Acute Urticaria

Differential Diagnosis:

- Atopic dermatitis (eczema)

- Contact dermatitis: allergic or irritant

- Non-urticarial drug eruptions

- Viral exanthems

- Insect bites (papular urticaria)

- Bullous pemphigoid

- Erythema multiforme minor

- Phyto(photo)dermatitis- Auriculotemporal (Frey’s) syndrome

- Sweet syndrome

- Schnitzler’s syndrome

- Cryopyrin-associated periodic fever syndromes

- Bradykinin-mediated angioedema

- Mast cell disorder

- Well’s syndrome

- Pregnancy: heat rash, prurigo of pregnancy, pruritic folliculitis of pregnancy, polymorphic eruption of pregnancy (PEP, aka pruritic urticarial papules and plaques of pregnancy/PUPPP), obstetric cholestasis, impetigo herpetiformis, pemphigoid gestationis

Mechanism:

- Urticaria is the end result of dermal edema from small vessel leakage caused by mast cell release of mediators such as histamine [17]

- Upstream mechanisms include:

• IgE-mediated: IgE bound to allergen cross links the FcƐ receptor on mast cells and basophils [18]

• NSAIDs:

  • Non-IgE mediated: class effect (all NSAIDs); inhibition of COX-1 shunts arachidonic acid to lipoxygenase pathway, increasing production of cysteinyl leukotrienes [19]

• IgE-mediated: specific IgE against a particular NSAID (reactivity limited to structurally cross-reactive NSAIDs)

• Infection-triggered hives can occur through multiple mechanisms [11]

  • Cross-reaction between antibodies against microbial antigens and mast cell FcɛRI

  • Immune complex formation triggers basophils or mast cells [20] to produce vasoactive amines and activate complement

  • Direct mast cell activation