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The artificial coloration of tissue by fluorescein. Under ultraviolet illumination, it stains dead or degenerated corneal epithelial cells due to abrasions, old age or following inadequate contact lens fit, a yellowish-green colour. It also stains the tears, thus facilitating the evaluation of tear drainage or the blood flow through the retina and choroid when injected intravenously. Corneal staining resulting from contact lens wear may present in various shapes, locations, depths or severity. A very common form is punctate staining as appears in punctate epithelial keratitis. There may be arcuate stains located in different parts of the cornea, some inferiorly (called inferior epithelial arcuate lesions) or superiorly (called superior epithelial arcuate lesions, acronym: SEAL, or epithelial splitting), which usually do not give rise to symptoms and appear mainly with soft or silicone hydrogel lenses. A very severe form of staining is called epithelial plug. It is typically round in shape and represents a loss of the full thickness of the epithelium. Corneal staining resulting from contact lens wear typically disappears after cessation of contact lens wear. See fluorescein angiography; dye dilution test; fluorescein test.