Orthokeratology and Myopia Control—What Do We Know?
By David A. Berntsen, OD, PhD, FAAO
Various types of optical correction have been studied to determine their ability to slow the progression of myopia in children. The most widely studied interventions to date have been bifocal spectacles and progressive addition lenses, but clinical trials of these modalities have had mixed results. Some of these trials have found no treatment effect, while others have found statistically significant but clinically small effects, with a few exceptions in certain subgroups of children (Cheng et al, 2011).
Most researchers consider a 50-percent reduction in myopia progression per year to be clinically meaningful. Therefore, in the United States where, on average, myopia increases by 0.50D per year (Goss and Winkler, 1983), a 0.25D effect per year would be considered clinically relevant. Various optical designs are being studied to assess their ability to slow myopia progression in children, and orthokeratology is one of those designs.
|Table 1. Mean Changes in Axial Length (AL) in Orthokeratology (OK) Studies|
|Study||Two-year AL Increase for Control Group||Two-year AL Increase for OK Group||Difference in AL Growth (Control - OK)||% Reduction in AL Growth in OK Group|
Chart Reviews Reveal Potential
Orthokeratology was first mentioned in the literature by Jessen in 1962, but it was initially plagued by unpredictable results and variable vision. Advances in lens materials and technology led to today's reverse geometry, overnight ortho-k lenses, which produce rapid, predictable reductions in myopic refractive error by reshaping the cornea.
Starting less than a decade ago, reports from retrospective chart reviews as well as case reports began suggesting that orthokeratology may reduce myopia progression (Reim et al, 2003; Cheung et al, 2004).
Several studies have now been published examining the effect of overnight orthokeratology on myopia progression in children. All to date have used either historical controls from a previous study or they have allowed children to decide whether they would be in the ortho-k group or in the control group. Although these studies were not clinical trials, in that they did not randomly assign patients to a group, the preliminary results are promising and support further investigation of orthokeratology as a possible myopia-controlling modality.
Because well-fitted ortho-k lenses result in an essentially emmetropic refractive error, most studies have examined the change in axial length or vitreous chamber depth to determine the effect of ortho-k on myopia progression. Table 1 shows the change in axial length and the percent reduction in axial growth reported by previously published studies. Although the reduction in axial growth ranges from 33 percent to 56 percent in these studies, they all report a reduction in eye elongation of about 0.22mm to 0.32mm over two years, which corresponds to a reduction in myopia progression of approximately 0.50D to 0.75D over that time.
Cho et al (2005) published the first study comparing axial eye elongation in children wearing ortho-k lenses to those in a control group; they reported a 46-percent reduction in axial elongation and a 52-percent reduction in vitreous chamber growth in the ortho-k group. Walline et al (2009) later reported a 56-percent reduction in axial elongation and a 43-percent reduction in vitreous chamber growth. These two studies used historical control groups from previous myopia clinical trials (single vision spectacle lens wearers and soft contact lens wearers, respectively). Importantly, both studies demonstrated a continued cumulative treatment effect after the first year of orthokeratology.
More recent studies by Kakita et al (2011) and Santodomingo- Rubido et al (2011) allowed children to select either single-vision spectacle lenses or ortho-k lenses. Both studies found a reduction in axial elongation of 0.22mm to 0.23mm in the ortho-k group over two years, although the overall percent reduction in eye elongation was slightly lower in these two studies. The latter study also reported that children being treated with ortho-k lenses had improved vision-related quality of life compared with children wearing spectacles.
Although these studies support a role for orthokeratology in decreasing eye elongation, a clinical trial is still the gold standard for confirming the efficacy of a treatment. The Retardation of Myopia in Orthokeratology (ROMIO) Study is the first clinical trial to randomize children to orthokeratology or single-vision spectacles. Although the ROMIO results are not yet published in the peer-reviewed literature, a January 2012 press release reported that axial length in children assigned to wear spectacles increased by 0.63mm over two years while the increase in children assigned to orthokeratology was only 0.36mm, a 43-percent reduction in axial elongation.
Mechanism of Action
In addition to knowing whether or not orthokeratology slows axial eye growth, understanding the mechanism by which these lenses act is also important. The prevailing hypothesis cited by the papers described above involves the myopic shift in peripheral retinal defocus caused by the change in corneal shape with orthokeratology. Work in animal models has shown that local retinal regions respond to local defocus signals and that inducing peripheral hyperopic defocus can accelerate central axial eye growth even when the fovea has a clear image (Smith, 2011). In humans, we know that myopic eyes are typically more hyperopic in the periphery than at the fovea, while hyperopic eyes are usually more myopic in the periphery than at the fovea (Millodot, 1981; Mutti et al, 2007). Children who have myopia and wear standard spectacle lenses experience peripheral hyperopic defocus (Lin et al, 2010), but myopic eyes fitted with orthokeratology experience a change from having peripheral hyperopic defocus (a potential “grow” signal) to peripheral myopic defocus (a stop signal), which may explain why orthokeratology slows myopia progression (Kang and Swarbrick, 2011). More work is needed to test the peripheral blur hypothesis to determine whether this change in peripheral defocus is, in fact, responsible for reduced eye growth when children wear ortho-k lenses.
Because ortho-k lenses are worn overnight, safety is a valid concern. A retrospective study of 639 adults and 677 children wearing ortho-k lenses estimated that the incidence of microbial keratitis in these patients was 7.7 per 10,000 years of wear, with an upper 95 percent confidence interval limit of 16.7 per 10,000 years of wear (Bullimore et al, 2009). Although the incidence of microbial keratitis for overnight orthokeratology wear is greater compared to the reported incidence for daily contact lens wear, the incidence of microbial keratitis in patients being treated with orthokeratology appears to be similar to that of other overnight modalities (Stapleton et al, 2008).
As we await the official published results from the ROMIO trial, we know early evidence suggests a role for orthokeratology in myopia control. More work is needed to evaluate the mechanism responsible for the reduced axial growth with orthokeratology, but these studies will help us educate our patients about the modality's potential benefits. CLS
To obtain references for this article, please visit http://www.clspectrum. com/references.asp and click on document #195.
|Dr. Berntsen is an assistant professor at the University of Houston College of Optometry. He is also a consultant to B+L.|