A red eye is an eye that appears red due to illness or injury. It is usually injection and prominence of the superficial blood vessels of the conjunctiva, which may be caused by disorders of these or adjacent structures. Conjunctivitis and subconjunctival hemorrhage are two of the less serious but more common causes. Management includes assessing whether emergency action (including referral) is needed, or whether treatment can be accomplished without additional resources. Slit lamp examination is invaluable in diagnosis but initial assessment can be performed using a careful history, testing vision (visual acuity), and carrying out a penlight examination. Diagnosis Particular signs and symptoms may indicate that the cause is serious and requires immediate attention. Seven such signs are: Reduced visual acuity Ciliary flush (circumcorneal injection) Corneal abnormalities including edema or opacities ("corneal haze") Corneal staining Pupil abormalities including abormal pupil size Abnormal intraocular pressure Severe pain The most useful is a smaller pupil in the red eye than the non-red eye (opposite eye) and sensitivity to bright light. Reduced visual acuity A reduction in visual acuity in a 'red eye' is indicative of serious ocular disease, such as keratitis, iridocyclitis, and glaucoma, and never occurs in simple conjunctivitis without accompanying corneal involvement. Ciliary flush Ciliary flush is usually present in eyes with corneal inflammation, iridocyclitis or acute glaucoma, though not simple conjunctivitis. A ciliary flush is a ring of red or violet spreading out from around the cornea of the eye. Corneal abnormalities The cornea is required to be transparent to transmit light to the retina. Because of injury, infection or inflammation, an area of opacity may develop which can be seen with a penlight or slit lamp. In rare instances, this opacity is congenital. In some, there is a family history of corneal growth disorders which may be progressive with age. Much more commonly, misuse of contact lenses may be a precipitating factor. Whichever, it is always potentially serious and sometimes necessitates urgent treatment and corneal opacities are the fourth leading cause of blindness. Opacities may be keratic, that is, due to the deposition of inflammatory cells, hazy, usually from corneal edema, or they may be localized in the case of corneal ulcer or keratitis. Corneal epithelial disruptions may be detected with fluorescein staining of the eye, and careful observation with cobalt-blue light. Corneal epithelial disruptions would stain green, which represents some injury of the corneal epithelium. These