The effect of daily transient +4 D positive lens wear on the inhibition of myopia in the tree shrew.

PURPOSE Negative-lens-induced defocus causes accelerated ocular elongation and myopia, whereas positive-lens-induced defocus produces reduced ocular elongation and hyperopia. Short durations of positive lens wear result in markedly stronger temporal effects than do short periods of negative lens wear in the chick model of refractive development. In mammalian and nonhuman primate models, there have been equivocal results in inhibiting myopia by short periods of positive lens wear when compared with data from the chick model. The purpose of the present study was an evaluation of full-time -9.5 D negative lens wear interrupted by short periods of daily +4 D positive lens wear in preventing experimental myopia in the tree shrew. METHODS One treatment group wore negative lenses (-9.5 D) binocularly for 23 hours a day (10 hours of which were spent in total darkness), interrupted by 1 hour of wearing positive lenses (+4 D) binocularly for 12 days. Another group of animals wore negative lenses (-9.5 D) binocularly for 23 hours a day, interrupted by two 30-minute periods of positive lens (+4 D) wear daily, again for 12 days. The animals were raised on a 14-hour/10-hour light-dark cycle. Animals wearing -9.5 D lenses binocularly, interrupted by 0-powered lenses for either 1 hour or two 30-minute periods daily for 12 days, acted as controls. RESULTS Continuous wear of -9.5 D lenses binocularly induced a -10.8 D myopic shift in refraction. Full-time wear of -9.5 D lenses binocularly, interrupted by 1 hour of 0-power lens wear binocularly, caused a myopic shift of 3.6 D over 12 days, whereas wearing -9.5 D lenses, interrupted by 1 hour every day of +4.0 D lens wear binocularly, whether it was continuous or divided into two 30-minute periods, caused a myopic shift of only 0.7 D over 12 days. CONCLUSIONS Daily intermittent +4 D positive lens wear effectively inhibits experimentally induced myopia and may prove a viable approach for preventing myopia progression in children.