Red Eye — Approach

A unilateral red eye needs a structured approach to separate benign, self-limiting causes from sight-threatening emergencies.

History

• Visual deterioration (any drop in acuity is a red flag)
• Trauma (including recent eye surgery, refractive surgery)
• Discharge — presence and type (purulent vs watery vs mucoid)
• Photophobia
• Contact lens wear
• Pain severity — mild grittiness vs severe pain with nausea/vomiting (raises acute glaucoma)

Basic examination

• General inspection of cornea, sclera and lids; evert the lids to check for a sub-tarsal foreign body; palpate for pre-auricular/submandibular lymphadenopathy
• Visual acuity in both eyes (with glasses/pinhole if worn)
• Pupils — shape, light reflex, relative afferent pupillary defect (RAPD)
• Cornea — fluorescein 1% + cobalt blue light for ulcers/abrasions

Differential diagnosis

Cause	Key features
Allergic conjunctivitis	Usually bilateral, itch, history of atopy
Viral conjunctivitis	Often starts unilateral then spreads; watery discharge; pre-auricular lymphadenopathy; recent viral illness. Consider herpes simplex if cold sores and/or a dendritic corneal lesion
Bacterial conjunctivitis	Purulent discharge; often bilateral
Corneal foreign body	Irritation, ± trauma/metalwork history, pain
Acute anterior uveitis	Photophobia, blurred vision, irregular pupil (posterior synechiae), cells in the anterior chamber on slit lamp
Acute angle-closure glaucoma	Severe pain and headache, photophobia, visual impairment, nausea/vomiting, fixed mid-dilated pupil, cloudy cornea
Microbial keratitis	Suspect especially in contact lens wearers (Pseudomonas/Acanthamoeba); may show a corneal infiltrate or abscess (white lesion) — needs prompt specialist assessment
Endophthalmitis	History of intraocular surgery, severe pain, hypopyon (pus in the anterior chamber); immunocompromised patients may present with endogenous disease — urgent specialist treatment within hours

Management

Viral conjunctivitis is self-limiting — most cases need no specific treatment. Comfort measures (cold compresses, artificial tears, topical antihistamines) are all that’s usually needed. Antibiotics don’t help, add cost, and contribute to resistance. If herpes simplex or zoster virus is suspected (cold sores/vesicular rash, or a dendritic corneal lesion), prescribe an antiviral — aciclovir ointment or ganciclovir gel — rather than an antibiotic. Refer to an ophthalmologist if viral conjunctivitis is severe, or if symptoms persist beyond resolution of the acute infection (to consider topical steroids and exclude a post-viral immune keratitis).

Bacterial conjunctivitis does benefit from topical antibiotics: a 2023 Cochrane review of 21 trials (8,805 participants) found antibiotics improved clinical cure by 26% relative to placebo (moderate-certainty evidence; roughly 15% more patients cured, number needed to treat ≈7) and increased microbiological cure, with no evidence of serious systemic side effects. A broad-spectrum topical antibiotic is recommended — there’s no evidence any one option is superior, so cost/convenience can guide the choice. Typical Therapeutic Guidelines dosing: chloramphenicol 0.5% drops, 1–2 drops every 2 hours for the first 24 hours then reducing to 6-hourly until discharge resolves (up to 7 days), or framycetin 0.5% drops on a similar reducing schedule; chloramphenicol 1% ointment can be added at bedtime. Gentamicin, tobramycin and quinolone drops aren’t recommended as first-line empiric treatment. Refer promptly if there’s no improvement within 5 days.

Don’t reach for chloramphenicol if microbial keratitis is a possibility (contact lens wearer, corneal infiltrate, severe pain) — starting it delays the specialist assessment and culture-directed treatment that keratitis actually needs, risking loss of vision or the eye.

Gonococcal and chlamydial conjunctivitis need same-day ophthalmology referral rather than primary-care treatment: gonococcal disease is treated with ceftriaxone plus saline eye lavage, and chlamydial disease with oral azithromycin or doxycycline (see the paediatric section below for neonatal disease specifically).

Warning features requiring prompt specialist referral

Contact lens wearer · previous eye/refractive surgery · decreased vision · severe pain · nausea and vomiting · cloudy or opaque cornea · dendritic ulcer · hypopyon · non-reactive pupil or RAPD · ocular trauma or worsening symptoms.

Paediatric red eye

The differential and referral thresholds above apply in children too, but a few things are specific to this age group: neonatal conjunctivitis (ophthalmia neonatorum, presenting in the first 4 weeks of life) always needs urgent assessment to exclude gonococcal and chlamydial infection (see gonococcal-conjunctivitis.md/trachoma-chlamydial-conjunctivitis.md), and periorbital swelling with a red eye in a child should always raise concern for (peri)orbital cellulitis rather than simple conjunctivitis given the more rapid progression risk in children.

COVID-era practical note

Viral conjunctivitis can be a presenting or accompanying feature of COVID-19 and other viral respiratory illnesses — a watery, itchy red eye in a patient with concurrent respiratory symptoms should prompt the same respiratory-illness precautions as any other presentation, not just ophthalmic assessment.