Bifocal soft lenses’ mode of action in doubt

By Joanna Lyford, Senior medwireNews Reporter

Bifocal soft contact lenses and corneal reshaping lenses vary in their ability to provide myopic defocus to the peripheral retina, according to US researchers writing in Optometry and Vision Science.

Indeed, their study failed to show that bifocal soft contact lenses can provide peripheral myopic defocus, raising questions about the mechanism by which these lenses slow axial elongation of the eye.

Anita Ticak (University of Houston, Texas) and Jeffrey Walline (Ohio State University, Columbus) studied 14 myopic individuals aged 8-35 years, of whom nine were female and 12 were White. The average spherical equivalent noncycloplegic manifest refraction for the right eye was -2.84 diopters.

Participants were evaluated for binocular logMAR visual acuity first while wearing bifocal soft contact lenses and again after wearing corneal reshaping contact lenses for a mean of 13.8 days.

At baseline, the average logMAR best spectacle-corrected, binocular, high-contrast visual acuity was -0.14 (Snellen 20/14). This value changed to -0.17 (20/13) while wearing bifocal soft lenses and to -0.09 (20/17) with corneal reshaping lenses.

There were significant interactions between lens type and degree of eccentricity and between degree of eccentricity and gaze direction, report Ticak and Walline. This indicates that bifocal soft and corneal reshaping contact lenses do not provide equivalent optical profiles in the periphery, they say.

Further assessment showed that myopic spherical equivalent refraction increased with increasing eccentricity with the corneal reshaping lenses wear but not bifocal soft contact lens wear.

Furthermore, corneal reshaping lenses provided relative myopia in the periphery and were more myopic than bifocal soft lenses at all 30-degree locations and at all 20-degree locations, say the researchers.

The team concludes that while the two types of lens are widely assumed to slow myopia progression in the same way, results from this study cannot confirm that bifocal soft contact lenses provide myopic defocus to the retinal periphery.

"Perhaps the bifocal soft contact lenses do not provide myopic defocus in the retinal periphery or perhaps an alternate study design, such as turning the head instead of the eyes to minimize contact lens decentration, is required to measure peripheral myopic defocus with soft contact lenses," they conclude.

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