0D on the cornea,” he said, generally preferring the Visian ICL phakic intraocular lenses (IOLs) for higher myopes.

But phakic IOLs may accelerate cataract formation, narrowing the target population, said Sonia Yoo MD, Professor of Ophthalmology at Bascom Palmer Eye Institute, University of Miami, USA. She also avoids lens extraction for safety reasons. She prefers phakic lenses in patients aged 30 to 50 years.

“While lens surgery is very very safe, there are some catastrophic things that can happen, particularly in high myopes, including retinal tears and detachments,” Dr Yoo said. By comparison, most complications with laser surgery are relatively easy to treat as long as reasonable treatment parameters are adhered to.

CHALLENGING

Laser surgery is also more economical and better at hitting the refractive target, Dr Yoo added. Multifocal lenses are particularly challenging in myopes because they tend to have retinal issues that lower contrast sensitivity, or maculas that are displaced relative to multifocal image projection, Dr Tassignon said.

Even extended-depth-of-focus lenses are problematic for myopic patients without cataracts, Dr Chee said. About 25 per cent are dissatisfied because of decreased contrast sensitivity even when refractive targets are achieved – which can be quite difficult in highly myopic patients.

The question often comes down to what the cornea can withstand under laser surgery and still deliver good quality of vision. New technology that measures corneal elasticity may help determine in advance what the cornea will do, as will a better understanding of the relationship between the anterior and posterior corneal surfaces.

“Besides biometric parameters we need more information about the biomechanics of the cornea. This is absolutely imperative but we do not have it yet… we hope such devices will come up in the near future,” said Dr Tassignon.

Marie-José Tassignon: marie-jose.tassignon@uza.be

Michael Mrochen: michael.mrochen@irocscience.com

Soon Phaik Chee: chee.soon.phaik@singhealth.com.sg

Abhay Vasavada: icirc@abhayvasavada.com

Sonia Yoo: syoo@med.miami.edu