Last updated: September 2026
"Red eye" is one of the most common ophthalmic presentations — and one of the most important triage skills is quickly separating benign, self-limited causes from sight-threatening emergencies that need immediate action.
Before anything else, note whether the injection is superficial (conjunctival) or deep (ciliary/pericorneal) — see our Types of Ocular Injection page for the bedside phenylephrine blanching test that distinguishes them quickly.
Any of these should prompt full slit lamp assessment with fluorescein staining and IOP measurement — not a quick reassurance-only exam.
| Category | Key features |
|---|---|
| Conjunctivitis (viral, bacterial, allergic) | Superficial injection, discharge, usually normal vision and no significant pain; itching favors allergic, purulent discharge favors bacterial |
| Subconjunctival hemorrhage | Well-demarcated, flat, painless blood under the conjunctiva; benign, resolves spontaneously — reassurance is usually all that's needed |
| Keratitis (infectious or non-infectious) | Deep injection, pain, photophobia, epithelial defect and/or infiltrate on fluorescein staining — a genuine emergency, especially in contact lens wearers |
| Anterior uveitis/iritis | Deep injection, pain, photophobia, miotic or irregular pupil, cells and flare in the anterior chamber on slit lamp |
| Acute angle closure | Deep injection, severe pain, markedly reduced vision, fixed mid-dilated pupil, hazy cornea from edema, very high IOP on palpation/tonometry — a true emergency requiring immediate IOP-lowering treatment |
| Scleritis | Deep, often violaceous injection, severe boring pain (frequently worse at night, may radiate), tender globe on palpation; associated with systemic autoimmune disease in a significant proportion of cases — warrants systemic workup |
| Episcleritis | Sectoral, milder injection, minimal or no pain, blanches with phenylephrine (unlike scleritis) — a useful bedside way to distinguish it from the more serious scleritis |
| Dry eye disease | Mild diffuse superficial injection, gritty/foreign-body sensation, often worse toward end of day |
When in doubt between episcleritis and scleritis, the phenylephrine blanching test is genuinely decisive: episcleritis blanches, scleritis does not. This one test can save a patient from an unnecessary systemic workup, or conversely prompt one that's genuinely needed.
📘 Want the exam-ready deep dive? This topic is covered in iNotes 2027: Basic Science & Optics (Ophthalmology Postgraduate Exam Notes) — available on Kindle. See all my books.
Experiencing symptoms like these? Learn about our Eye Department or book a consultation with Dr. Dhaval Patel.