The Red Eye
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Direct answer
Most red eyes that walk into an Indian outpatient department are conjunctivitis, but the five that matter — keratitis, anterior uveitis, acute angle-closure glaucoma, scleritis and endophthalmitis — announce themselves through four discriminators: visual acuity, the pattern of injection, the pupil, and intraocular pressure. Discharge with normal vision and a comfortable pupil is conjunctivitis; a hazy, stained cornea with ciliary flush is keratitis; a small painful pupil with cells and flare is uveitis; a fixed semidilated pupil with a steamy cornea and pressure above 40 mmHg is acute glaucoma; and a red eye after surgery or trauma with hypopyon is endophthalmitis until excluded. Fluorescein staining, tonometry and a slit lamp convert a symptom into a diagnosis within minutes.
What you must remember
- Reduced vision is the single red flag that removes a red eye from the "simple" category — conjunctivitis sees 6/6; keratitis, uveitis and glaucoma do not.
- Diffuse injection maximal in the fornices suggests conjunctivitis; deep ciliary flush around the limbus signals corneal or intraocular disease; pupil clues follow — small, irregular and sluggish in anterior uveitis, fixed and mid-dilated in acute angle closure, normal in conjunctivitis and episcleritis.
- Fluorescein staining finds the corneal epithelial break — dendritic in herpes simplex, a round infiltrate in bacterial keratitis, a linear abrasion after trauma.
- Acute angle-closure glaucoma presents with pain, haloes around lights, nausea and a hard eye; it is the red eye most often mistaken for migraine or gastritis.
- Scleritis is a deep, boring, night-time pain in a sectoral violet discolouration, often with systemic disease such as rheumatoid arthritis or vasculitis; episcleritis is mild, sectoral and self-limiting.
- Postoperative or post-traumatic red eyes are endophthalmitis until proven otherwise — pain and hypopyon demand urgent vitreous assessment.
- Subconjunctival haemorrhage — a flat, bright-red patch with normal vision and a clear cornea — is dramatic but benign, and recurrent bilateral cases warrant a bleeding history and blood pressure check.
- Never pad a discharging eye, never prescribe steroids for an undiagnosed red eye, and never dilate an eye with a shallow anterior chamber and a suspect angle.
Reasoning through four red eyes side by side
Place the four classic differentials in a row and let the discriminators work. First, a young man with bilateral watery redness, morning lid sticking, tender preauricular node and 6/6 vision: injection is fornix-deep, the cornea stains clear, the pupils react — adenoviral conjunctivitis; treatment is hygiene and reassurance. Second, a contact lens wearer with unilateral pain, photophobia and 6/18 vision: the flush is limbal, fluorescein shows a central stain with an infiltrate and a hypopyon — bacterial keratitis; scrape and start fortified antibiotics hourly. Third, a woman with unilateral aching photophobia and 6/24 vision: the pupil is small and sticky, keratic precipitates line the endothelium and cells thread the aqueous — anterior uveitis; cycloplegia and topical steroids, then a cause hunt. Fourth, an elderly woman with evening pain, vomiting and blurred vision with rainbow rings: the cornea is oedematous, the pupil fixed at mid-dilation, the anterior chamber shallow and the pressure 50 mmHg — acute angle closure; acetazolamide, timolol and mannitol before pilocarpine, then iridotomy. Notice that the same triad — pain, redness, reduced vision — resolves differently by asking only four questions: how well does the eye see, where is the redness deepest, what is the pupil doing, and what does the tonometer say.
Where candidates slip
The recurring clinical error in Indian practice is the over-the-counter steroid-combination drop for an undiagnosed red eye, which converts adenoviral conjunctivitis into a prolonged keratitis and a herpetic dendrite into a geographic ulcer. The exam version of the same error is listing drug names before naming the diagnosis. A second slip is dilating every red eye for fundus examination — in a shallow-chamber elderly eye, mydriasis can precipitate the very angle closure being excluded. The third is forgetting the systemic thread: a boring scleritis with joint pain is rheumatoid arthritis speaking, and a red eye with oral ulcers and genital ulcers is Behçet disease, answers that earn the extra mark.
Frequently asked questions
Which single finding makes a red eye dangerous?
Reduced visual acuity, which excludes simple conjunctivitis and points to keratitis, uveitis, glaucoma or endophthalmitis.
How does the pupil differ in uveitis and acute angle closure?
Uveitis constricts the pupil — small, irregular and sluggish with posterior synechiae — while angle closure fixes it in mid-dilation with a steamy cornea and raised pressure.
What does ciliary flush indicate?
Deep injection concentrated around the limbus, signifying corneal or intraocular inflammation such as keratitis or anterior uveitis, not surface conjunctival disease.
Which red eye is mistaken for an acute abdomen or migraine?
Acute angle-closure glaucoma, with severe headache, nausea and vomiting accompanying a painful red eye with haloes.
How are episcleritis and scleritis distinguished?
Episcleritis is mild, sectoral, pink and self-limiting; scleritis is a deep, boring pain with violet discolouration, tenderness and associations such as rheumatoid arthritis and Wegener granulomatosis.