Visual control of eye growth and risk factors for myopia development
Introduction: Chickens and monkeys raised with spectacle lenses develop predictable refractive errors. In the case of negative lenses, which place the plane of sharp focus behind the retina, the eye elongates and becomes myopic and, in the case of positive lenses, it develops axial hyperopia. The following questions emerge: (1) how can the eye determine the position of the plane of sharp focus and how is this information transmitted to the underlying sclera? (2) how could the growth commands be pharmacologically suppressed? (3) which consequences have the spectacle lens experiments for the understanding of human myopia and how could the critical visual experience be prevented?
Methods: By using hemi-field lenses, which defocus only part of the visual field, it was tested in the chicken model of myopia whether refractive errors can be locally induced. By raising chickens with lenses under cycloplegia and with only one possible viewing distance, it was studied whether the retina can determine the sign of imposed image defocus. By quantitative immunohistochemistry it was studied which retinal cells and after which exposure time carry the information on the position of the image plane. By automated infrared photoretinoscopy, the precision of accommodation during reading was studied in human subjects.
Results: (1) Visual eye growth control is mainly a matter of the retina. Already after 30 minutes of treatment with spectacle lenses, the glucagonergic amacrine cells in the chick retina carry the information on the average position of the image plane. (2) Possible candidates für the signal transduction are glucagon and retinoic acid. Glucagon antagonists inhibit hyperopia development, glucagon itself, and inhibitors of retinoc acid synthesis, suppress myopia development. (3) Since axial eye growth is stimulated when the image plane is behind the retina, it is important to avoid this situation in daily visual experience. During near work, like reading, human subjects display considerable under-accommodation (up to 1 D). That this could add up to a critical daily dose (comparable with negative lens wear) is suggested by animal expriments.
Discussion: In addition to a genetical pre-disposition, visual experience is a critical factor in the development of myopia. Myopia development could be blocked by pharmacological inhibition of the visual fedback loops controlling axial elongation or by avoiding the critical visual exposure. Related studies in children are currently underway.
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