Share this article / Partagez l'article

Facebook

Twitter

Linkedin

Google

Pinterest

Email

By Shirley Ha, BSc. (Hons), O.D.

Myopia, or nearsightedness, affects approximately 30 per cent of Canadians and more than 1.6 billion people worldwide. The World Health Organization estimates that the number of myopes will grow to 2.5 billion globally by 2020.

Myopia often starts in early childhood and, if undiagnosed or uncorrected, it can impact a person’s educational success and job prospects. It is associated with a greater risk for sight-threatening complications such as cataract, glaucoma and retinal detachment, especially in axial myopia.

Various research projects are being undertaken around the world to reduce and/or find preventative measures for progressive myopia in children. Techniques that are investigated include atropine/pirenzepine, bifocals and progressive addition lenses (PALs), under-correction, soft multi-focal contact lenses, modified spectacle lenses and the controversial orthokeratology (OK).

For decades, eyecare practitioners have used OK to correct moderate myopia and low with-the-rule astigmatism in children and adults. These customized reverse-geometry rigid gas permeable (RGP) contact lenses, also known as non-surgical corneal reshaping (CR), are worn during sleep and removed on wakening and can provide patients with clear vision and all-day freedom from glasses and contact lenses, usually within one to two weeks. The added possibility of myopia control makes OK a very attractive mode of correction, especially in children.

In recent years, multiple studies have provided strong evidence that OK can effectively reduce myopia progression in children and slow axial elongation.[1]^,[2] Impressively, the third-year report of the five-year longitudinal Stabilization of Myopia through Accelerated Reshaping Technologies (SMART) Study[3] and the Controlling Astigmatism and Nearsightedness in Developing Youth (CANDY) Study[4] both showed greater than 80 per cent decrease in axial length in children with OK compared with the soft contact lens control groups.

Researchers theorize that the focus or defocus of the peripheral retina dictates the development of a person’s refraction. The “traditional” contact lens or flat-form spectacle lens encourages ametropia because it does not address the increasing defocus/blur from the centre to the equatorial/peripheral retina created by the difference between the curvatures of the optical lens and the retina. This peripheral defocus can impede the normal correction of ametropia in children called emmetropization and can send a signal within the retina to keep elongating in a myope.[5]

The key to myopia control is to develop treatment modalities that can manipulate the peripheral images back onto, or in front of, the peripheral retina to stop the retinal signal for ocular growth.

Scientists at the Vision Cooperative Research Centre (Vision CRC) in Australia followed their own model and pioneered a new generation of optical products to reduce peripheral hyperopic defocus: the MyoVision™ spectacle lens and silicone hydrogel contact lenses. Their respective clinical trials and studies of their modified products confirmed their hypotheses that decreasing peripheral hyperopia also reduces the rate of myopia progression.[6]^,[7] Both Zeiss and Ciba Vision were given exclusive licensing rights from Vision CRC to incorporate the myopic control technology into their products.

The same myopia control theory can be said to occur in OK as well – the flatter central base curve compressing the central cornea during sleep also displaces the anterior corneal layers outward into the mid-peripheral annular steeper radius to create an artificially more myopic peripheral retina, correcting the hyperopic peripheral defocus and eliminating the stimulus for axial growth.

Unlike refractive surgery, OK is not permanent and discontinuing lens wear will cause the cornea to revert to its original shape. If OK can, in fact, control and decrease the progression of axial length growth and stop the eyes from becoming more nearsighted, then children fitted in early childhood will have their eyes return to the way they were before treatment – when their myopia levels were low – not to where their eyeball lengths would otherwise have been years later when they stop lens wear. This is indeed promising.

The risks associated with wearing OK lenses are similar to those associated with other contact lenses. Doctor supervision and strict adherence to proper wearing schedules and lens maintenance will minimize side effects and make OK safe and effective.

The Canadian Ophthalmological Society does not currently endorse overnight OK and cites microbial keratitis as a major risk factor; OK is recommended by the American Academy of Optometry as an alternative to refractive surgery; the Canadian Association of Optometrists does not have a policy or position paper on the matter.

There are two current FDA-approved technologies for overnight OK lenses: Paragon Vision Sciences’ Paragon CRT^® and Bausch and Lomb’s portfolio of Vision Shaping Treatment (VST) designs. Certification from the manufacturers is required for all practitioners. At present, there are no written guidelines to help practitioners implement the results of OK research into actual clinical practice, except those provided by the manufacturers.

While OK might be highly controversial to some, the rising rates of progressive myopia in children present an incredible growth opportunity for those who are prepared to take on the OK specialty and « reshape » the future of their contact lens practice.