What are the causes and treatments of unilateral red eye?

Causes of Unilateral Red Eye

Unilateral red eye should immediately raise concern for serious vision-threatening conditions that require urgent ophthalmologic evaluation, particularly when accompanied by pain, photophobia, decreased vision, or corneal involvement. 1, 2, 3

Critical Red Flags Requiring Emergency Referral (Within 24 Hours)

Use the RAPID acronym to identify patients needing urgent ophthalmology consultation 4:

• Redness plus any of the following:
• Acuity loss or worsening
• Pain (moderate to severe ocular pain, not just irritation)
• Intolerance of light (photophobia)
• Damaged cornea (visible opacity, ulceration, or fluorescein uptake)

Always examine the cornea with fluorescein staining in any case of purulent conjunctivitis to detect early corneal involvement. 1

Vision-Threatening Causes of Unilateral Red Eye

Infectious Keratitis

• Presents with severe pain, photophobia, decreased vision, and corneal opacity 4
• Herpes simplex virus keratitis is characteristically unilateral and can progress to stromal keratitis, corneal scarring, perforation, uveitis, and retinitis 4, 1
• Bacterial keratitis can cause rapid corneal destruction, particularly in contact lens wearers 2

Acute Angle-Closure Glaucoma

• Sudden onset of severe unilateral pain, decreased vision, halos around lights, and mid-dilated fixed pupil 2, 5
• Represents a true ophthalmologic emergency requiring immediate intervention 3
• Elevated intraocular pressure can cause acute corneal edema 4

Anterior Uveitis (Iritis)

• Unilateral eye pain, photophobia, blurred vision, and ciliary injection (redness concentrated around the cornea) 2, 5
• Pupil may be small and poorly reactive 5
• Requires topical steroids and urgent ophthalmology referral 2

Scleritis

• Deep, boring ocular pain that may radiate to the face and awaken patients from sleep 4
• Associated with systemic vasculitis and autoimmune conditions 4
• Can lead to vision loss if untreated 2

Serious Bacterial Infections

Gonococcal Conjunctivitis

• Can cause corneal perforation within 24-48 hours 1
• Presents with copious purulent discharge and severe conjunctival injection 1
• Requires immediate systemic antibiotics in addition to topical therapy 1, 6
• In neonates, represents an emergency that can lead to septicemia, meningitis, and death 1

Chlamydial Conjunctivitis

• Presents with follicular conjunctivitis, with distinctive follicles on the bulbar conjunctiva and semilunar fold 1
• Requires systemic antibiotics, not just topical treatment 1
• Consider sexual abuse in children with chlamydial conjunctivitis 1

Common Non-Emergent Causes

Viral Conjunctivitis

• Usually starts unilaterally but often becomes sequentially bilateral 4
• Watery discharge, follicular reaction, preauricular lymphadenopathy 4
• Self-limited within 5-14 days, but epidemic keratoconjunctivitis can cause subepithelial infiltrates and prolonged symptoms 4

Bacterial Conjunctivitis

• Purulent or mucopurulent discharge, conjunctival injection 4
• Most cases are self-limiting but can be treated with broad-spectrum topical antibiotics like gatifloxacin 6, 2
• Dosing: one drop every 2 hours while awake (up to 8 times) on day 1, then 2-4 times daily on days 2-7 6

Subconjunctival Hemorrhage

• Bright red blood under the conjunctiva, typically painless with normal vision 2
• Usually benign and self-resolving 2

Corneal Abrasion/Foreign Body

• History of trauma, foreign body sensation, tearing, photophobia 2
• Fluorescein staining reveals epithelial defect 2

Special Considerations

Unilateral Symptoms in Dupilumab Users

• Unilateral eye symptoms are unlikely to represent dupilumab-related ocular surface disorders and should be managed via existing pathways for other causes 4

Contact Lens Wearers

• Higher risk for bacterial keratitis, particularly with Pseudomonas 2
• Patients should not wear contact lenses during treatment of bacterial conjunctivitis 6

Varicella Zoster Virus (Shingles)

• Unilateral vesicular dermatomal rash involving the eyelid, severe pain 4, 1
• Can cause corneal scarring, uveitis, retinitis, and late corneal anesthesia 1
• Requires systemic antiviral therapy 1

Clinical Pitfalls to Avoid

• Do not assume unilateral red eye is simple conjunctivitis—always assess for pain, vision changes, and corneal involvement 2, 3
• Ciliary injection (redness concentrated around the cornea) indicates intraocular inflammation, not simple conjunctivitis 5
• Never prescribe topical steroids without ruling out herpes simplex keratitis, as steroids can worsen herpetic disease 2
• Patients with severe pain not relieved by topical anesthetics require immediate ophthalmology referral 2