GPAtlas🧭 When to suspect
"Red eye" is a presenting symptom, not a diagnosis. The clinical task in primary care is to separate the common, benign, self-limiting causes – infective and allergic conjunctivitis, subconjunctival haemorrhage, episcleritis – from the small number of sight-threatening emergencies: 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; 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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