Orthokeratology and Its Effects on Children
BY LORETTA B. SZCZOTKA-FLYNN, OD, PHD, MS, FAAO
Early evidence indicates that corneal reshaping GP contact lenses, or modern orthokeratology lenses, may slow myopia progression in children. But are the risks worth the benefits? This review will highlight results of the studies examining orthokeratology and myopia control in children performed to date as well as the questions that remain unanswered.
Risks and Benefits
The control of myopia has been the quest of many researchers for decades. In the United States alone, 100 million people are myopic, and most became myopic during childhood. In countries such as Hong Kong, China, Singapore, and Taiwan, the prevalence of myopia is even higher.
Figure 1. This corneal topography image shows the oblate corneal shape produced by orthokeratology.
Because myopic progression occurs through axial eye growth, the control of myopia—hence reduced eye growth—has the potential to decrease ocular morbidity secondary to complications such as retinal detachment, glaucoma, and chorioretinal degeneration. In addition, patients who have lower myopia have better spectacle cosmesis and more predictable refractive surgery results compared to patients who have high myopia.
Various interventions have been attempted to control myopia progression. However, treatments such as alignment fitted GPs, multifocal spectacles, and candidate pharmaceutical agents have had little to no clinical effect in the ability to slow eye growth or myopia progression.
Overnight orthokeratology has demonstrated effectiveness in reducing myopia in adults and adolescents. Note that reduction refers to a decrease in existing myopia, and control refers to less progression of myopia compared to a control group.
However, there are known risks to overnight use of orthokeratology lenses. As in overnight use of soft lenses, sleeping in contact lenses increases the risk of microbial keratitis. In a summary of microbial keratitis related to overnight orthokeratology, Watt et al (2007) reported that about 50 percent of those affected were children younger than 16 years old, and about 75 percent were in East Asia. Therefore, if orthokeratology is effective in controlling myopia in children, then its benefit must well exceed the risk associated with this mode of lens wear in this age group.
Children and Myopia Control
Because of the previously unknown risk-to-benefit ratio, there are no reports in the literature of completed randomized, controlled clinical trials of orthokeratology versus a conventional method of myopia correction to slow myopia progression in children. However, there is anecdotal evidence that orthokeratology lenses slow the progression of myopia in children, and two small pilot studies utilizing historical controls that confirm the effect.
The first study was performed by Reim et al and published in 2003. They used changes in refractive error and base curve of corneal reshaping lenses to monitor myopia progression after three years of therapy in 164 eyes of adolescents. There was no control group. They reported that myopia increased by −0.37D after three years, which appears substantially less that reported for single vision spectacle wearers (about −0.50D to −0.78D per year in their comparisons).
Cheung et al (2004) reported a case of an 11-year-old child in Hong Kong who had monocular myopia—the other eye was nearly emmetropic—and was fit with an orthokeratology lens in the more myopic eye. Over two years of monitoring the axial length, the eye wearing the orthokeratology lens increased by 0.13mm, and the unaided eye increased by 0.34mm with a corresponding increase in myopia.
The two controlled pilot studies were published by Cho et al in 2005 and by Walline et al in 2009. Cho fit 35 7-to-12-year-old children with orthokeratology lenses and monitored eye growth for two years, then compared the study group to a historical single vision spectacle group. Walline fit 40 8-to-11-year-old children with orthokeratology lenses and monitored eye growth for two years, then compared them to a historical soft contact lens-corrected control group.
Both studies reported that the vitreous chamber depth grew significantly less in the orthokeratology treated children compared to the control groups. Walline reported that the orthokeratology eyes grew 0.10mm less per year, and Cho reported that the orthokeratology eyes grew 0.25mm less over two years.
These initial studies suggest that eyes of children treated with orthokeratology lenses grow about 50 percent less compared to eyes wearing traditional myopic corrections. What does this translate to in diopters? A difference in eye elongation of 0.25mm translates to about a 0.50D difference in refractive error. Thus, the roughly 0.25mm difference between the treatment and control groups over two years in the above studies would predict that the children wearing orthokeratology lenses will have 0.50D less myopia (compared to the control group) after two years of treatment.
If extrapolated further, let's presume an 8 year old was fit in orthokeratology lenses for 10 years. Is it possible his myopia would be 2.50D less than it would have been if he had worn conventional correction by the time he was 18? That question remains unanswered. Furthermore, Cho's study revealed that there was tremendous variation in changes in eye length among children and that there was no way to predict the effect for an individual subject.
The prevailing theory for the treatment effect is that the corneal abberations induced by orthokeratology lenses reduce relative hyperopic defocus of the peripheral retina, which is a stimulus for eye elongation. Thus the oblate corneal shape produced by orthokeratology (Figure 1) causes the peripheral light rays to focus anterior to the peripheral retina, which could slow axial eye growth.
Only larger, and ideally randomized, trials that follow subjects over a longer period of time will tell us whether overnight orthokeratology will decrease myopic progression in children. One study, the Stabilizing Myopia by Accelerating Reshaping Technique (SMART) Study, is already underway and will monitor subjects over five years. The unique feature of the SMART Study is that Ascan measurements will be taken after regression to a stable refraction and topography.
Another study is being conducted in Spain (Santodomingo et al, 2009) and yet another randomized trial has begun in China (Cheung, Cho, 2009)
Until the results of these studies are known, evaluate the risk-to-benefit ratio and properly consent your patients using published evidence before initiating this therapy for the control of myopia in children. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #170.
Dr. Szczotka-Flynn is an associate professor at the Case Western Reserve University Dept. of Ophthalmology & Visual Sciences and is director of the Contact Lens Service at University Hospitals Case Medical Center. She has received research funding from Ciba, Vistakon, Alcon, and CooperVision.