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Daniel L. Adams, Jonathan C. Horton; Shadows from retinal blood vessels cause local amblyopia by deprivation of photoreceptors. Journal of Vision 2002;2(7):123. doi: 10.1167/2.7.123.
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Cataracts deprive the whole retina, rendering the entire eye amblyopic. It is unknown if restricted portions of the retina can become amblyopic from local occlusion. We examined the effect of shadows cast by retinal blood vessels on the geniculocortical projection in 12 squirrel monkeys. Under general anesthesia and neuromuscular blockade, the animals' retinas were photographed and major vascular landmarks were calibrated in the visual fields. Next, one eye was enucleated and the remaining eye (in 2 cases) was injected with [3H]proline. After 10 days, the animals were perfused and striate cortex was processed for cytochrome oxidase (CO) and autoradiography. A CO pattern resembling the retinal vessels was present in the cortex of 9 monkeys. Each element in the cortical representation of the Purkyne Tree could be matched with its corresponding retinal vessel. Autoradiographs showed that cortical territory representing retina hidden by blood vessels was innervated exclusively by geniculocortical afferents serving the other eye. Remodeling of afferents has been described in classic studies of amblyopia from lid suture, but never locally in the cortex from a media opacity (in this case, a natural one) confined to a small area of the retina. Several retinas were plastic-embedded and thin-sectioned to measure blood vessel diameter and distance from blood vessel to photoreceptor layer. Mean pupil diameter was measured in living baby squirrel monkeys. The distance from pupil aperture to retinal vessels was also measured. Vessels as small as 30 µm in diameter were represented in the cortex, indicating that they cast shadows dense enough to cause local amblyopia. Optical analysis revealed that many vessels produced shadows that were entirely penumbra, demonstrating that partial eclipse of the pupil is amblyogenic. We conclude that focal occlusion from retinal blood vessels can produce local amblyopia in normal subjects.
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