A prescription for bifocals may help children with rapidly worsening nearsightedness maintain better vision, a new study suggests.
Children with nearsightedness, or myopia, have trouble seeing objects clearly at a distance, and for some, the condition progresses quickly. The tendency toward rapid progression is believed to be inherited, and it has been unclear whether anything can be done to slow it.
One potential tactic is to prescribe children bifocals — lenses that are split into two sections, with the upper half for distance vision and the lower half for near vision. But studies so far have yielded mixed results.
For the new study, reported in the Archives of Ophthalmology, researchers recruited 135 Chinese-Canadian children with rapidly progressing nearsightedness. Nearsightedness is particularly common among people of Asian descent, affecting up to 90 percent of some populations.
The researchers randomly assigned the children to either keep wearing single-focus lenses or get a new prescription for bifocals; half of the latter group were given prism bifocals, which help the eyes work together.
Over the next two years, children wearing either type of bifocal showed a moderately slower rate of nearsightedness progression, the researchers found.
Nearsightedness is described in terms of diopters; at the start of the study, the children's nearsightedness averaged about -3.00 diopters, which is generally considered to be moderate myopia. All had been showing a rate of progression of at least -0.5 diopters in the year before the study.
Over the two-year treatment period, children who stayed with single-focus lenses had an average rate of progression of -1.55 diopters. Those wearing bifocals or prism bifocals, meanwhile, fared better, with rates of progression of -0.96 and -0.7, respectively.
Myopia that reaches levels of -6.00 diopters or beyond is considered more severe. In the long term, more-severe myopia carries greater risks of retinal detachment, glaucoma and cataract, noted Dr. Desmond Cheng, the lead researcher on the study.
The benefits of bifocals in slowing myopia progression were "modest" and it remains to be seen whether the treatment effect lasts or tapers off, according to Cheng, an optometrist in Ontario, Canada, and an adjunct associate professor at Hong Kong Polytechnic University.
For now, he told Reuters Health in an email, the potential benefits of bifocals must be weighed against the downsides — including the lenses' greater expense and "poor cosmetic appearance."
As for why bifocals might help these children, Cheng explained that, for most people, objects viewed at a close distance do not focus completely on the retina, but slightly behind it — in what is called hyperopic defocus.
If that situation is maintained for long periods — as may be the case when a person does a lot of reading or other "near work" — it may actually contribute to nearsightedness progression, according to Cheng. Bifocals, he said, interrupt this hyperopic defocus, and may thereby help slow myopia progression.
SOURCE: Archives of Ophthalmology, January 2010.