All outcomes but lens thickness demonstrated a statistically significant treatment-by-visit interaction, indicating that the pattern of growth between different visits was dependent on the assigned treatment group. The average axial elongation was 0.06 mm/y faster for soft single-vision contact lens wearers than it was for soft multifocal contact lens wearers (visit-by-treatment interaction, p = 0.0048). Post hoc analysis showed that the rate of axial elongation was greater for soft single-vision contact lens wearers after 2 years, but the yearly comparisons between the two groups were not statistically significantly different. The rate of vitreous chamber depth increase was an average of 0.08 mm/y faster for soft single-vision contact lens wearers than it was for soft multifocal contact lens wearers (visit-by-treatment interaction, p < 0.0001), and it was faster during both the first year and the second year. The overall average rate of spherical equivalent refractive error progression was 0.26 D/y faster for soft single-vision contact lens wearers than soft multifocal contact lens wearers (visit-by-treatment interaction, p < 0.0001), and it was greater during both the first and second years of the study. No changes in lens thickness were exhibited by either group (no visit effect, treatment effect, or visit-by-treatment interaction) (Table 4). Anterior chamber depth showed a statistically significant growth in the soft multifocal contact lens wearers, with a difference of 0.05 mm over 2 years (visit-by-treatment interaction, p = 0.0299). The difference occurred between years 1 and 2 when the anterior chamber depth of the soft single-vision lens wearers did not grow while the anterior chamber depth of the soft multifocal contact lens wearers did.
There was a significant correlation between the baseline refractive error and axial elongation for the soft multifocal contact lens wearers (r = −0.39, p = 0.04), but not soft single-vision contact lens wearers who wore lenses for 2 years (r = −0.02, p = 0.94) (Fig. 3).
Soft multifocal contact lenses with a distance center design slowed the growth of the eye by approximately 29%, and they slowed the progression of refractive error by approximately 50%. It is unknown why progression of myopia was slowed nearly twice as much as axial elongation. Perhaps the less precise measurement of axial elongation with a-scan ultrasound, as compared to partial coherence interferometry, was less capable of determining the true axial elongation exhibited by participants, perhaps soft multifocal lenses result in changes to eye shape that affect ciliary tonus and therefore refractive error more than axial length, or perhaps it is due to measurement error.
These findings support the effect of a distance center (with concentric rings of relative plus power) design of soft multifocal contact lens on myopia control reported over a shorter period by Anstice and Phillips,24 an introductory case report by Aller and Wildsoet23 (with concentric rings of relative plus power), and a 1-year trial reported by Sankaridurg et al. (with a progressive increase in relative plus power).25 The results of these four reports indicate a strong need for a long-term randomized clinical trial to determine the effect of soft multifocal contact lenses on myopia control and to determine the mechanism of treatment effect. Sankaridurg et al.25 examined the potential role of peripheral refraction while wearing the contact lenses and found a significant correlation between relative peripheral refractive error and progression of myopia when measured through the multifocal lenses at 30° and 40° in the nasal retina and 40° in the temporal retina. However, they did not rule out the potential myopia control effect of reduced accommodative effort or error (lag of accommodation). Anstice and Phillips24 found significantly slower progression of myopia for the soft multifocal contact lens–wearing eye, even after switching the multifocal contact lens to the contralateral eye. The slower progression of myopia solely in the soft multifocal contact lens–wearing eye indicates the effect may be due to the peripheral refractive error, but that was not directly measured. They also reported that the children accommodated for near, even when they were wearing the multifocal lens. However, the study did not measure peripheral refractive error, so the relative influence of peripheral refractive error and altered accommodation cannot be examined. None of the current studies have measured both peripheral refraction and accommodation while wearing the soft multifocal contact lenses, so more information is needed to determine how soft multifocal contact lenses may slow eye growth. Furthermore, differences in myopia control may be observed for the different soft multifocal contact lens designs (concentric rings of relative plus power versus progressive increase in relative plus power).
