PURPOSE. To address the question of whether negative lenses and diffusers stimulate increased eye growth by the same or different mechanisms. METHOD. White-Leghorn chicks were used in this study. Their right eyes were fitted with either A. diffusers, B.-15 D lenses or C. diffusers exchanged with-15 lenses (n=14 per group). This combination was chosen because diffusers and-15 D lenses produce similar responses in young chicks over the 4 day treatment period used here. For group C, treatments were switched every 4 hr across the 12 hr day, with equal numbers of chicks starting the day with each of the treatments. To control for the brief exposure to normal vision during switching, a similar regimen of removal and replacement was followed for the other two groups. Treatment effects were monitored by high frequency A-scan ultrasonography under halothane anesthesia.
RESULTS. The consistent trend in the results was for the group subjected to the exchange paradigm to show a reduced response compared to the other two treatment groups although only the axial length effect proved to be significantly different. Changes in interocular differences (mm) over the treatment period for -15 D lens, diffuser, -15 D lens/diffuser respectively were: Vitreous chamber: 0.46 ±0.12, 0.50 ±0.18, 0.43 ±0.11; Choroidal thickness: -0.079 ±0.063, -0.089 ±0.053, -0.062 ±0.077; Axial length*: 0.52 ±0.14, 0.58 ±0.21#, 0.42 ±0.13#.
ANOVA:* F2,39= 3.51, p<0.05; #significantly different, p<0.05
CONCLUSIONS. These data argue that negative lenses and diffusers represent different treatment paradigms. This result, considered in the context of findings from human myopia studies, argues for the lens paradigm to be used in modeling normal human myopia.