Codes

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Diagnostic Pearls

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Differential Diagnosis & Pitfalls

• Serum sickness / serum sickness-like reaction – Associated with fever, lymphadenopathy, arthralgias (look for refusal to use an extremity), dusky skin lesions, and recent drug (ie, beta-lactam) or sera exposure.
• Kawasaki disease – The child appears ill and is febrile.
• Urticarial vasculitis – Individual lesions last longer than 24 hours and are associated with pain, purpura and/or arthralgias or arthritis (look for joint swelling or refusal to use extremities).
• Mastocytosis (urticaria pigmentosa) – Has persistent yellow-brown macules and plaques that urticate with stroking.
• Acute hemorrhagic edema of infancy – Large annular purpuric papules and plaques, fever, and edema in an otherwise well infant between the ages of 4 months and 3 years.
• Viral exanthem – Nonspecific viral exanthems tend to be more macular and papular with a diffuse distribution.
• Cryopyrin-associated periodic syndromes – Muckle-Wells syndrome, familial cold autoinflammatory syndrome (familial cold urticaria), and neonatal-onset multisystem inflammatory disease.
• Alpha-gal syndrome

• Angioedema – Edema of the subcutaneous or submucosal tissues rather than edema of the dermis with urticaria. It is not pruritic and commonly affects the face (eyelids, earlobes, lips); familial angioedema usually does not have individual small hives.
• Erythema multiforme – Fixed for several days, does not respond to antihistamines, and associated with dusky, necrotic centers (rather than the pale edematous center of urticaria).
• Papular urticaria / insect bites – Often excoriated and lasts longer than 24 hours.
• Dermatographism may mimic urticaria and is induced by firmly stroking the skin. It lasts from 0.5-2 hours.
• Contact dermatitis (irritant, allergic) may have an unusual geometric shape correlating to the inciting irritant and often develops blisters.
• Lupus erythematosus – Often with epidermal changes (scaly, atrophic, or ulcerated).
• Herpes zoster – May initially be urticarial, but lesions are painful and evolve into blisters and crusts.
• Erythema annulare centrifugum – Often with epidermal changes (scale), and lesions persist for weeks.
• Cellulitis
• Fixed drug eruption
• Exanthematous drug eruption
• Lyme disease
• Erythema marginatum
• Juvenile idiopathic arthritis
• Bedbug bite
• Loiasis
• Schistosomiasis
• Strongyloidiasis
• Toxocariasis – A common cause of chronic urticaria.
• African trypanosomiasis

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