What is the most likely diagnosis for unilateral lower eyelid conjunctival injection with minimal edema and a unilateral periorbital dark circle without discharge?

Differential Diagnosis: Unilateral Lower Lid Conjunctival Injection with Minimal Edema and Dark Circle

This presentation most likely represents either herpes simplex virus (HSV) conjunctivitis or chronic allergic conjunctivitis, with HSV being the primary concern given the strictly unilateral presentation without discharge.

Primary Diagnostic Considerations

Herpes Simplex Virus Conjunctivitis (Most Concerning)

HSV conjunctivitis characteristically presents as unilateral bulbar conjunctival injection with watery discharge and mild follicular reaction, and may have a palpable preauricular node 1. The key features that align with your presentation include:

• Unilateral presentation is typical for HSV, distinguishing it from most other viral causes 1
• The absence of discharge in your case does not exclude HSV, as the watery discharge may be minimal or intermittent 1
• Eyelid edema can occur with HSV conjunctivitis 1
• The periorbital dark circle may represent early eyelid involvement or vascular congestion 1

Critical examination steps you must perform immediately:

• Examine the eyelids carefully for vesicular rash or ulceration, which are distinctive signs of HSV 1
• Perform fluorescein staining of the cornea to detect dendritic epithelial keratitis, as HSV can progress to keratitis, stromal keratitis, corneal scarring, and perforation 1, 2
• Palpate for preauricular lymphadenopathy 1
• Evert the lower lid to examine for follicular reaction on the tarsal conjunctiva 1

Chronic Allergic Conjunctivitis (Alternative Diagnosis)

Unilateral forms of chronic allergic conjunctivitis can occur when non-specific local factors are present, such as eye rubbing, local trauma, or asymmetric allergen exposure 3. Features supporting this diagnosis include:

• Itching would be the most consistent symptom if allergic, though you don't mention this 4, 5
• Minimal edema and conjunctival injection are consistent with allergic conjunctivitis 5
• The unilateral presentation suggests a non-specific triggering factor on that side (trauma, asymmetric allergen exposure, or eye rubbing) 3
• The dark circle could represent chronic venous congestion from repeated rubbing or "allergic shiners" 5

Critical Red Flags to Rule Out

You must immediately exclude these vision-threatening conditions:

• Examine for corneal involvement with fluorescein staining in any conjunctivitis case to detect early corneal pathology 2
• Look for corneal infiltrates or ulcers that could indicate bacterial superinfection 1
• Assess visual acuity to ensure no vision loss 6
• Check for severe pain, which would suggest more serious pathology requiring immediate ophthalmology referral 6

Recommended Diagnostic Approach

Follow this algorithmic evaluation:

1. Perform fluorescein staining first to rule out corneal involvement 2
2. Examine eyelid margins and skin for vesicular lesions or ulceration suggesting HSV or VZV 1
3. Evert the lower lid to assess for follicular reaction on tarsal conjunctiva 1
4. Palpate preauricular nodes bilaterally 1
5. Question specifically about:
   • Recent stress, fever, or viral illness (HSV triggers) 1
   • History of cold sores or prior HSV infection 1
   • Itching (strongly suggests allergy) 4, 5
   • Eye rubbing or trauma to that eye 3
   • Known allergies or seasonal patterns 3, 5

Management Based on Diagnosis

If HSV is suspected (vesicular lesions, dendritic keratitis, or high clinical suspicion):

• HSV usually subsides without treatment within 4-7 days unless complications occur 1, 7
• However, if corneal involvement is present, immediate ophthalmology referral is mandatory 2
• Isolation until vesicular lesions have crusted over and active discharge has resolved 7

If chronic allergic conjunctivitis is diagnosed:

• Topical antihistamines with mast cell-stabilizing activity are first-line treatment 6, 4
• Identify and eliminate triggering factors on the affected side 3
• Cold compresses and preservative-free artificial tears for symptomatic relief 6

Common Pitfalls to Avoid

• Never assume unilateral presentation excludes viral causes—HSV is characteristically unilateral 1
• Always perform fluorescein staining even in seemingly benign conjunctivitis to avoid missing corneal involvement 2
• Do not dismiss the possibility of early HSV just because classic vesicular lesions are not yet visible 1
• Consider that the "dark circle" may represent early periorbital involvement requiring closer monitoring 1

If examination reveals any corneal staining, vesicular lesions, or if symptoms worsen, immediate ophthalmology referral is indicated 6.