Myopia Update: The Case for More Myopic Defocus
BY DAVID A. BERNTSEN, OD, PHD, FAAO
In the January 2015 issue of Contact Lens Spectrum, an article titled “Controversies in Contact Lens Care” described survey results from an American Optometric Association (AOA) event where vision care professionals were polled on various topics.
One of the questions asked the practitioners which myopia control method they prescribed most often. The top two responses were soft bifocal contact lenses (32%) and orthokeratology (29%); however, bifocal/multifocal spectacles were a close third, (26%), followed by GP contact lenses (12%).
Finding that GP contact lenses made the list of myopia control options is a bit surprising, because two randomized clinical trials found that alignment-fitted GPs do not slow axial eye growth (Katz et al, 2003; Walline et al, 2004). And, even though low concentrations of atropine (as low as 0.01%) have been shown to slow myopia progression (Chia et al, 2014), it was chosen by less than 1% of the respondents.
Of the optical corrections reported, three optical modalities made up 87% of the responses. What is the clinical evidence for each of these modalities? As you will see, when looking at treatment effects across optical corrections, clinical studies seem to suggest that the more retina experiencing myopic defocus, the better. Let’s explore this a little bit further.
Progressive Addition Lenses (PALs)
Bifocal spectacles are among the most widely studied optical corrections for slowing myopia progression in children. Multiple clinical trials using PALs have found clinically small treatment effects. The Correction of Myopia Evaluation Trial (COMET) found a 0.20D (14%) reduction in myopia progression over three years with PALs (Gwiazda et al, 2003).
Even in myopic children with high accommodative lag and esophoria and who were, at one time, thought to benefit most from PALs, COMET2 (2011) found that PALs slowed myopia progression by 0.14D after one year and by 0.28D over three years (24%).
The Study of Theories about Myopia Progression (STAMP), which also enrolled only children who had high accommodative lag, also found a 0.18D treatment effect after one year of PAL wear (Berntsen et al, 2012).
Because of the small effect that mainly occurs in the first year of wear, these studies did not recommend PALs for the purpose of myopia control. However, peripheral myopic defocus on the superior retina caused by the PAL near addition was found to be associated with slower myopia progression (Berntsen et al, 2013).
If the large amount of myopic defocus on the superior retina is responsible for the small PAL-effect, what happens when the proportion of the peripheral retina exposed to myopic defocus increases?
A recent study examining the effect of executive bifocals with a +1.50D add found a treatment effect of 0.81D (39%) over three years (Cheng et al, 2014), with some evidence of a slightly higher effect when adding prism to the bifocal. Although masking was not utilized in this study (outcome data were collected by an investigator who knew the child’s treatment assignment), this result suggests that more peripheral myopia is better. Because the plus addition on an executive bifocal covers the entire inferior portion of the lens, it will cause a myopic shift in defocus on a much larger percentage of the peripheral retina compared to PALs.
This creation of myopic defocus on a larger proportion of the retina could potentially explain why a larger treatment effect was found with executive bifocals using a +1.50D add compared to multiple PAL studies using a higher add power (+2.00D). If exposing more of the retina to myopic defocus is good, this naturally leads us to consider optical designs that allow the creation of myopic defocus throughout the peripheral retina.
There is a growing body of literature demonstrating that overnight orthokeratology slows axial eye growth. Non-randomized studies have reported orthokeratology treatment effects of between 36% to 55% (Cho et al, 2005; Kakita et al, 2011; Walline et al, 2009).
The first randomized clinical trial utilizing ortho-k (the Retardation of Myopia in Orthokeratology [ROMIO] Study) found a 43% reduction in axial eye growth over two years (Cho and Cheung, 2012). Myopic eyes with peripheral hyperopic defocus before beginning ortho-k have been shown to have peripheral myopic defocus after initiating ortho-k (Kang and Swarbrick, 2011).
Interestingly, a randomized study utilizing partial-reduction ortho-k for children who have high myopia found an even larger treatment effect of 63% (Charm and Cho, 2013). In that study, children with –5.00D or more myopia had roughly 4.00D of their myopia corrected with ortho-k and wore spectacles during the day to correct the remaining myopia.
This larger treatment effect could be due to a greater peripheral myopic shift being caused by more significant midperipheral corneal steepening associated with correcting higher amounts of myopia with ortho-k lenses. Eyes that experience larger peripheral corneal power changes after ortho-k are reported to experience slower axial eye growth (Zhong et al, 2014). Because ortho-k induces midperipheral corneal steepening in an annular pattern, the resulting changes in peripheral defocus will occur throughout the peripheral retina.
This leads us to the final, and most popular modality from the AOA survey—soft bifocal contact lenses.
Soft Bifocal Contact Lenses
Rotationally symmetric soft contact lens designs that incorporate an annulus of plus power can create myopic peripheral defocus (Berntsen and Kramer, 2013) similar to the optical effect of ortho-k. Several studies have evaluated the ability of soft bifocal contact lenses to slow myopia progression using various center-distance lens designs (peripheral plus and concentric rings).
Compared to ortho-k studies, the results of soft multifocal contact lens studies are more varied, with some studies reporting treatment effects as low at 25% (Lam et al, 2014), others reporting effects of 72% (Aller et al, 2014), and others falling in between with effects of around 50% (Walline et al, 2013).
One factor likely contributing to this variability in effect size is the contact lens design. The amount of peripheral plus, the size of the central distance zone, and the size of the optic zone all influence the power profile of the lens. Wear time is also a likely contributor to those differences.
An advantage of ortho-k over soft bifocal contact lenses is that the optical benefits of ortho-k are present throughout the day. With soft bifocal contact lenses, the optical benefit is only present as long as the contact lens is worn.
In the study by Lam et al (2014), despite an overall treatment effect of 25%, the effect size was 50% over two years when looking only at children who wore their lenses six or more hours per day. These results support the continued investigation of soft bifocal contact lens designs for myopia control.
More Is Better, It Seems
The results of spectacle and contact lens studies support the continued investigation of optical designs that provide clear central vision while maximizing the amount of peripheral retina experiencing myopic defocus. More work is needed to understand the mechanism regulating eye growth. For now, it seems that the more retina with myopic defocus, the better. CLS
For references, please visit www.clspectrum.com/references and click on document #235.
Dr. Berntsen is an assistant professor at the University of Houston College of Optometry. He has received research funding from the Johnson & Johnson Vision Care Institute and Alcon.