Red Eye Presentation
A red, irritated eye triggers a three-way diagnostic workup.
- ~6M: Annual conjunctivitis visits (US) (Primary care and ED)
- ~80%: Viral share of cases (Most common cause)
- ~50-70%: Bacterial share (children) (Higher in pediatrics)
- ~50%: Correct diagnosis by exam alone (Overlap is common)
Why differential matters
Wrong category means wrong or wasted treatment choice.
Three broad categories
Bacterial, viral, and allergic causes share a red-eye look.
History and discharge character separate the three quickly.
Discharge Character Assessed
Discharge texture and color are the single best early clue.
- Purulent: Bacterial discharge (Thick, yellow-green)
- Watery: Viral discharge (Clear, serous)
- Stringy mucoid: Allergic discharge (Ropey, clear)
- Moderate: Exam reliability (Best single feature)
Purulent discharge
Thick yellow-green pus suggests bacterial infection strongly.
Watery discharge
Clear watery tearing often accompanies a viral cause.
Stringy mucoid discharge
Ropey, elastic mucus is classic for allergic conjunctivitis.
Discharge character alone narrows the differential quickly.
Itch & Bilaterality Check
Itch severity and one- vs two-eye spread refine the guess.
- Intense: Allergic itch (Hallmark symptom)
- Mild: Viral irritation (Gritty, not itchy)
- Minimal: Bacterial itch (Burning more common)
- Bilateral: Allergic laterality (Usually both eyes)
Itch as a discriminator
Intense itch points strongly toward an allergic cause.
Spread pattern
Viral often starts in one eye, then spreads.
Bilateral intense itch is nearly diagnostic for allergy.
Associated Findings
Extra clues outside the eye narrow the diagnosis further.
- Preauricular node: Viral clue (Tender, palpable)
- Recent URI: Viral clue 2 (Cold-like symptoms)
- Seasonal pattern: Allergic clue (Pollen, other allergy)
- Lids matted shut: Bacterial clue (On waking)
Viral associated signs
Preauricular lymphadenopathy and URI support a viral cause.
Allergic associated signs
Seasonal timing and other allergy symptoms support allergy.
Bacterial associated signs
Eyelids matted shut on waking supports bacterial cause.
Combine findings — no single sign is fully definitive.
Treatment Implication
The likely category determines what treatment is reasonable.
- Antibiotic drops: Bacterial treatment (Topical therapy)
- Supportive: Viral treatment (Self-limited course)
- Antihistamine drops: Allergic treatment (Plus allergen avoidance)
- Common: Overtreatment risk (Antibiotics overused)
Bacterial management
Topical antibiotic drops shorten bacterial conjunctivitis course.
Viral management
Viral cases are self-limited; supportive care is enough.
Allergic management
Antihistamine or mast-cell stabilizer drops relieve allergic symptoms.
Matching treatment to category avoids unnecessary antibiotic use.