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Red Eye

Definition

Red eye is a non-specific clinical sign referring to redness of the eye due to dilation and engorgement of the superficial blood vessels of the conjunctiva, episclera, or sclera. It is one of the most common ocular complaints encountered in primary care and emergency practice. While it is often due to a benign, self-limiting condition such as conjunctivitis or subconjunctival hemorrhage, it can occasionally signal a sight-threatening emergency.

Textbook of Family Medicine 9e, p. 346

Causes / Classification

The Wills Eye Manual classifies red eye causes into four main groups:

1. Adnexal causes Trichiasis, distichiasis, floppy eyelid syndrome, entropion or ectropion, lagophthalmos, blepharitis, meibomitis, acne rosacea, dacryocystitis, canaliculitis.

2. Conjunctival causes Ophthalmia neonatorum (infants), bacterial/viral/allergic/chemical/atopic/vernal conjunctivitis, subconjunctival hemorrhage, inflamed pinguecula, superior limbic keratoconjunctivitis, giant papillary conjunctivitis, conjunctival foreign body, Stevens-Johnson syndrome, conjunctival neoplasia.

3. Corneal causes Infectious or inflammatory keratitis, contact lens-related problems, corneal foreign body, recurrent corneal erosion, pterygium, neurotrophic keratopathy, UV or chemical burns.

4. Other causes Trauma, dry eye syndrome, anterior uveitis (iritis), episcleritis, scleritis, endophthalmitis, acute angle-closure glaucoma, carotid-cavernous fistula (corkscrew vessels), cluster headache, prostaglandin analog eye drops.

Wills Eye Manual, p. 42

Clinical Features and Evaluation

History

Onset: Sudden onset (e.g., foreign body, subconjunctival hemorrhage) vs. gradual onset (viral/allergic conjunctivitis, iritis).
Pain: A gritty "grain of sand" sensation suggests a superficial corneal irritant. Deep, dull, aching pain suggests iritis, iridocyclitis, or a deep penetrating foreign body.
Photophobia: A danger symptom. It occurs with corneal inflammation, iritis, and angle-closure glaucoma. Its absence makes serious intraocular pathology less likely.
Discharge: Purulent (creamy/yellow) discharge indicates bacterial infection; serous/clear discharge suggests viral; scanty, stringy, white discharge is typical of allergy. Absence of discharge with redness suggests a non-infectious cause (e.g., iridocyclitis, UV keratitis, angle-closure glaucoma).
Visual acuity change: Any reduction in vision is a serious danger sign requiring immediate evaluation.
Textbook of Family Medicine 9e, p. 347

Physical Examination

Type of injection: Conjunctival injection shows individually visible branching vessels extending from the periphery toward the cornea. Ciliary (perilimbal or circumcorneal) injection - a red ring surrounding the cornea with indistinct vessels - indicates deeper inflammation (iritis, keratitis) and is more serious.
Cornea: Normally perfectly transparent. Haziness or opacification indicates corneal edema (seen in angle-closure glaucoma, keratitis). Fluorescein staining under UV light reveals corneal abrasions and dendritic ulcers (herpes simplex).
Pupil: Constricted and irregular in iritis; partially dilated and non-reactive in acute angle-closure glaucoma; normal in conjunctivitis.
Anterior chamber depth: A shallow chamber assessed by side illumination suggests angle-closure glaucoma.
Intraocular pressure (IOP): Elevated in acute angle-closure glaucoma; low in iritis and perforating injuries; normal in conjunctivitis.
Preauricular lymphadenopathy: Characteristic of viral conjunctivitis; absent in bacterial conjunctivitis.
Textbook of Family Medicine 9e, p. 347-348

Differential Diagnosis at a Glance

Parameter	Bacterial Conjunctivitis	Iritis	Keratitis	Acute Angle-Closure Glaucoma
Vision	Normal	Blurred	Blurred	Markedly blurred
Pain	None	Moderate, intermittent	Sharp, severe	Severe ± nausea/vomiting
Photophobia	None	Moderate	Moderate	Moderate
Discharge	Purulent, with lash crusting	None	None to mild	None
Injection	Diffuse	Circumcorneal	Circumcorneal	Diffuse
Cornea	Clear	Clear	Cloudy	Cloudy (hazy)
Pupil	Normal	Constricted	Normal	Dilated, fixed
IOP	Normal	Normal or low	Normal	Elevated

Source: Textbook of Family Medicine 9e, adapted from AAO, p. 347

Red Flags (Refer to Ophthalmologist Immediately)

The triad of red eye + pain + loss of vision always signals a potentially blinding condition. Refer urgently if any of the following are present:

Reduced visual acuity
Significant ocular pain
Photophobia
Corneal staining (fluorescein positive)
Pupil abnormality (unequal size, irregular shape)
Elevated intraocular pressure
Shallow anterior chamber
Ciliary (circumcorneal) injection

Textbook of Family Medicine 9e, p. 348

Special Situations

Red Eye in Infants

Requires special attention to distinguish:

Ophthalmia neonatorum (neonatal conjunctivitis): chemical (silver nitrate drops), gonococcal (within 2-5 days), or chlamydial (5-14 days after birth).
Congenital glaucoma: presents with photophobia, epiphora, blepharospasm, and enlarged cloudy cornea.
Dacryocystitis: lacrimal sac infection with medial canthal swelling and purulent discharge.
Textbook of Family Medicine 9e, p. 348

Management Principles

Bacterial conjunctivitis: Topical antibiotic drops (e.g., polymyxin-trimethoprim, fluoroquinolones for contact lens wearers). Often self-limiting.
Viral conjunctivitis: Supportive - cold compresses, lubricating drops. Highly contagious (adenovirus); strict hygiene advised.
Allergic conjunctivitis: Topical antihistamines, mast cell stabilizers, avoidance of allergens.
Subconjunctival hemorrhage: Spontaneously resolves in 2-3 weeks; reassure the patient.
Herpetic keratitis: Topical antiviral (acyclovir/ganciclovir); refer to ophthalmologist. Do NOT use topical steroids without specialist guidance.
Acute angle-closure glaucoma: Ophthalmic emergency. Immediate IOP reduction with IV acetazolamide, topical beta-blockers, pilocarpine; urgent ophthalmology referral for laser iridotomy.
Iritis/anterior uveitis: Topical steroids and cycloplegic drops; refer to ophthalmologist to investigate systemic cause.

Summary

Red eye is a common but potentially serious presenting complaint. A systematic approach focusing on the presence or absence of pain, visual change, photophobia, type of discharge, and the pattern of injection helps distinguish benign conditions (conjunctivitis, subconjunctival hemorrhage) from sight-threatening emergencies (acute glaucoma, herpetic keratitis, endophthalmitis, anterior uveitis). Any red flag feature mandates prompt referral to an ophthalmologist.

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