Failure of the eyes to align properly to focus on an object. The affected eye may deviate in any direction, including inward (cross-eye) or outward (walleye). Problems with photoreception or the nerves that relay images to the brain cause a constant degree of deviation (comitant); defects in the nerves that control the muscles that move the eyes cause deviation that varies with the direction of gaze (noncomitant). Both types impede development of a child's ability to focus the eyes and merge images from the two retinas into one (fusional reflex). The brain suppresses the image from the deviant eye, which may become functionally blind. Treatment may involve exercises to strengthen the weak eye or surgery or both.
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