What Are Hives (Urticaria)?
Hives, also known as urticaria, are erythematous, raised, itchy wheals (welts) with central pallor that blanch with pressure and typically resolve within 2-24 hours without leaving a trace. 1
Clinical Characteristics
Physical Appearance:
- Hives present as erythematous, edematous, circumscribed lesions with extreme pruritus 2
- The lesions are raised plaques that blanch with pressure and often show central pallor 3, 2
- Individual wheals in ordinary urticaria last 2-24 hours before resolving without scarring 1
- Lesions usually resolve spontaneously within 2-3 hours without a trace 3
Distribution and Pattern:
- Urticarial lesions are often generalized with multiple lesions in no specific distribution 2
- The rash typically involves skin and mucosa 4
Associated Features
Pruritus (Itching):
- Extreme pruritus is the pathognomonic symptom of urticaria 5
- The severity of itching can be graded from mild (present but not annoying) to intense (sufficiently troublesome to interfere with normal daily activity or sleep) 6
Angioedema:
- Urticaria is closely associated with angioedema in 40% of individuals 2
- Angioedema involves temporary swelling of deep dermis and subcutaneous tissue, most commonly affecting the lips, eyes, tongue, hands, and feet 6
- Unlike wheals, angioedema tends to be localized and may involve mucous membranes with pain and burning sensation 3
- Some forms of swelling can be associated with hives 6
Pathophysiology
Mechanism:
- The pathogenesis involves mast cell activation with subsequent release of histamine and other vasoactive mediators 2, 7
- This leads to increased vascular permeability of postcapillary venules and development of edema, erythema, and pruritus 2
- Mast cells are the primary effector cells in urticaria, producing and secreting inflammatory mediators responsible for cutaneous swelling and pruritus 7
Classification by Duration
Acute vs. Chronic:
- Acute urticaria lasts less than 6 weeks 5, 2
- Chronic urticaria is defined as lasting more than 6 weeks 5, 2
Important Clinical Pitfall
Duration as a Red Flag:
- Wheals persisting beyond 24 hours suggest urticarial vasculitis rather than ordinary urticaria 1
- Urticarial vasculitis is distinguished by wheals persisting for days and requires skin biopsy for histologic confirmation 1
Disease Activity Assessment
Severity Grading:
- Disease activity can be quantified using the 7-Day Urticaria Activity Score, which grades both wheals and pruritus 6
- Mild disease: <20 wheals per 24 hours with mild pruritus 6
- Moderate disease: 20-50 wheals per 24 hours with troublesome pruritus that doesn't interfere with daily activities 6
- Intense disease: >50 wheals per 24 hours or large confluent areas with severe pruritus interfering with normal daily activity or sleep 6