Urticaria (Hives)
Urticaria is a common reaction pattern characterized by wheals (transient, pruritic, erythematous, edematous plaques). The primary mechanism is mast cell degranulation in the superficial dermis.
Key Characteristics
- Transient Nature: Individual lesions usually resolve within 24 hours. If a lesion persists >24 hours, consider Urticarial Vasculitis.
- Pruritus: Almost universal and can be intense.
- Angioedema: Deep dermal/subcutaneous swelling; involves deeper structures (e.g., lips, eyelids, tongue).
Differential Diagnosis
| Type | Trigger/Mechanism |
|---|---|
| Acute | < 6 weeks. Often IgE-mediated (food, drugs, infections). |
| Chronic | > 6 weeks. Often autoimmune, idiopathic, or physical. |
| Physical | Dermographism (stroking), cold, pressure, or solar exposure. |
High-Yield Exam Pearl:
- Anaphylaxis: Urticaria + angioedema + respiratory distress or hypotension is an emergency. Always manage with intramuscular epinephrine.
- ACE Inhibitors: Can cause angioedema (bradykinin-mediated) without associated urticaria. Always screen for this in patients presenting with isolated lip/tongue swelling.