What is the best course of treatment for a patient with acute urticaria, presenting with pruritic rashes, erythematous lesions, and mild leukocytosis, who has already been given cetirizine (antihistamine)?

Differential Diagnoses for Acute Urticaria with Target-Like Lesions

Primary Working Diagnosis: Acute Urticaria (Most Likely)

This patient most likely has acute urticaria, but the presence of target-like lesions and persistence beyond initial treatment raises concern for urticarial vasculitis or erythema multiforme, which cannot yet be ruled out. 1

Why Acute Urticaria is Ruled In:

• Sudden onset of pruritic, erythematous, migratory wheals appearing 2 hours after eating (string beans and egg) strongly suggests acute urticaria 2, 3
• Lesions are migratory - moving from extremities to abdomen, face, and scalp, which is classic for urticaria 1
• Associated with possible food trigger (last meal: string beans and egg), though food-induced acute urticaria is actually uncommon (only 0.9% in one study) 4
• Self-limited course - acute urticaria typically resolves within days to 3 weeks 4
• Mild leukocytosis (14.39 × 10⁹/L) is consistent with reactive process seen in acute urticaria 3
• No eosinophilia (0%) makes parasitic or severe allergic causes less likely 3

Why Acute Urticaria Cannot Be Completely Confirmed Yet:

• Presence of "target-like lesions" is atypical for simple urticaria and raises concern for other diagnoses 1
• Persistence despite cetirizine treatment - while some patients don't respond to antihistamines, this warrants consideration of alternative diagnoses 1
• Recurrence after initial ER visit (10/18 to 11/03) suggests either inadequate treatment or a more persistent condition 3

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Critical Differential: Urticarial Vasculitis (Cannot Be Ruled Out)

Why This Must Be Considered:

• Individual lesions lasting >24 hours is the KEY distinguishing feature of urticarial vasculitis versus ordinary urticaria (where lesions last 2-24 hours) 5, 1
• "Target-like lesions" could represent the fixed nature of vasculitic lesions 5
• Persistence of symptoms over multiple days despite antihistamine treatment 5
• Undocumented fever on both presentations - fever can accompany urticarial vasculitis 1

Why It Cannot Be Ruled Out Yet:

• No lesional skin biopsy performed - this is ESSENTIAL to confirm or exclude urticarial vasculitis, showing leucocytoclasia, endothelial damage, perivascular fibrin deposition, and red cell extravasation 1, 5
• Duration of individual lesions not documented - need to specifically ask if individual wheals last >24 hours 5
• No complement levels checked (C3, C4) - needed to distinguish normocomplementemic from hypocomplementemic disease, which carries worse prognosis 1, 5
• No vasculitis screen performed 1, 5

What to Look For:

• Mark individual lesions with a pen to determine if they last >24 hours 5
• Look for post-inflammatory hyperpigmentation or purpura - suggests vasculitis rather than urticaria 1
• Ask about burning sensation rather than just pruritus - more common in vasculitis 5
• Check for systemic symptoms - arthralgias, abdominal pain, which can accompany urticarial vasculitis 1, 5

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Critical Differential: Erythema Multiforme (Cannot Be Ruled Out)

Why This Must Be Considered:

• "Target-like lesions" are the HALLMARK of erythema multiforme 2
• Distribution on face, trunk, and extremities fits erythema multiforme pattern 2
• Recent upper respiratory infection possible (undocumented fever) - infections, especially HSV and Mycoplasma, are common triggers 2
• Age group - erythema multiforme can occur in young children 2

Why It Cannot Be Ruled Out Yet:

• Lesions described as "migratory" - true erythema multiforme lesions are typically fixed, not migratory 2
• No mucosal involvement documented - check oral mucosa, conjunctiva, and genitalia carefully 2
• "Target-like" is vague - true targets have three distinct zones (central dusky area, pale edematous ring, erythematous outer ring) 2
• Pruritus is prominent - erythema multiforme is typically more burning than pruritic 2

What to Look For:

• Examine lesions carefully for true target morphology with three distinct zones 2
• Check all mucosal surfaces - oral mucosa, eyes, genitalia 2
• Ask about preceding HSV infection or other viral prodrome 2
• Determine if lesions are truly fixed (lasting days in same location) versus migratory 2

