Conjunctival injection mainly affects the posterior conjunctival blood vessels. Because these vessels are more superficial than the ciliary arteries, they produce more redness, move with the conjunctiva, and constrict with topical vasoconstrictors. Certain signs help to distinguish among the various causes of a red eye. Characterized by vascular dilation, cellular infiltration and exudation Allergic disease often has papillary projections and pruritus in individuals with a history of allergic disease. Viral infections tend to have lymphoid follicles on the undersurface of the lid and enlarged tender preauricular nodes.
Bacterial disease tends to have more purulent discharge.
Differentiating these different types is imprecise, requiring the physician to assume that a bacterial etiology is involved when unclear. Patients may have decreased visual acuity, photophobia. They often complain of severe eye pain. An epithelial defect may be evident on slit lamp exam or may require staining with fluorescein. Any opacification of the cornea in a red eye is an infection of the cornea until proven otherwise.