The Latest on PALs and Myopia Control
By David A. Berntsen, OD, PHD, FAAO
Bifocal spectacles and progressive addition lenses (PALs) have been the most widely studied optical corrections for slowing myopia in children. In the last decade, the results of multiple clinical trials conducted in the United States and abroad have been reported. In general, studies using PALs have found treatment effects that were clinically small and have not warranted a change in clinical practice.
The largest PAL clinical trial in the United States was the Correction of Myopia Evaluation Trial (COMET). This well-done, multi-center study found a 0.20D (14 percent) reduction in myopia progression over three years in PAL-wearing children (+2.00-D add) compared to children wearing single vision lenses (SVLs) (Gwiazda et al, 2003). Figure 1 shows a summary of the percentage decrease in myopia progression reported by several clinical trials.
Figure 1. Percent reduction in myopia progression reported by several bifocal and PAL clinical trials.
Although the results from COMET and other bifocal studies have not suggested that we fit all myopic children with PALs, certain subgroups of COMET children who had high accommodative lag were reported to have a greater treatment effect (Gwiazda et al, 2004). PAL-wearing children in COMET who had a high accommodative lag and near esophoria had 0.39D less myopia progression than did SVL-wearing children after one year and 0.64D less progression over three years. PAL-wearing children who had high accommodative lag and low myopia (spherical equivalent of –2.25D or less myopia) also had significant one- and three-year treatment effects in COMET of 0.28D and 0.48D, respectively.
Others have also reported that PALs were more effective in children who have a high accommodative lag (Hasebe et al, 2008). These findings supported further investigation of whether certain myopic children who have high accommodative lag may clinically benefit from wearing PALs.
PAL Trials in Children Who Have High Accommodative Lag
So what can we expect when children who have a high lag of accommodation wear PALs? Thus far, two clinical trials have looked at this question. Both studies limited enrollment to various subsets of myopic children who have high accommodative lag that were previously reported to have a clinically meaningful treatment effect.
COMET2 was a three-year, multi-center clinical trial that limited enrollment to myopic children who had high accommodative lag, near esophoria, and low myopia. The study found that PALs (+2.00-D add) slowed myopia progression by 0.14D compared to wearing SVLs after one year and by 0.28D over three years (COMET2, 2011).
The Study of Theories about Myopia Progression (STAMP) was a two-year clinical trial that randomly assigned myopic children to wear PALs (+2.00-D add) or SVLs for one year. All children wore SVLs for a second year to determine the permanence of the treatment effect. Children in STAMP had a high lag of accommodation and were also required to have near esophoria and/or low myopia. The study found that PALs slowed myopia progression by 0.18D after one year (Berntsen et al, 2012).
Though previous studies suggested that a clinically meaningful treatment effect (0.25D or greater after one year) would be found in these children who have high accommodative lag, the one-year PAL effect sizes for COMET2 (0.14D) and STAMP (0.18D) were not as large as expected. The one-year effect sizes in these two studies of children who have high accommodative lag were similar to the original COMET one-year effect size of 0.18D reported for all myopic children. Although the COMET2 three-year reduction in myopia progression was statistically significant, the 0.28D (24 percent) treatment effect was clinically small.
Treatment Effect Permanence?
Regardless of the treatment effect size, any effect of wearing optical lenses must be permanent for it to ultimately be of benefit in controlling myopia.
Until recently, no study had examined treatment effect permanence after ceasing to wear an optical treatment (such as PALs) in a fully randomized sample of children. All children in STAMP wore SVLs during the second year of the study. Although the treatment effect after one year of PAL wear in STAMP was small, no difference in the progression rate of myopia was found between the SVL control group and the former PAL-wearing group during the second year of the study (Berntsen et al, 2012). At least for the first year after discontinuing PAL wear, the treatment effect was maintained, which supports the continued investigation of new optical treatments that might be better at slowing the rate of myopia progression.
Origin of the Treatment Effect
The small but consistent PAL treatment effect seen across multiple clinical trials has traditionally been thought to result from decreased hyperopic retinal blur during near work due to PALs reducing accommodative lag; however, the few studies that have looked for an association between accommodative lag and myopia progression in myopic children have not found a significant relationship (Weizhong et al, 2008; Berntsen et al, 2011; Berntsen et al, 2012).
Based on work in animals showing that optically induced hyperopic defocus in the peripheral retina causes accelerated axial eye growth (Smith, 2011), it is possible that the small reduction in myopia progression with PALs may be due to the PAL add corridor causing myopic defocus on the superior retina.
More work and research is needed to better understand the mechanism behind the small but consistent PAL treatment effect.
While PALs have not yielded the reductions in myopia progression that we had once hoped for, other optical treatments show promise. Success has been reported with orthokeratology lenses. While we wait for the publication of the peer-reviewed results from the first orthokeratology clinical trial (Retardation of Myopia in Orthokeratology or ROMIO), a press release from the ROMIO study reported a treatment effect of 43 percent over two years.
Novel contact lenses that attempt to induce peripheral myopia are also currently being investigated (Sankaridurg et al, 2011).
Time—and long-term research studies—will tell whether optical designs can be optimized to maximize the reduction in myopia progression. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #197.
|Dr. Berntsen is an assistant professor at the University of Houston College of Optometry. He is also a consultant to B+L.|