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Blind spot (scotoma) or area in the normal field of vision. It may be persistent or temporary and shifting, as in a migraine aura. The field may narrow, as in glaucoma. The normally small blind spot corresponding to the point where the optic nerve enters the eye can enlarge if fluid accumulates at that point (papilledema). Methanol or quinine poisoning, diseases of the nerve sheaths, deficiency diseases, and atherosclerosis can also cause blind spots. Tumours pressing on the optic nerves in different locations can cause loss of different halves or quarters of the visual field of each eye. See alsomacular degeneration.
This entry comes from Encyclopædia Britannica Concise. For the full entry on visual-field defect, visit Britannica.com.
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