Although the add power of all three studies was +2.00 D, the results varied. To readily compare studies, the results are often described in terms of percentage change in progression by dividing the difference in progression of the experimental and control groups by the progression of the control group. In terms of progression of myopia, this study found a 51% treatment effect over 2 years, compared to 36% (−0.69 D versus −0.44 D over 10 months) and 34% (−0.86 D versus −0.57 D over 1 year) for Anstice and Phillips24 and Sankaridurg et al.,25 respectively. We found 29.3% slower axial elongation over 2 years, compared to 50% slower axial elongation (0.22 versus 0.11 mm over 10 months) reported by Anstice and Phillips24 and 33% slower axial elongation (0.40 versus 0.27 mm over 1 year) reported by Sankaridurg et al. The multifocal contact lenses varied by design (progressive increase in add power for the study of Sankaridurg et al. and this study, compared to concentric rings of relative plus power for the study of Anstice and Phillips), as well as the size of the central distance zone. The contact lenses used in this study had a 2.3-mm diameter central zone, compared to a 3.0-mm-diameter reported by Sankaridurg et al. and a 3.36-mm-diameter reported by Anstice and Phillips. Each of these factors could potentially play a role in slowing the progression of myopia, so an investigation to optimize myopia control with a soft multifocal contact lens should be conducted.
Thirteen (32.5%) participants did not provide two complete years worth of data to the study, but the reasons for withdrawals and data on contact lens wear compliance were not collected. Anecdotally, subjects did not complain of poor vision while wearing the soft multifocal contact lenses, and this is confirmed by the fact that subjects who withdrew from the study exhibited better than 20/20 vision at distance and near during their last visit. Although those 13 were similar to the soft multifocal contact lens wearers who remained, there is a greater potential for bias when participants do not complete the study. The treatment groups were matched on age and gender to a historical control group, and statistical adjustment controlled for differences in anterior chamber depth and lens thickness as well as potential differences in spherical equivalent refractive error. However, the treatments were not randomly allocated, which increases the chance of bias due to unmeasured variables.
Data were collected on the historical controls 5 years before the enrollment of the soft multifocal contact lens wearers, and the 2-year data on the progression of myopia were available at the time of matching but were not evaluated before the matching process. The progression of myopia of the soft single-vision contact lens wearers does not explain the observed treatment effect because they progressed precisely at the rate expected for young myopes, −0.50 D per year in the United States.6,10 Furthermore, the mechanism of the treatment effect was not examined, so changes in accommodative effort or error (lag of accommodation) cannot be ruled out as potential causes of slowed progression of myopia.
In a small study such as this one, the presence of subjects with results farther from the mean has the potential to influence the results disproportionately. While within the range of axial length change reported within other studies, we investigated the impact of two potential outliers on the results identified by using an interquartile range. To accomplish this, the 25th and 75th percentiles for the distribution of axial elongation were determined. The difference between the two percentiles represents the interquartile range. To determine the limits for potential outliers, 1.5× the interquartile range is subtracted from the 25th percentile to find the lower bound. This quantity is added to the 75th percentile to find the upper bound. One observation was outside the upper bound. The other fell on the upper bound. The elimination of either outlier with its accompanying matched pair did not change the results as reported. Elimination of both outliers changed the statistical significance for the visit-by-treatment interaction for axial length (now p = 0.086) and anterior chamber depth (now p = 0.056). The difference in axial elongation between the two groups over the 2 years was still statistically significant, but the difference between the two groups from baseline and 1 year decreased from 0.065 mm to 0.051 mm with the elimination of the two subjects. Given that statistical interaction tests possess lower statistical power, it is not surprising the deletion of data from four subjects (two pairs) could result in a change in statistical significance.
Soft multifocal contact lenses with a distance center design may slow the average growth of the myopic eye, and this study suggests that the treatment effect for axial elongation continues to accrue beyond the first year of treatment. The average myopia control effect is similar in magnitude to corneal reshaping contact lens wear and greater than several other types of myopia control. Although this was not a randomized clinical trial, it agrees with the results of Anstice and Phillips24 as well as those of Sankaridurg et al.25 Eye care practitioners may tell parents that soft multifocal contact lenses provide clear vision, and they may also provide myopia control, although not for everyone.
Jeffrey J. Walline
The study was supported by the provision of materials from CooperVision, Johnson & Johnson Vision Care, and Alcon.
Received: December 4, 2012; accepted May 3, 2013.
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