Potency of myopic defocus in spectacle lens compensation.

PURPOSE Previous studies have shown that chick eyes compensate for positive or negative lenses worn for brief periods if the chicks are in darkness the remainder of the time. This study was undertaken to determine whether chicks can compensate for brief periods of lens wear if given unrestricted vision the remainder of the time. Previous studies have also shown that chick eyes alternately wearing positive and negative lenses for brief periods compensate for the positive lenses. The current study sought to determine whether brief periods of positive lens wear can outweigh daylong wearing of negative lenses. METHODS Chicks wore +6 D or +10 D lenses for between 8 and 60 min/d, in two to six periods and wore either no lenses or negative lenses for the remainder of the 12-hour daylight period. Refraction and ultrasound biometry were performed before and after the 3-day-long experiments. RESULTS Wearing positive lenses for as little as 12 min/d (six periods of 2 minutes) with unrestricted vision the remainder of the time caused eyes to become hyperopic and reduced the rate of ocular elongation. These effects also occurred when the scene viewed was beyond the far point of the lens-wearing eye and thus was myopically blurred. Even when chicks wore negative lenses for the entire day except for 8 minutes of wearing positive lenses, the eyes compensated for the positive lenses, as though the negative lenses had not been worn. When chicks wore binocular negative lenses for the entire day except for 8 minutes of wearing a positive lens on one eye and a plano lens on the other, the eye wearing the positive lens became less myopic than the eye wearing the plano lens. CONCLUSIONS Brief periods of myopic defocus imposed by positive lenses prevent myopia caused by daylong wearing of negative lenses. This implies that periods of myopic and hyperopic defocus do not add linearly. If children are like chicks and if the hyperopic defocus of long daily periods of reading predisposes a child to myopia, regular, brief interruptions of reading might have use as a prophylaxis against progression of myopia.