GPAtlas🧭 When to suspect
"Red eye" is a presenting symptom, not a diagnosis.
Separate the common, benign, self-limiting causes from the small number of sight-threatening emergencies:
• Benign: infective and allergic conjunctivitis, subconjunctival haemorrhage, episcleritis.
• Sight-threatening: acute angle-closure glaucoma, microbial keratitis, anterior uveitis, scleritis, and penetrating or chemical injury.
Two screening questions do most of the diagnostic work: is the eye painful? and is vision affected?
• A painless eye with normal acuity is almost always benign.
• Any of pain, photophobia, reduced vision, or contact-lens wear should raise concern.
• Visual acuity (VA) is the vital sign of the eye: document it in both eyes at every red-eye consultation, as it is the single best discriminator between surface irritation and serious intraocular disease.
| Likely cause | Pain | Vision | Key distinguishing feature |
|---|---|---|---|
| Infective conjunctivitis | Gritty, no true pain | Normal | • Diffuse redness with discharge (purulent → bacterial; watery → viral) • Often bilateral or sequential |
| Allergic conjunctivitis | Itch, no pain | Normal | • Bilateral, itch-dominant, watery, stringy mucus • History of atopy |
| Subconjunctival haemorrhage | None | Normal | • Flat, bright-red patch with a defined edge • White sclera elsewhere • No discharge |
| Episcleritis | Mild ache or none | Normal | • Sectoral redness • Vessels blanch with topical phenylephrine |
| Scleritis | • Severe, deep, boring • Wakes from sleep |
May reduce | • Violaceous deep redness that does not blanch • Associated systemic autoimmune disease |
| Anterior uveitis (iritis) | Aching, photophobia | Blurred/reduced | • Circumlimbal (ciliary) flush • Small or irregular pupil |
| Microbial keratitis | Painful, foreign-body sensation | Reduced | • Contact-lens wearer • Corneal white spot or opacity • Fluorescein-staining defect |
| Acute angle-closure glaucoma (AACG) | Severe, with headache, nausea, vomiting | • Markedly reduced • Haloes around lights |
• Fixed mid-dilated oval pupil • Hazy cornea • Hard, tender globe |
| Herpes zoster ophthalmicus (HZO) | Pain, dysaesthesia | Variable | • Unilateral V1 vesicular rash • Hutchinson's sign (lesion on the nose tip) |
Source: NICE · College of Optometrists
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