Myopia is reaching worldwide epidemic proportions, with an average annual increase in prevalence from 1% to 2.5% in various countries and ethnicities.^1-4 It is estimated that approximately half of the population will be myopic by 2050 if the current trend of the increase holds.¹ What is more concerning is that the age of onset of myopia is becoming significantly younger, rendering a faster rate of progression, older age of stabilization from progression, and significantly higher risk of irreversible vision loss due to complications such as degenerative and traction maculopathy, retinal detachment, and glaucoma as well as earlier onset of cataract, etc.^5-7

Fortunately, converging evidence from both experimental myopia models as well as clinical studies have shown that at least for the subtype of the condition that is classified as juvenile myopia—which tends to have the age of onset several years after birth­, a relatively lower level of myopia at the time of diagnosis, and a lack of obvious association with other syndromic disorders—optical interventions, such as overnight orthokeratology (ortho-k) and daytime multifocal contact lenses (MFCLs), as well as pharmaceutical treatments such as atropine, may slow down the progression of myopia and axial elongation significantly.^8-12 Moreover, due to the irreversible nature of the pathological changes associated with myopia progression, the earlier the intervention, the better the accumulative benefit for minimizing the risks of serious complications.⁸

This article focuses on using daytime MFCLs, which have been shown to slow down myopia progression by a range of 20% to 70% per year,^13-22 as a myopia control treatment.

PROPOSED MECHANISMS

Although the exact mechanism is not completely understood, MFCLs may inhibit axial growth and myopia progression through one or more of the following factors:

1. imposing relative myopic defocus overlying a simultaneously in-focus image on the retina;^22-24
2. introducing positive spherical aberration to the central retina;²⁵
3. modifying accommodative and vergence response;^18,25,26 and
4. improving behavioral changes, such as increased outdoor exposure, by providing clear vision and improved self-esteem without the constraint from glasses.^27-28

Due to complex interactions among the visual and behavioral impacts of MFCLs, it will be extremely challenging to design animal and clinical studies to tease out the exact amount of influence from each individual factor. Nonetheless, more and more clinical trials with rigorous study design and larger sample sizes are underway to better determine the dose- or design-dependent relationship between MFCLs in varying add powers and optical designs and their efficacies in myopia retardation.

EFFICACY

Multiple studies have investigated the efficacy of myopia control by a variety of MFCL designs, with follow-up durations ranging from 10 months to two years.^13-22 Note that most of the earlier studies utilized MFCLs that were cleared by the U.S. Food and Drug Administration for the correction of presbyopia and were off-label for myopia control in a pediatric population.13,18,22 With more accumulative evidence confirming the consistent anti-myopia effects of MFCLs in children of various ethnicities and countries of residency, more myopia-control-specific MFCLs are under clinical investigation.^14-17,19-21

Overall, the anti-myopia efficacies of MFCLs have been reported to be about 0.31D/year (95% CI, 0.05D to 0.57D) measured by relative refractive change and 0.12mm/year (95% CI, 0.07mm to 0.18mm) measured by axial inhibition between the treatment and the control groups.²² There are significant discrepancies in the percentage efficacy outcomes reported as relative change in refraction versus that in axial elongation in many clinical studies, which can be largely explained by the confounding effect due to the physiological axial growth after birth.

For children who ranged in age from 4 to 8 years old, a physiological axial growth of 0.30mm ± 0.17mm (n = 243) could be accompanied by no change of refraction, and the similar physiological axial elongation was also observed in older children from 8 to 12 years old to a lesser degree (0.12 ± 0.12mm, n = 404).²⁸ This resulted in an under-estimation of the myopia control efficacy using relative axial change as the outcome measure when physiological axial growth was not being accounted for in both the treatment and the control groups, as demonstrated in the hypothetical example in Table 1.

Due to the complexity of the study designs and the significantly higher cost, most of the clinical studies published did not have sufficient power to test dose- or design-dependent efficacies, as single-vision soft contact lenses (SCLs) were used as the control lenses rather than another anti-myopia lens design.

