Abstract
Abstract: :
Purpose: The recent finding that interchanging diffusers and -15 D lenses reduces the response relative to either device alone in normal chick eyes (Wong et al., 2001) implies that different mechanisms underlie lens- and deprivation-induced ocular growth increases. This study was undertaken to investigate whether these mechanisms are also different in optic nerve-sectioned eyes. Methods: Optic nerve sectioned (ONS) eyes of 11 day-old chicks were fitted with either a diffuser, a -15 D lens or the two devices interchanged at 3 hr intervals across each 12 h-day (n=6 per group). In the first two cases, the devices were also removed but then replaced at the same time intervals. Measurements were made after 4 days using retinoscopy for refractive errors (RE) and high frequency A-scan ultrasonography for axial ocular components. Results: ONS eyes were initially more hyperopic (mean, SD: +6.51 3.23 D) than their fellow eyes (+3.48 0.69 D). All 3 treatment groups became progressively myopic thereafter (p<0.0001). RE changes in the diffuser group (-13.79 3.15 D) were significantly greater (p<0.05) than those for the lens group (cf. -7.23 1.13 D), while those for the exchange group (-8.61 2.81 D) lay between the two. The myopia was axial in origin in all cases. Thus all groups showed greater vitreous chamber growth in their ONS eyes relative to their fellow eyes (p<0.0005) with the changes in growth showing similar intergroup differences to the refraction data (diffuser: 0.67 0.13 mm; lens: 0.34 0.06 mm; exchange: 0.47 0.13 mm). Changes in axial length also mirrored the refraction data (diffuser: 0.85 0.24 mm, lens: 0.59 0.23 mm; exchange: 0.73 0.13 mm; ). Conclusions: The findings that diffusers induced greater changes than lenses in ONS eyes and that the exchange group effects lay between the other groups together suggest that mechanisms of myopia induction are different in ONS eyes compared to normal eyes. The data also suggest that the mechanisms underlying form deprivation- and lens-induced myopia are different.
Keywords: 481 myopia • 316 animal model • 315 anatomy