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S. A. McFadden, L. Gulliver, A. Leotta, M. H. C. Howlett; Range of Spectacle Lens Compensation in the Guinea Pig. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3713.
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The guinea pig eye can compensate for both plus and minus defocus imposed by wearing a spectacle lens1,2 and provides a fast and effective mammalian model of myopia3. In this study we determined the full range of spectacle lens compensation (SLC) in the tri-coloured guinea pig.
SLC was induced with a spectacle lens of power -10D,-8D, -6D, -4D, +4D , +6D or ramped to +6D (+2D, +4D, +6D each worn for 4 days) worn over one eye from 6-18 days of age. Comparison was made to a previous experiment in which guinea pigs had worn -4D, -2D, 0D, +2D or +4D lenses over a similar period. Each group contained 8-12 guinea pigs. At the end of the lens-wear period measures were made of: corneal power using videokeratometry; refractive error under cycloplegia using streak retinoscopy and Nidek autorefraction; and axial ocular parameters using high frequency ultrasound (resolution 30 µm) under isoflurane anaesthesia.
Eyes wearing a minus lens developed elongated ocular lengths and myopic refractive errors relative to their fellow untreated eye. Conversely, positive lenses caused the eye to slow its rate of ocular elongation and develop a relative hyperopic refractive error. The thickness of the choroid significantly increased as refractive errors became relatively hyperopic and as vitreous depth and ocular length decreased. The sclera thinned the greater the axial elongation. Ocular compensation to spectacle lenses depended on the sign of defocus, and also the magnitude of the imposed defocus. However, the response was asymmetric, with robust SLC compensation up to -8D, but poor compensation observed to +6D lenses. The asymmetry was exaggerated when estimates of the imposed effective power were calculated. Even when the power of the lens was gradually increased to +6D over the lens-wear period (ramp design) no evidence was found for SLC, suggesting that powers beyond the resolvable range probably induce blur and myopia as occurs in form deprivation.
The guinea pig eye can compensate for -8D to +4D. Such robust and rapid SLC allows the guinea pig to be used as a primary mammalian model to test the underlying biology of myopia and its reversal.1. McFadden, S.A. and Wallman, J. (1995) Guinea pig eyes grow to compensate for spectacle lenses. IOVS (ARVO Abstract), 36(4): 3504.2. McFadden, S.A., Howlett, M.H.C., and Mertz, J.R. (2004). Retinoic acid signals the direction of ocular elongation in the guinea pig eye. Vision Research, 44, 643-653.3. Howlett, M.C., McFadden S.A. (2002) A Fast and Effective Mammalian Model to Study the Visual Regulation of Eye Growth. IOVS (ARVO Abstract), 41 (4), 2928.
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