Red eye: what is serious and what is not
- Clinical approach
Most red eyes are conjunctivitis or a small bleed and settle alone. Reduced vision, real pain, light sensitivity or an odd pupil mean a same-day referral.
What it is
Red eye is the most common eye complaint in primary care. Most cases are conjunctivitis, subconjunctival haemorrhage or dry eye, which are benign. The job is to pick out the few that threaten sight: keratitis, acute angle closure, anterior uveitis, scleritis, endophthalmitis and trauma. Visual acuity, the pattern of redness, the cornea and the pupil sort almost every case.
Red flags
- Reduced visual acuity that does not improve with a pinhole.
- Moderate or severe pain, not just grittiness or itch.
- Photophobia, especially pain in the red eye when light is shone into the other eye.
- Ciliary flush: redness concentrated in a ring around the cornea.
- Corneal opacity, a white spot or a fluorescein-staining defect; contact lens wear.
- A fixed, mid-dilated or irregular pupil; recent eye surgery, injection or injury; a newborn.
Step-by-step approach
- Measure visual acuity in each eye before anything else, with glasses or a pinhole.
- Ask: pain or grittiness, itch, type of discharge, light sensitivity, contact lenses, injury, recent surgery.
- Look at the pattern: diffuse conjunctival redness, ciliary flush, a sector, or flat blood under the conjunctiva.
- Examine the cornea with a penlight and fluorescein: clarity, staining, a branching (dendritic) ulcer.
- Check the pupils (size, shape, reaction) and the anterior chamber (pus, blood, shallow depth).
- If acute angle closure is possible, estimate the pressure: a stony-hard eye on gentle palpation or tonometry.
- Any red flag: same-day ophthalmology. No red flags: treat the likely benign cause and give return advice.
Causes of red eye and how urgent they are
| Cause | Pain and vision | Key sign | Urgency |
|---|---|---|---|
| Viral conjunctivitis | Gritty; vision normal | Watery discharge, follicles, tender node in front of the ear | Routine |
| Bacterial conjunctivitis | Mild; vision normal | Purulent discharge, lids stuck together in the morning | Routine |
| Allergic conjunctivitis | Itch; vision normal | Both eyes, watery, history of atopy | Routine |
| Subconjunctival haemorrhage | None | Flat bright red patch with a sharp edge | Routine; check blood pressure and anticoagulants |
| Episcleritis | Mild ache | Sector of redness that blanches with phenylephrine drops | Routine |
| Scleritis | Severe, boring, wakes the patient | Deep violet redness, tender globe | Same day |
| Keratitis | Pain, photophobia, blur | Corneal opacity or staining; contact lens wear | Same day |
| Anterior uveitis | Ache, photophobia, mild blur | Ciliary flush, small pupil | Same day |
| Acute angle closure | Severe pain, halos, nausea, blur | Fixed mid-dilated pupil, hazy cornea, hard eye | Emergency |
Keys to the differential
- The pupil helps: normal in conjunctivitis, small in anterior uveitis, mid-dilated and unreactive in acute angle closure.
- A contact lens wearer with a painful red eye has bacterial keratitis until proven otherwise (Pseudomonas can perforate the cornea within days).
- Topical steroids given without a slit lamp exam can turn a herpes simplex dendritic ulcer into a deep corneal ulcer.
- Recurrent unilateral red painful eye in a young adult with inflammatory back pain suggests HLA-B27 anterior uveitis.
Next step / Initial management
- No red flags and normal acuity: manage in primary care (hygiene and cool compresses for viral conjunctivitis, a topical antibiotic for bacterial) and review if it is not better in a few days.
- Any red flag: same-day ophthalmology. Do not start steroid drops and never send the patient home with anaesthetic drops.
- Suspected acute angle closure: emergency; start pressure-lowering treatment while transfer is arranged.
- Contact lens wearer: remove the lens, keep the lens and its case for culture, refer the same day.
- Newborn with conjunctivitis: urgent assessment for gonococcal and chlamydial infection, which need systemic treatment.
Test yourself
Think of the answer, then tap the question to see it.
Which findings turn a red eye into a same-day referral?
Reduced visual acuity, moderate or severe pain, photophobia, ciliary flush, a corneal opacity or staining defect, or an abnormal pupil.
Why should steroid eye drops not be started without a slit lamp exam?
Because they can make herpes simplex keratitis and bacterial or fungal keratitis much worse, and they raise eye pressure.
What is the pupil like in acute angle closure compared with anterior uveitis?
In angle closure it is fixed and mid-dilated; in anterior uveitis it is small and may be irregular from adhesions to the lens.
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Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.
Sources: Conjunctivitis Overview (StatPearls Publishing (NCBI Bookshelf), 2026); Acute Angle-Closure Glaucoma (StatPearls Publishing (NCBI Bookshelf), 2026); Uveitis (StatPearls Publishing (NCBI Bookshelf), 2023); Bacterial Keratitis (StatPearls Publishing (NCBI Bookshelf), 2023).
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