Not all red eyes are conjunctivitis, and the differential for a red eye is broad, including infectious, allergic, traumatic/irritant, rheumatologic, and oncologic etiologies.
Conjunctivitis is inflammation and injection of the usually transparent conjunctival tissue that covers the outer portion of the eye and inner eyelids. It is often a diagnosis of exclusion after ensuring more severe or vision-threatening etiologies of red eye are not present.
The most common etiology for conjunctivitis in children is viral and treated with supportive care. Viral conjunctivitis often begins unilaterally with serous or mucoid discharge, and then may become bilateral. Bacterial conjunctivitis is most often unilateral with purulent discharge and most often treated with topical antibiotics.
Indications for urgent referral and treatment include a red eye with one or more of the following: severely painful eye movements or severe foreign body sensation, photophobia, decreased visual acuity, fixed pupil, proptosis, redness in a "ciliary flush" pattern (redness localized to or more pronounced in a ring around the limbus), corneal ulceration or opacity, suspected Neisseria gonorrhoeae infection or herpes simplex virus conjunctivitis, or history of chemical burn or penetrating trauma.