What are the differential diagnoses and first‑line management for an adult presenting with unilateral eye itching (pruritus) and watery discharge?

Differential Diagnosis for Unilateral Pruritus and Watery Eye

The most likely diagnosis is viral conjunctivitis (particularly adenoviral or HSV), but you must immediately rule out HSV keratitis with fluorescein staining because untreated HSV can progress to corneal scarring and perforation. 1

Immediate Red Flag Assessment

Before considering benign causes, perform fluorescein staining in every case to detect corneal involvement that may not be clinically apparent. 1, 2 Urgent ophthalmology referral is mandatory if you find:

• Dendritic lesions on fluorescein staining (HSV keratitis) 1
• Visual acuity changes or moderate-to-severe pain 1, 2
• Vesicular lesions on the eyelid or periocular skin 1, 2
• Corneal infiltrates or ulceration 1
• Proptosis or painful extraocular movements 2

Most Likely Differential Diagnoses

1. Viral Conjunctivitis (Most Common)

Adenoviral Conjunctivitis:

• Presents with watery discharge, bulbar conjunctival injection, and follicular reaction on inferior tarsal conjunctiva 1, 3
• Often starts unilaterally but becomes bilateral within days 2
• Palpable preauricular lymphadenopathy is common 1, 3
• Recent exposure to infected individuals or concurrent upper respiratory infection supports this diagnosis 1, 3
• Self-limited course with resolution in 5-14 days 1, 3

Herpes Simplex Virus (HSV) Conjunctivitis:

• Critical to identify: Typically unilateral with watery discharge and mild follicular reaction 1, 2
• Preauricular lymphadenopathy is commonly present 1, 2
• Key distinguishing feature: Look for vesicular rash or ulceration on eyelid skin 1
• Must perform fluorescein staining to detect dendritic epithelial keratitis—the hallmark that predicts progression to stromal keratitis, scarring, or perforation 1
• Triggered by stress, fever, UV exposure, or trauma 1

2. Allergic Conjunctivitis

• Hallmark symptom: Pronounced itching is the dominant feature 4, 5, 6
• Typically bilateral but can present asymmetrically 5, 7
• Watery or stringy/ropy discharge with milky conjunctival appearance 6
• Papillary hypertrophy of tarsal conjunctiva (not follicles) 6
• Family history of allergy, asthma, or eczema 8, 6
• Look for allergic facies: dark infraorbital circles, horizontal skin folds (Morgan-Dennie lines) on lower eyelid, pale boggy nasal turbinates 2

3. Bacterial Conjunctivitis (Less Likely with Watery Discharge)

• Purulent or mucopurulent discharge that mats eyelids on waking is typical 1, 3
• Can present unilaterally initially 1
• Watery discharge makes this less likely unless very early in course 2
• Absence of follicles distinguishes bacterial from viral 3

4. Other Important Considerations

Molluscum Contagiosum:

• Unilateral follicular reaction with shiny, dome-shaped umbilicated lesions on eyelid skin or margin 1, 2
• More common in immunocompromised patients 1, 2

Parinaud Oculoglandular Syndrome:

• Unilateral granulomatous follicular conjunctivitis with ipsilateral regional lymphadenopathy 1
• Associated with cat-scratch disease or tularemia 1, 2
• Fever and eyelid swelling may be present 2

Diagnostic Examination Algorithm

Step 1: Rule Out Vision-Threatening Emergencies

• Check visual acuity 2
• Perform fluorescein staining (mandatory in all cases) 1, 2
• Assess pupil reactivity and extraocular movements 2
• Examine for proptosis 2

Step 2: Characterize the Presentation

• Inspect eyelids carefully for vesicular lesions (HSV/VZV) or umbilicated papules (molluscum) 1, 2
• Evert lower lid to visualize tarsal conjunctiva: follicles suggest viral, papillae suggest allergic 1, 3, 6
• Palpate preauricular lymph nodes: enlargement suggests viral (especially HSV) or Parinaud syndrome 1, 2, 3
• Assess discharge character: watery (viral/allergic), purulent (bacterial), stringy/ropy (allergic) 1, 3, 6

Step 3: Obtain Targeted History

• Recent exposure to infected individuals or upper respiratory infection (adenovirus) 1, 3
• History of cold sores, stress, fever, or UV exposure (HSV) 1
• Personal or family history of allergies, asthma, eczema (allergic) 8, 6
• Contact lens wear and hygiene practices 8
• Recent trauma, chemical exposure, or ocular surgery 8

First-Line Management

For Presumed Viral Conjunctivitis (No Red Flags)

• Supportive care: Artificial tears, cold compresses, topical antihistamines for symptomatic relief 3
• Do not prescribe topical antibiotics—viral conjunctivitis does not respond to antibacterial agents 3
• Strict hygiene measures to prevent spread 3
• Mandatory follow-up in 3-4 days to assess improvement 2
• Expect resolution in 5-14 days 1, 3

For Suspected HSV (Even Without Corneal Involvement)

• Immediate ophthalmology referral if fluorescein shows dendritic lesions or any corneal involvement 1
• Mild HSV conjunctivitis without corneal involvement may resolve spontaneously in 4-7 days with supportive care 1
• Never initiate corticosteroids before ophthalmology consultation—they can exacerbate herpetic infections 2

For Allergic Conjunctivitis

• Begin with conservative measures: allergen avoidance, environmental control, ocular irrigation, cold compresses 4
• Topical antihistamines (levocabastine, emedastine) or mast cell stabilizers (sodium cromoglycate, olopatadine) for mild-to-moderate cases 4, 5
• Oral antihistamines for moderate-to-severe cases 5
• Reserve topical corticosteroids for refractory cases due to serious side effects 4, 5

For Bacterial Conjunctivitis

• Delayed antibiotic prescribing is preferred for uncomplicated cases (wait 2-3 days if no improvement) 1
• Immediate topical antibiotics (moxifloxacin 0.5% three times daily for 4 days) if rapid resolution desired 1, 3

Critical Pitfalls to Avoid

• Never assume "no discharge" excludes infection—early viral conjunctivitis may have minimal watery discharge 2
• Never skip fluorescein staining—early HSV keratitis may not be clinically apparent without staining 1
• Never start corticosteroids empirically—they worsen HSV and can mask serious pathology 2
• Never dismiss unilateral presentation as non-infectious—HSV, molluscum, and early adenovirus commonly present unilaterally 1, 2
• Consider sexual abuse in children with gonococcal or chlamydial conjunctivitis 1