What are the possible causes of unilateral lateral eye redness, irritation, and tearing?

Differential Diagnosis for Lateral Eye Redness, Irritation, and Tearing

The most likely diagnoses for unilateral lateral eye redness with irritation and tearing include viral conjunctivitis (particularly COVID-19 or EBV), chlamydial inclusion conjunctivitis, episcleritis, floppy eyelid syndrome, and medication-induced keratoconjunctivitis—with the specific diagnosis depending on associated clinical features such as discharge character, follicular reaction, lymphadenopathy, and eyelid abnormalities.

Infectious Causes

Viral Conjunctivitis

• COVID-19 (SARS-CoV-2) presents characteristically with unilateral more than bilateral conjunctival injection, follicular reaction, chemosis, watery discharge, and superficial keratitis, typically occurring early in disease with self-limited duration of 5-21 days 1
• Epstein-Barr virus causes follicular conjunctivitis that is typically unilateral with ipsilateral lymphadenopathy, subconjunctival hemorrhage, periorbital edema, and tearing—often self-limiting but can present as oculoglandular syndrome 1
• Look for systemic symptoms: respiratory symptoms or exposure history with COVID-19; generalized fatigue, fever, pharyngitis, and splenomegaly with EBV 1

Bacterial Conjunctivitis

• Chlamydial inclusion conjunctivitis presents as unilateral or bilateral follicular conjunctivitis with chemosis, papillary hypertrophy, and distinctive follicles on the bulbar conjunctiva and semilunar fold—highly communicable via hands and fomites 1
• Gonococcal conjunctivitis shows marked eyelid edema, significant purulent discharge (not just tearing), and preauricular lymphadenopathy—requires urgent evaluation for corneal involvement 1
• Parinaud oculoglandular syndrome causes unilateral granulomatous follicular conjunctivitis with ipsilateral preauricular/submandibular lymphadenopathy, most commonly from cat scratch disease (Bartonella henselae) 1

Non-Infectious Inflammatory Causes

Episcleritis

• Presents with acute onset of lateral or sectoral redness, lacrimation, and photophobia in patients aged 40-50 years, more common in women 2
• Critical distinction: Eye pain or tenderness should raise concern for scleritis, which requires urgent ophthalmologic referral 2
• Most cases are idiopathic and resolve within 1-3 months; nodular type tends to be more recurrent and painful 2

Mechanical/Anatomic Causes

• Floppy eyelid syndrome shows upper eyelid edema that easily everts, horizontal lid laxity, diffuse papillary reaction of superior tarsal conjunctiva, punctate epithelial keratopathy, and mild discharge—bilateral but often asymmetric 1
• Associated with obesity, sleep apnea, upper-eyelid laxity, and thyroid disease (commonly Hashimoto's thyroiditis) 1
• Chronic ocular irritation occurs due to nocturnal eyelid ectropion causing upper-tarsal conjunctiva contact with bedding 1

Medication-Induced Keratoconjunctivitis

• Laterality based on drug use pattern—conjunctival injection, punctal edema, inferior fornix and bulbar conjunctival follicles, keratitis (especially inferonasal) 1
• Most common with topical glaucoma medications, NSAIDs, antibiotics, antivirals, or preservatives in eye medications—particularly with multiple medications and frequent dosing 1
• May present with contact dermatitis of eyelids with erythema and scaling 1

Chronic Inflammatory Conditions

Blepharitis-Associated Conjunctivitis

• Rosacea conjunctivitis presents with bilateral (but can be asymmetric) chronic blepharitis, eyelid margin telangiectasias, meibomian gland inspissation, conjunctival injection, and evaporative dry eye with inferior involvement 1
• Chronic inflammatory condition with episodic flares; ocular involvement present in approximately 75% of rosacea patients 1
• Worsening of symptoms in the morning is typical of blepharitis, whereas worsening later in the day suggests aqueous deficient dry eye 3

Contact Lens-Related Keratoconjunctivitis

• Ranges from mild to diffuse conjunctival injection with focal or diffuse corneal neovascularization and superficial punctate keratopathy 1
• Occurs as reaction to mechanical irritation, chronic hypoxia, or preservatives—related to duration of wear and hygiene 1

Critical Red Flags Requiring Urgent Evaluation

• Corneal infiltrate or ulcer (particularly with purulent discharge)—suggests gonococcal conjunctivitis with risk of perforation 1
• Eye pain or tenderness—raises concern for scleritis rather than episcleritis 2
• Unilateral presentation with lymphadenopathy—consider Parinaud oculoglandular syndrome, EBV, or chlamydial infection 1
• Loss of eyelashes or conjunctival cicatricial changes refractory to therapy—consider carcinoma, immune-mediated diseases (including pemphigus vulgaris), or severe blepharitis 3

Diagnostic Approach

Key clinical features to differentiate:

• Discharge character: Watery (viral, episcleritis) vs. purulent (bacterial) vs. mucoid (chronic inflammatory) 1
• Follicular reaction: Present in viral (COVID-19, EBV), chlamydial, and medication-induced causes 1
• Lymphadenopathy: Ipsilateral preauricular/submandibular nodes suggest Parinaud syndrome, EBV, or gonococcal infection 1
• Eyelid abnormalities: Easy eversion (floppy eyelid), telangiectasias (rosacea), contact dermatitis (medication-induced) 1, 3
• Temporal pattern: Acute onset (viral, episcleritis) vs. chronic/recurrent (blepharitis, floppy eyelid, medication-induced) 1, 2