From Medscape Medical News
January 11, 2010 — Bifocal lenses can moderately slow myopic progression in children with high rates of myopic progression after 24 months, according to new research published in the January issue of the Archives of Ophthalmology.
"Myopia is a common refractive problem, particularly in East Asia, where reported prevalence values in children can be as high as 50% to 60% by the age of 12 years. Prevalence of myopia is also high among Asian children living in Western countries," write Desmond Cheng, OD, MSc, PhD, formerly from Queensland University of Technology, Brisbane, Australia, and currently from Hong Kong Polytechnic University, Hong Kong, and colleagues.
"A number of well-designed prospective studies have investigated the effect of positive lenses, in bifocal or multifocal form, on myopic progression in children. However, bifocals and multifocals have proven to be relatively ineffective myopia-control treatments in children," the authors write.
Myopic progression rate appears to be an important factor in determining the effectiveness of multifocal lens treatment, the authors point out.
The aim of this study was to determine whether bifocal and prismatic bifocal spectacles could control myopia in children with high rates of myopic progression, defined as 0.5 diopters or more in the preceding year.
The study randomly assigned 135 Chinese Canadian children (73 girls and 62 boys) with myopia of at least 1.00 diopter to receive single-vision lenses (n = 41), +1.50-diopter executive bifocals (n = 48), or +1.50-diopter executive bifocals with a 3-prism diopter base-in prism in the near segment of each lens (n = 46).
The authors explain that their randomization was done by putting subjects' file numbers on slips of paper and drawing them from a container at random. In addition, subjects and the investigator were aware of the treatment assignments, as blinding was difficult to achieve because the lens treatments were visually very different.
The mean age of the children was 10.3 years (standard error [SE], 0.15 years), and their mean visual acuity was −3.08 diopter (SE, 0.10 diopter).
Myopic progression was measured by an automated refractor under cycloplegia, and increase in axial length was measured by A-scan ultrasonography at 6-month intervals for 24 months.
Of the original 135 children, 131 (97%) completed the trial at 24 months.
The results showed that myopic progression averaged −1.55 diopter (SE, 0.12 diopter) for children who wore single-vision lenses, −0.96 diopter (SE, 0.09 diopter) for those who wore bifocals, and −0.70 diopter (SE, 0.10 diopter) for those who wore prismatic bifocals.
Significant Effect of Lens Design
There was a significant effect of lens design on the degree of myopic progression (P < .001), the authors report. Compared with the single-vision lens group, the magnitude of mean myopic progression was −0.59 diopter (P < .001) in the bifocal lens group and −0.85 diopter (P < .001) less in the prismatic bifocal lens group.
The study also found that axial length increased an average of 0.62 mm (SE, 0.04 mm), 0.41 mm (SE, 0.04 mm), and 0.41 mm (SE, 0.05 mm) in the single-vision lens, bifocal lens, and prismatic bifocal lens groups, respectively.
The treatment effect of bifocals (0.59 diopter) and prismatic bifocals (0.85 diopter) was significant, with a P value less than .001, and both bifocal groups had less axial elongation than the single-vision lens group (0.21 mm; P < .001), the authors report.
The findings of this study could be generalized to children with rapidly progressing myopia, irrespective of ethnicity, although this clearly needs to be tested, the authors state.
Limitations of the study include the use of an atypical randomization scheme to assign subjects to treatment groups and potential bias because the investigator was not masked.
The authors concede that the treatment effect of bifocal and prismatic bifocal lenses of 38% and 55%, respectively, that they found in their study — although greater than those in other studies — is still modest.
"Whether or not the effect tapers off will decide clinical significance," they write. "If the treatment effects continued over time, then the treatment could have a significant role in preventing the development of very high pathologic myopia."
For now, they conclude, bifocals should be offered to myopic children "with caution" in clinical practice.
Dr. Cheng has reported no relevant financial relationships.
Arch Ophthalmol. 2010;128:12-19.