Regardless, when compared to the studies investigating the efficacies of overnight ortho-k in myopia retardation, clinical trials involving MFCLs have the advantages of being able to better mask the subjects and the investigators of the treatment allocation, hence less risks of bias, more balanced retention rate at the end of the trial period between the treatment and the control groups, and, finally, better validity of the study results.

Additionally, the absence of a corneal molding effect with daytime MFCLs offers the ability to monitor both refractive and axial change, which provides a more comprehensive picture of the overall myopia control efficacy compared to just the axial change alone, as with ortho-k.

CLINICAL CONSIDERATIONS

Age The most important factors with respect to the appropriate age for daytime contact lens wear have nothing to do with a number. Patients should be motivated to wear lenses, have a good sense of responsibility for lens wear and care, exhibit an ability to verbalize questions and concerns to parents when necessary, and be able to comply with routine follow-up care, all of which vary considerably due to each patient’s maturity and parental support. We did not find physical age to be a reliable predictor of long-term success with MFCL treatment.

Initial Add Power Selection Currently, there is no clear guideline regarding which add power or which design to select for each patient when using the lenses for myopia control. Although preliminary evidence from animal studies suggested a positive dose-response relationship—in that the higher the add power and the bigger the imposed retinal area of relative plus defocus, the better the myopia inhibiting effect^24-26—caution needs to be taken when selecting a high add power due to its potential compromise on distance visual performance, which has a significant impact on children’s adherence to MFCL wear. At the University of California Berkeley Myopia Control Clinic, we usually start with an add power that is 0.50D to 0.75D more than a patient’s rate of myopia progression in the previous year. This seemed to be a reasonable starting point that offered a good balance between progression risk consideration and the impact of the add power on visual performance.²⁹

Design As there is currently no clear evidence for design-specific differences in myopia control efficacy among various lenses, our priorities when choosing specific designs are lens centration, oxygen permeability of the material, and the replacement modality.

In our experience, temporal lens decentration has a significant impact on distance vision, especially when using a center-distance design. In those cases, patients often need to be over-corrected by –0.75D or more to achieve reasonable distance acuity. This over-minused correction may potentially impact the myopia controlling efficacy of the lens, as the increased minus power is also incorporated in the near optics of the lens, resulting in an overall decrease in the imposed plus defocus to the retina. As a result, for those patients who cannot achieve good acuity with no more than –0.50D of over-correction, a different lens design is often considered.

Monitoring for Rate of Progression and Efficacy of Myopia Control A snapshot of patients’ refractive status and/or the axial lengths provide little information on the effectiveness of any myopia control treatment. Rather, continuous and long-term follow-up visits are critical in monitoring the efficacy of the treatment. Our routine follow-up schedule for MFCLs in myopia control is one to two weeks after initial dispensing, then every six months for those who are progressing slowly (previous annual rate of progression ≤ –0.75D) and every three months for those who are progressing faster.

Despite the inconsistency in the efficacy outcomes reported as relative change in refraction versus axial elongation in many clinical studies, monitoring for changes in both is equally important, as they both provide critical information in understanding the overall ocular growth. While refractive change offers the most direct understanding of the rate of myopia progression, axial changes provide combined insights on both physiological and myopia progression-specific elongation of ocular structure.

Progressive Myopes Who Have Significant With-the-Rule Corneal Toricity In cases in which satisfactory visual performance cannot be achieved with soft toric multifocal lenses, large-diameter bitoric corneal GP lenses with a front-aspheric multifocal design are a viable alternative to provide optimal centration, good comfort and fitting, and anti-myopia optics on the lenses.

SUMMARY

Overall, MFCLs provides both statistically and clinically significant benefit in myopia retardation compared to single-vision spectacles or SCLs. The relatively simple fitting procedures and shorter chair time compared to that of overnight ortho-k treatment offer a strong argument of MFCLs as one of the primary options for myopia control.

We are eager to see more clinical studies providing better insights on the “optimal optical dosage” for myopia control and novel designs that are specifically custom-tailored for progressive myopic children. CLS

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