Optical correction of induced axial myopia in the tree shrew: Implicationsfor emmetropization

Purpose. To determine whether an active emmetropization mechanism is involv ed in the recovery from axial myopia through the use of a mammalian model o f refractive development. Specifically, we sought to establish whether the emmetropization mechanism is visually guided by the level of clarity of the image falling on the retina, or if recovery is driven by a mechanism sensi tive to abnormal eye shape. Methods. Young tree shrews had axial myopia ind uced by monocular deprivation (MD) of pattern vision and then the myopic ey e was either: (1) accurately corrected with a negative lens or (2) had a ze ro-powered lens placed in front of it. Their emmetropization response was m onitored, both through the use of ocular refractive and biometric measures, as well as through the assessment of scleral dry weight and glycosaminogly can synthesis, as indicators of scleral metabolism, Results. Corrective len ses prevented recovery from induced myopia (-6.8 +/- 0.7 D after 5 days MD vs. -6.6 +/- 0.6 D after 5 days of lens wear), whereas animals fitted with zero-powered lenses displayed near full recovery from the induced myopia (- 6.6 +/- 0.6 D vs. -1.7 +/- 0.3 D). Significant reductions in scleral dry we ight (-4.6 +/- 1.3%) and glycosaminoglycan synthesis (-28.6 +/- 7.3%) were found in the posterior sclera of animals wearing corrective lenses. Convers ely, animals wearing zero-powered lenses displayed elevated levels of glyco saminoglycan synthesis (+62.3 +/- 11.1%) in conjunction with scleral dry we ights that did not differ significantly between treated and fellow control eyes (-1.5 +/- 2.6%), Conclusions. Accurate correction of induced axial myo pia prevents the refractive, biometric and scleral metabolic responses that are normally observed in tree shrew eyes recovering from induced myopia. T hese findings support the hypothesis that recovery is driven by an active e mmetropization response dependent on the clarity of image falling on the re tina and not by a mechanism that is sensitive to abnormal eye shape.