How can viral conjunctivitis be distinguished from allergic conjunctivitis?

Distinguishing Viral from Allergic Conjunctivitis

Viral conjunctivitis presents with watery discharge, follicular reaction on the inferior tarsal conjunctiva, preauricular lymphadenopathy, and often concurrent upper respiratory infection, while allergic conjunctivitis is distinguished by bilateral itching as the predominant symptom, papillary (not follicular) reaction on the superior tarsal conjunctiva, and absence of preauricular lymphadenopathy. 1

Key Clinical Differentiators

Discharge Pattern

• Viral: Watery, clear discharge with photosensitivity and pain 1, 2
• Allergic: Watery discharge with mild mucous component, but itching dominates the presentation 1

Conjunctival Reaction Pattern

• Viral: Follicular reaction on the inferior tarsal conjunctiva—these are small, dome-shaped elevations that represent lymphoid hyperplasia 3, 1
• Allergic: Papillary reaction on the superior tarsal conjunctiva—these are flat-topped elevations with central vascular cores 4

Lymphadenopathy

• Viral: Preauricular lymph node enlargement is common, especially with adenoviral infections 3, 1
• Allergic: Preauricular lymphadenopathy is absent 1

Laterality and Onset

• Viral: Often starts unilateral but becomes sequentially bilateral within days; abrupt onset 3, 1
• Allergic: Bilateral from onset; may be seasonal or perennial depending on allergen exposure 1

Associated Features

• Viral:

  • Concurrent upper respiratory infection (rhinorrhea, sore throat, fever) 3
  • Subconjunctival hemorrhages may be present 1, 4
  • Chemosis and eyelid swelling can be marked 3, 1
  • Duration typically 5-14 days, self-limited 1

• Allergic:

  • History of atopy, asthma, or eczema 1
  • Concurrent allergic rhinitis common 1
  • Itching is the most consistent and distinguishing feature 1
  • No matted eyelids upon waking 1

Symptom Severity

• Viral: Photosensitivity (photophobia) and pain suggest corneal involvement, particularly with adenoviral keratoconjunctivitis 2
• Allergic: Itching predominates; pain and photophobia are absent unless there is secondary corneal involvement from chronic rubbing 1

Diagnostic Testing When Needed

Viral Conjunctivitis

• Rapid in-office immunodiagnostic tests for adenovirus have 88-89% sensitivity and 91-94% specificity 3
• PCR testing can detect viral DNA but availability varies by laboratory 3
• Testing is not routinely necessary but may prevent misdiagnosis and unnecessary antibiotic use 3

Allergic Conjunctivitis

• Conjunctival scrapings reveal eosinophils and mast cells on Giemsa stain in vernal conjunctivitis 3
• Skin prick testing or specific IgE measurement can identify allergen triggers 5
• Diagnosis is primarily clinical; laboratory testing is rarely needed 1

Common Pitfalls to Avoid

• Do not prescribe antibiotics for viral conjunctivitis—they provide no benefit and cause unnecessary toxicity 1, 2
• Do not use topical corticosteroids indiscriminately—they prolong adenoviral infections, worsen herpes simplex virus infections, and increase intraocular pressure 1
• Do not miss bacterial superinfection—if purulent discharge develops in a patient with presumed viral conjunctivitis, consider bacterial co-infection 1
• Do not overlook red flags requiring urgent ophthalmology referral: visual loss, moderate-to-severe pain, corneal involvement, or history of herpes simplex virus eye disease 1, 2

Management Implications

Viral Conjunctivitis

• Supportive care only: preservative-free artificial tears, cold compresses 1, 2
• Topical antihistamines (e.g., olopatadine) for symptomatic relief of itching 1
• Strict hand hygiene and avoidance of shared towels/pillows for 10-14 days 1
• Patient education about high contagiousness is essential 1

Allergic Conjunctivitis

• First-line: Topical antihistamines with mast cell-stabilizing activity (olopatadine, ketotifen) 1
• Environmental modifications: sunglasses as barrier to airborne allergens, cold compresses, refrigerated artificial tears 1
• Avoid eye rubbing 1
• Consider allergen-specific immunotherapy for severe cases 1