What is urticaria?

What is Urticaria?

Urticaria is a disease characterized by short-lived, itchy weals (hives), angioedema, or both, resulting from mast cell activation and histamine release in the skin and mucous membranes. 1

Core Clinical Features

Weals (hives) are the pathognomonic lesions—erythematous, edematous, itchy, transient plaques that typically resolve within 2-24 hours without leaving a trace. 1 The lesions can appear anywhere on the body and are characterized by:

• Intense pruritus (itching) as the primary symptom 2
• Transient nature—individual lesions lasting 2-24 hours in ordinary urticaria 1
• Central pallor with surrounding erythema (wheal-and-flare reaction) 3

Angioedema involves deeper dermal and subcutaneous tissue, causing swelling of areas like the eyelids, lips, and mucous membranes, often with pain and burning rather than itching. 4, 5 When angioedema affects the respiratory tract, it becomes life-threatening and requires urgent treatment. 5

Clinical Classification

The British Association of Dermatologists provides a structured classification based on presentation rather than etiology, which is more clinically useful: 1

Ordinary Urticaria (most common pattern)

• Acute: Continuous activity up to 6 weeks 1
• Chronic: 6 weeks or more of continuous activity 1
• Episodic: Acute intermittent or recurrent activity 1

Physical Urticarias

Reproducibly triggered by specific physical stimuli (mechanical, thermal, or other): 1

• Symptomatic dermographism (pressure/scratching)
• Delayed pressure urticaria (takes 2-6 hours to develop, lasts up to 48 hours) 1
• Cholinergic urticaria (triggered by sweating stimulus)
• Cold contact urticaria
• Solar urticaria
• Aquagenic urticaria

Other Important Patterns

• Contact urticaria: Only occurs when eliciting substance is absorbed through skin or mucous membranes; can progress to anaphylaxis in highly sensitized individuals (e.g., latex allergy) 1
• Urticarial vasculitis: Presents clinically as urticaria but shows small vessel vasculitis on biopsy; weals persist for days rather than hours 1
• Angioedema without weals: May be caused by ACE inhibitors or C1 esterase inhibitor deficiency 1

Pathophysiology

Mast cells are the primary effector cells, producing and secreting inflammatory mediators, predominantly histamine, which cause cutaneous swelling and pruritus. 2, 6 The mechanisms of mast cell activation include:

• IgE-mediated reactions to allergens (foods, drugs, latex) 1
• Autoimmune mechanisms: Approximately one-third of chronic urticaria patients have functional autoantibodies against the high-affinity IgE receptor or against IgE itself 4
• Non-immunological degranulation from drugs like codeine, NSAIDs, or radiocontrast media 1
• Physical stimuli in physical urticarias 1

Epidemiology and Natural History

• Lifetime prevalence: 20% for acute urticaria, 1% for chronic urticaria 2
• Acute urticaria is most prevalent in the pediatric population and is often recurrent 4
• Most patients do not have systemic reactions, though allergic and some physical urticarias may occasionally progress to anaphylaxis 1

Diagnostic Approach

The diagnosis is primarily clinical based on patient history. 1 Key distinguishing features include:

• Duration of individual weals: 2-24 hours in ordinary urticaria, up to 2 hours in contact urticaria, within 1 hour for most physical urticarias (except delayed pressure), and days for urticarial vasculitis 1
• Pattern of occurrence: Spontaneous vs. induced by specific triggers 1
• Associated symptoms: Angioedema, systemic symptoms, or features suggesting vasculitis 1

Investigations should be guided by history and are not required for all patients. 1 No routine blood tests are needed for mild disease responding to antihistamines. 1

Important Clinical Pitfalls

• Distinguish angioedema without weals from urticaria with angioedema, as the former may indicate ACE inhibitor use or C1 esterase inhibitor deficiency requiring different management 1
• Recognize urticarial vasculitis when weals persist beyond 24 hours, as this requires skin biopsy and evaluation for systemic involvement 1
• Consider anaphylaxis risk in contact urticaria and certain physical urticarias, particularly in highly sensitized individuals 1
• Chronic urticaria etiology remains uncertain in many cases despite thorough evaluation (idiopathic), though autoimmune mechanisms, chronic infections, or food additive intolerance may be identified 1, 3