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Less Likely but Consider: Drug Reaction

Why to Consider:

• Paracetamol and cetirizine given before second presentation - though drug reactions to these are uncommon 1
• NSAIDs can worsen urticaria in aspirin-sensitive patients 1

Why Less Likely:

• Symptoms preceded medication administration on first presentation 4
• No history of drug allergies 3
• Cetirizine is the TREATMENT for urticaria, not a cause 1

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Less Likely: Viral Exanthem

Why to Consider:

• Undocumented fever on both presentations 2
• Upper respiratory infections are the most common association with acute urticaria (39.5% of cases) 4
• Age group - viral exanthems common in toddlers 2

Why Less Likely:

• No respiratory symptoms (no cough, colds, or difficulty breathing documented) 4
• Marked pruritus - viral exanthems are typically less pruritic 2
• Migratory wheals more consistent with urticaria than fixed viral exanthem 4

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Immediate Diagnostic Steps Required:

To Distinguish Urticaria from Urticarial Vasculitis:

1. Mark 2-3 representative lesions with a pen and photograph them - reassess in 24 hours to determine if individual lesions persist >24 hours 5
2. If lesions last >24 hours: perform lesional skin biopsy to confirm or exclude vasculitis 1, 5
3. Order complement levels (C3, C4) if vasculitis suspected 1, 5

To Distinguish from Erythema Multiforme:

1. Carefully examine lesion morphology - look for true three-zone targets 2
2. Thoroughly examine all mucosal surfaces - oral cavity, conjunctiva, genitalia 2
3. Document if lesions are fixed or migratory 2

Laboratory Work NOT Needed for Simple Acute Urticaria:

• Routine laboratory investigation is NOT cost-effective in acute urticaria unless clinical features suggest autoimmune disease or systemic involvement 3
• The CBC already done shows only mild reactive leukocytosis - no further workup needed if this is simple acute urticaria 3

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Treatment Approach While Differentiating:

Current Treatment is Inadequate:

The patient is on cetirizine 1mL BID, which is likely underdosed for this severity of urticaria. 1

Recommended Immediate Management:

1. Increase cetirizine dose up to 4 times the standard dose if inadequate response after 2-4 weeks - this is common practice and safe 1, 3
2. Consider switching to alternative second-generation H1-antihistamine (loratadine, desloratadine, fexofenadine, levocetirizine) as individual responses vary 1, 6
3. Add sedating antihistamine at night (chlorphenamine 4mg or hydroxyzine 10mg) if sleep is disrupted, though this adds little to urticaria control if H1 receptor already saturated 1

Corticosteroids - Controversial:

• Recent systematic review shows NO clear benefit of adding prednisone to antihistamines in acute urticaria (2 out of 3 RCTs showed no improvement) 7
• However, one older study showed faster resolution with prednisolone 50mg for 3 days (93.8% complete remission in 3 days vs 65.9% with loratadine alone) 4
• Short course (3-4 days) of prednisolone 0.5-1mg/kg may be considered for severe symptoms, but avoid prolonged use 1, 7

Avoid:

• NSAIDs and aspirin - can worsen urticaria 1
• Hot water, tight clothing, overheating - aggravate symptoms 1, 3

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Common Pitfalls to Avoid:

1. Assuming all urticaria is allergic - most acute urticaria is idiopathic (>50% of cases have no identifiable trigger) 4
2. Over-investigating simple acute urticaria - extensive laboratory workup is not cost-effective unless systemic disease suspected 3
3. Missing urticarial vasculitis by not documenting individual lesion duration - always mark and time lesions if uncertain 5
4. Dismissing "target-like lesions" as just urticaria - this requires careful examination to exclude erythema multiforme 2
5. Underdosing antihistamines - up to 4x standard dose is safe and often necessary 1, 3
6. Using prolonged corticosteroids - should be avoided except in very selected cases under specialist supervision 1

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Prognosis and Follow-up:

• Acute urticaria is self-limited - longest episode typically 3 weeks 4
• 50% of patients with wheals alone clear by 6 months 1
• Patients with wheals AND angioedema have >50% still active after 5 years 1
• If symptoms persist >6 weeks, reclassify as chronic urticaria and consider referral to dermatology or allergy 2, 3