What are the causes of chronic urticaria?

Causes of Chronic Urticaria

Most cases of chronic urticaria remain idiopathic despite thorough evaluation, but when identifiable causes exist, they fall into autoimmune mechanisms (30-50% of cases), drug reactions, infections, physical triggers, and rarely systemic diseases. 1

Primary Etiological Categories

Autoimmune Mechanisms (Most Common Identifiable Cause)

• Autoimmune chronic urticaria accounts for approximately 30-50% of chronic urticaria cases with circulating functional autoantibodies against the high-affinity IgE receptor (FcεRI) or against IgE itself on mast cells and basophils 2, 3
• Thyroid autoimmunity is significantly more prevalent in chronic urticaria patients (14%) compared to population controls (6%), particularly asymptomatic Hashimoto's thyroiditis and rarely Graves' disease 1
• The mechanism by which these autoantibodies are produced remains unknown, and their presence does not clearly alter disease course or treatment response 1, 4

Idiopathic (Most Common Overall)

• Despite thorough evaluation, the majority of chronic urticaria cases remain unexplained and are classified as chronic idiopathic urticaria when no evidence of functional autoantibodies exists 1
• This represents the largest proportion of chronic urticaria cases in clinical practice 1, 5

Drug-Induced Mechanisms

• Nonimmunological mast cell degranulation can occur with codeine, radiocontrast media, and other agents independent of IgE receptor activation 1
• Aspirin and NSAIDs cause or aggravate urticaria through mechanisms probably involving leukotriene formation and histamine release 1
• Dietary pseudoallergens including salicylates, azo dyes, and food preservatives may trigger symptoms through similar pathways 1
• ACE inhibitors cause angioedema through inhibition of kinin breakdown rather than histamine-mediated mechanisms 1

Infectious Associations

• Helicobacter pylori infection shows variable association; meta-analysis demonstrates that chronic urticaria resolution is more likely when antibiotic eradication therapy is successful (Quality of evidence I, Strength of recommendation B) 1
• Proposed associations with occult infections (dental abscess, gastrointestinal candidiasis) have little supporting evidence (Quality of evidence III) 1
• In children, various infections may trigger chronic urticaria, though controlled studies are lacking 4, 6

Physical Urticarias

• Physical triggers reproducibly induce urticaria through mechanical stimuli (delayed pressure urticaria, symptomatic dermographism, vibratory angioedema), thermal stimuli (cholinergic, cold contact, localized heat urticaria), or other stimuli (aquagenic, solar urticaria, exercise-induced anaphylaxis) 1
• These represent 5-10% of chronic urticaria cases in children 4

Rare Systemic Causes

Urticarial vasculitis presents clinically as urticaria but shows small vessel vasculitis on skin biopsy, with potential joint and renal involvement 1

Autoinflammatory syndromes include:

• Hereditary: Cryopyrin-associated periodic syndromes (CAPS), familial cold autoinflammatory syndrome, Muckle-Wells syndrome 1
• Acquired: Schnitzler syndrome, systemic-onset juvenile idiopathic arthritis, adult-onset Still disease 1

C1 esterase inhibitor deficiency causes angioedema without wheals through complement activation and bradykinin formation rather than histamine 1

Associated Conditions (Not Direct Causes)

• Celiac disease shows significantly higher prevalence in children and adolescents with severe chronic urticaria compared to case-matched controls 1
• No statistical association exists between malignancy and urticaria (Quality of evidence II-ii), despite individual case reports 1

Critical Clinical Pitfalls

Avoid extensive laboratory testing in all patients—investigations should be guided by history and clinical presentation, not performed routinely 1. For mild disease responding to H1 antihistamines, no investigations are required 1.

Do not assume food allergies are common triggers in chronic urticaria; IgE-mediated reactions to environmental allergens are more relevant in acute/episodic urticaria than chronic forms 1.

Recognize that individual wheals lasting >24 hours or persisting for days suggest urticarial vasculitis rather than ordinary chronic urticaria, requiring skin biopsy for diagnosis 1.