What causes intermittent rash in a child?

Intermittent Rashes in Children: Causes and Clinical Approach

The most common causes of intermittent rashes in children are urticaria (hives), atopic dermatitis (eczema), viral exanthemas, and psoriasis—each distinguished by the duration of individual lesions and specific triggering factors.

Urticaria (Hives)

Urticaria is the classic intermittent rash in children, with individual wheals lasting 2-24 hours before completely resolving without scarring 1, 2. This temporal pattern is the key diagnostic feature that distinguishes ordinary urticaria from other conditions 1.

Clinical Patterns by Duration:

• Ordinary urticaria: Individual wheals last 2-24 hours 1, 2
• Contact urticaria: Wheals resolve within 2 hours 1, 2
• Physical urticaria: Lesions disappear within 1 hour (except delayed pressure urticaria, which takes 2-6 hours to develop and up to 48 hours to fade) 1, 2
• Urticarial vasculitis: Weals persist for days—if lesions last beyond 24 hours, consider this diagnosis 1, 2

Common Triggers in Children:

• Acute episodic urticaria (most common in pediatrics): Triggered by viral infections, allergic reactions to foods and drugs, contact with chemicals/irritants, or physical stimuli 3
• Idiopathic urticaria: No identifiable cause in many instances 3
• Chronic urticaria: Physical factors cause 5-10% of cases; other triggers include infections, foods, additives, aeroallergens, and drugs 3

Diagnostic Approach:

• No investigations required for acute/episodic urticaria unless history suggests specific triggers 1, 2
• For suspected IgE-mediated reactions to environmental allergens (latex, nuts, fish), confirm with skin-prick testing and CAP fluoroimmunoassay 1
• For chronic urticaria not responding to H1 antihistamines: Full blood count with differential, erythrocyte sedimentation rate, thyroid autoantibodies, and thyroid function tests 2

Atopic Dermatitis (Eczema)

Atopic dermatitis causes intermittent flares driven by the interplay between genetic skin barrier dysfunction, immune dysregulation, and environmental triggers 4, 5.

Pathophysiology of Flares:

• Skin barrier disruption (genetic and environmental) allows penetration of allergens, irritants, and microbes, triggering inflammatory cascades 4, 5
• Itch-scratch cycle: Scratching damages the barrier further, creating self-perpetuating inflammation and worsening pruritus 4, 5
• Immune activation: Acute flares show Th2-dominant inflammation with elevated IL-4, IL-5, IL-13, and IL-31; chronic phase adds Th1 response 4, 5

Specific Triggers for Flares:

• Exposure to allergens, fungal elements, tobacco smoke, and air pollutants 4, 5
• Emotional stress 1
• Secondary bacterial infection (suggested by crusting or weeping) 1
• Contact dermatitis (new allergen exposure) 1

Clinical Pitfall:

Deterioration in previously stable eczema warrants evaluation for secondary bacterial infection or development of contact dermatitis 1. Send swabs for bacterial culture if infection suspected 1.

Psoriasis

Pediatric psoriasis flares intermittently in response to specific triggers, particularly infections and stress 1.

Triggering Factors:

• Infections: Pharyngeal and perianal group A β-hemolytic Streptococcus infection, Kawasaki disease 1
• Stress: Emotional stress, physiologic stressors 1
• Environmental: Increased BMI, second-hand cigarette smoke 1
• Cutaneous trauma: Koebner phenomenon 1
• Medication-related: Withdrawal of systemic corticosteroids, paradoxically TNF inhibitor medications 1

Clinical Approach:

Elicit potential triggering factors during history and physical examination, and address objectively when possible (e.g., culture-directed antibiotics for positive streptococcal cultures) 1.

Viral Exanthemas

Viral exanthemas are common intermittent rashes in children that often occur while taking medications, mimicking drug hypersensitivity in 10% of cases 6.

Key Features:

• Roseola: Rash presents after resolution of high fever 7
• Erythema infectiosum (fifth disease): Viral prodrome followed by "slapped cheek" facial rash, often with fever 7
• Common viral triggers: Epstein-Barr virus, human herpesvirus 6, cytomegalovirus, and Mycoplasma pneumoniae 6

Diagnostic Challenge:

Distinction between virus-induced and drug-induced eruption during acute phase is usually not possible 6. Serological and PCR assays can help, though concomitant acute infection does not exclude drug hypersensitivity 6.

Critical Pitfalls to Avoid

• Do not routinely recommend dietary restrictions without professional supervision—evidence shows little benefit in most cases and risks nutritional deficiency 1, 4
• Do not attempt house dust mite eradication as a preventive measure—current evidence does not support this intervention 1, 4
• Do not assume all intermittent rashes have identifiable triggers—the multifactorial nature means some children develop flares despite minimal environmental exposures 4
• Recognize that angioedema may last up to 3 days without treatment, distinguishing it from ordinary urticaria 1