In this randomized trial of 118 subjects aged 8 to <12 years with low myopia, high accommodative lag, and near esophoria, the PALs were found to slow the progression of myopia by 0.28 D (95% CI, 0.01–0.55 D) over 3 years. This result, while small in magnitude, is statistically significant and, combined with findings in other studies, suggests a true biological effect of slowed eye growth in children wearing PALs.14
The most prominent hypothesis for the development and progression of myopia is that children with insufficient accommodation when engaged in near work activities may experience retinal defocus that may lead to axial elongation and myopia.21,22
Because PALs would be expected to provide clearer vision across a range of viewing distances, one would expect that they would reduce defocus and might lead to slower eye growth.
This hypothesis was in part the rationale for the original COMET and its exploratory analyses of accommodative lag, which showed that the 3-year treatment benefit of PALs on progression of myopia was larger in subgroups of children with higher accommodative lag and near esophoria (0.64 D; 95% CI, 0.08–1.19 D; n
= 76) and in children with higher accommodative lag and lower baseline myopia (0.48 D; 95% CI, 0.02–0.93 D; n
= 104) than in the overall cohort (0.20 D; 95% CI, 0.06–0.33 D; n
Analysis of data from a smaller subgroup of 35 COMET subjects meeting the COMET2 eligibility criteria for age, baseline myopia, accommodative lag, and near esophoria showed a 0.71-D (95% CI, 0.22–1.20 D) 3-year treatment effect of PALs (COMET, unpublished data, 1997–2001). The treatment effect found in COMET2 is not significantly different from that found in each of these three subgroup analyses from the original COMET study14
= 0.25, 0.46, and 0.12 for Cochrane's Q tests for heterogeneity comparing COMET2 results versus the results in each COMET subgroup, respectively).
Two other recent clinical trials of spectacle lenses for myopia control also reported statistically significant treatment effects in subgroups of children with high accommodative lag.15,16
Hasebe et al.,15
in a study of myopic Japanese children randomized to PALs or SVLs for 18 months, reported that a subgroup of 36 children with accommodative lag of at least 1.80 D had an 18-month treatment effect of 0.61 D (95% CI, 0.30–0.92 D), with myopia in the PAL group progressing by 0.87 D and in the SVL group by 1.48 D. The use of standard bifocals versus SVLs in a study by Cheng et al.16
of Chinese Canadian children with high rates of previous myopic progression significantly reduced progression over 2 years (by 0.88 D; 95% CI, 0.45–1.31 D) in a subgroup of 43 children with lags of accommodation of at least 1.00 D (progression in the standard bifocal group was 0.88 D and in the SVL group was 1.76 D). For comparison, in the present study the treatment effect of PALs was 0.17 D (95% CI, 0.02–0.32 D) at 18 months and 0.23 D (95% CI, 0.02–0.45 D) at 2 years. The treatment effect found in COMET2 is significantly smaller than those found in the subgroup analyses in Cheng et al.16
and Hasebe et al.15
at the same time points (P
= 0.01 for Cochrane's Q tests for both comparisons).
Methodological differences among all the studies may have affected the results. First, the lens type used was different in each study: the Ellipse in the present study, the Varilux Comfort in COMET (Essilor of America),13
the Sola MCLens (Sola International, San Diego, CA) in Hasebe et al.,15
and Essilor Executive bifocals (Essilor of America) in Cheng et al.16
While many details of the lens designs are proprietary, it is known that the Varilux Comfort lens has a wider distance and intermediate zone and more induced astigmatism than does the Ellipse lens.23
In these lenses, the widths of the near zones vary greatly with fitting height; the Comfort's near zone is narrower than the Ellipse's for a fitting height of 14 mm, which is the minimum recommended for the Ellipse.23
Also, the present study and the original COMET study13
used a +2.00-D near addition, whereas Cheng et al.16
and Hasebe et al.15
used a +1.50-D addition. These lens variations may have produced differences in peripheral aberrations and in the power of the addition used for near work, both of which could affect progression.24,25
Second, high accommodative lag was defined as 1.00 D or more in the current study and in Cheng et al.,16
greater than 0.43 D in the original COMET study,13
and 1.80 D or more in Hasebe et al.15
Third, the other studies had higher rates of myopia progression, most likely because they included children who were younger15
factors that are associated with increased progression.26,27
Also, Cheng et al.16
was limited to children who had previously demonstrated fast progression. It might be expected that treatments would work better when myopia is still progressing rather than when it is close to reaching a plateau.
We found no statistically significant associations between baseline factors and treatment effect; however, our study was underpowered to detect these types of associations unless they were very strong. Although not statistically significant, there may be some evidence in our data that children with lower myopia might benefit more from PALs. This finding is consistent with those in the COMET which showed greater treatment benefit among subjects with baseline myopia of −1.25 to −2.24 D compared with those with −2.25 to −4.50 D, both in the overall cohort and in the subgroup of subjects with higher accommodative lag.14
In addition, although COMET2 enrolled only subjects with larger lags of accommodation, there is a suggestion that, within this group, the children with higher lag (≥1.50 D) had an increased treatment effect, consistent with the COMET study's finding that higher lag was associated with increased treatment effect across a wider range of lags.
Strengths of the present study include the use of a double-masked design with random assignment to treatment groups and training and certification of study investigators. In addition, missing data did not appear to influence the results. Another strength is an adequately powered sample size and a high rate of follow-up overall, although a limitation is that there was higher loss to follow-up in the PAL (n
= 7) group than in the SVL (n
= 1) group. Another limitation is that compliance with lens wear was reported to be worse in the PAL group, a factor that could have diluted the treatment effect of the PALs. However, this difference in compliance did not appear to affect progression. In the present study, as in other studies of PALs or bifocals for control of myopia,15,16
we prescribed a consistent near addition power for all children, +2.00 D. It is possible that if this power were customized for individual children based on the level of myopia, near accommodative response, and phoria, then the treatment benefit of the lenses might be enhanced. In addition, although we collected estimates of compliance with spectacle wear, we do not have data on what part of the lens the children were actually looking through during near work, and so it is possible that the full benefit of the near addition was not realized.
The treatment effect in this study increased over the 3 years rather than maintaining the same level as found in the first year, the latter of which is a more typical finding with various lens treatments.17
The pattern in the present study is similar to that in the subgroup of COMET subjects with higher lag,13,14
but different from that in the overall COMET cohort.13
Given the current result, a longer trial might find a greater treatment effect of PALs.
The consistent presence of an effect of PALs or bifocals in different studies suggests that the visual environment plays a definite, but perhaps modest, role in myopia progression, although it is not completely clear what in the environment is being manipulated with the lenses. COMET2 was designed to examine the effects of high accommodative lag and the resultant central hyperopic defocus. Recent work28
indicates that an important additional mechanism may be peripheral hyperopic defocus, which was reduced in the lower visual field only with the PALs, in which case the effect might be stronger if defocus was reduced in the entire visual periphery. However, a recent study evaluating three different spectacle lens designs for reducing peripheral hyperopic defocus versus SVLs found no statistically significant differences in the 1-year progression of myopia.29
These results suggest that there is much more to be learned about the mechanisms of eye growth. It is hoped that when the mechanisms are better understood, more powerful, clinically useful lens treatments for slowing the progression of myopia may be developed.
In conclusion, the PALs used in this study were found to have a statistically, but not clinically significant effect on slowing myopia progression in children with high accommodative lag and near esophoria.