15.2 Urticaria & Angioedema
Key Takeaways
- Hereditary angioedema, drug-induced urticaria and angioedema, other causes and contrast-related allergy are the enumerated topics.
- Bradykinin-mediated angioedema lacks urticaria and pruritus and does not respond to epinephrine, antihistamines or corticosteroids.
- Angiotensin-converting enzyme inhibitor angioedema may begin months to years after starting the drug and requires permanent discontinuation of the class.
- Hereditary angioedema shows low C4 with low C1 inhibitor level or function, and is treated with bradykinin-targeted therapies.
- Acquired C1 inhibitor deficiency additionally shows a low C1q level and is associated with lymphoproliferative disease.
Last updated: August 2026
1. Urticaria & Angioedema
Cutaneous wheals (urticaria) and deep tissue swelling (angioedema) are common clinical entities with distinct pathophysiologic pathways (histaminergic mast-cell mediated versus bradykinin-mediated) that dictate completely different therapeutic approaches.
Urticaria (Hives)
- Pathophysiology: Dermal microvascular hyperpermeability driven by mast cell and basophil degranulation with local release of histamine, prostaglandin D2, and leukotrienes.
- Acute Urticaria (< 6 Weeks): Usually self-limiting; commonly triggered by viral infections (upper respiratory or gastrointestinal), food/drug allergies, or insect stings.
- Chronic Spontaneous Urticaria (CSU, >= 6 Weeks): Recurrent wheals with or without angioedema lasting >= 6 weeks. Etiology is autoimmune in > 40% to 50% of cases, driven by functional IgG autoantibodies directed against either IgE or the high-affinity IgE receptor alpha subunit (FcεRIα) on cutaneous mast cells.
- Clinical Characteristics of True Urticaria:
- Intensely pruritic, erythematous, raised, edematous wheals with central pallor.
- Crucial Diagnostic Rule: Individual lesions are evanescent (fleeting) and resolve completely within < 24 hours without leaving bruising, scarring, or residual pigmentation, while new lesions appear elsewhere.
- Urticarial Vasculitis (Critical Mimicker):
- Suspect when individual lesions persist for > 24 to 48 hours in the exact same location, are described as painful, burning, or tender rather than pruritic, and leave residual purpura, ecchymosis, or hyperpigmentation upon fading.
- Workup: Full-thickness punch biopsy of the skin lesion showing leukocytoclastic vasculitis (neutrophilic infiltration with nuclear debris/leukocytoclasia, fibrinoid necrosis of postcapillary venule walls, and RBC extravasation). Check serum complement levels (C3, C4, CH50) and anti-C1q antibodies; rule out systemic lupus erythematosus (SLE) and hepatitis C.
- Stepwise Management Strategy for Chronic Spontaneous Urticaria (CSU):
- Step 1: Standard-dose Second-Generation H1 Antihistamines (e.g., Cetirizine 10 mg, Loratadine 10 mg, Fexofenadine 180 mg, Levocetirizine 5 mg daily).
- Step 2: Up-titrate the second-generation H1 antihistamine dose up to 4 times (4x) the standard licensed dose (e.g., Cetirizine 20 mg PO BID or 40 mg PO daily).
- Step 3: Add Omalizumab (anti-IgE humanized monoclonal antibody, 300 mg subcutaneous injection every 4 weeks).
- Step 4: Add Cyclosporine A (calcineurin inhibitor, 3–5 mg/kg/day divided BID; monitor blood pressure and renal function).
- Board Pearl: Avoid long-term maintenance systemic corticosteroids for CSU due to significant cumulative toxicities and rebound flares upon steroid withdrawal.
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Angioedema: Histaminergic vs. Bradykinin-Mediated
Angioedema represents localized, non-pitting edema of the deep dermis, subcutaneous tissue, or submucosal layers. Correctly distinguishing between Mast Cell / Histaminergic and Bradykinin-Mediated angioedema is a critical lifesaving skill.
| Feature | Histaminergic (Mast Cell) Angioedema | Bradykinin-Mediated Angioedema |
|---|---|---|
| Pathogenic Mediator | Histamine, Leukotrienes, Prostaglandins from mast cells | Bradykinin (excess production or impaired degradation) |
| Associated Cutaneous Signs | Urticaria (hives), pruritus, erythema, flushing | ABSENCE of urticaria (no hives); no pruritus |
| Gastrointestinal Symptoms | Rare | Frequent: Severe episodic colicky abdominal pain, nausea, vomiting, transient ascites, and bowel wall edema on CT scan |
| Speed of Onset | Rapid (peaks in minutes to 1–2 hours) | Slower, progressive swelling (peaks over 12–36 hours, resolves over 48–72 hours) |
| Response to Epinephrine, Antihistamines, & Steroids | EXCELLENT RESPONSE (standard first-line therapy) | ZERO RESPONSE (completely ineffective) |
| Clinical Subtypes | Allergic anaphylaxis, acute viral urticaria, physical triggers | ACE Inhibitor-Induced, Hereditary Angioedema (HAE), Acquired C1-INH Deficiency |
Bradykinin-Mediated Angioedema Subtypes
1. ACE Inhibitor-Induced Angioedema
- Mechanism: Angiotensin-Converting Enzyme (ACE / kininase II) is the primary enzyme responsible for the physiological degradation of bradykinin. Pharmacologic ACE inhibition leads to impaired bradykinin breakdown and localized accumulation, activating bradykinin B2 receptors to cause potent vasodilation and increased vascular permeability.
- Epidemiology: Occurs in 0.1% to 0.7% of patients taking ACE inhibitors (up to 4- to 5-fold higher incidence in Black patients due to genetic polymorphisms in alternative bradykinin degradation enzymes like aminopeptidase P and neutral endopeptidase).
- Timing: Can occur within hours/days of initiating an ACE inhibitor or after years of completely uneventful therapy.
- Clinical Presentation: Asymmetric, painless, non-pruritic swelling of the tongue, lips, uvula, soft palate, and larynx. Can cause rapid life-threatening airway obstruction.
- Management:
- Permanently discontinue the ACE inhibitor. Mark ACE inhibitors as a severe drug allergy in the medical record.
- Airway Management: Early fiberoptic nasopharyngoscopy and elective endotracheal intubation (or emergent cricothyroidotomy/tracheostomy) if laryngeal edema or stridor is present.
- Antihistamines, corticosteroids, and epinephrine are ineffective but frequently given while securing the airway.
- Angiotensin Receptor Blocker (ARB) Use: ARBs do not inhibit kininase II and have very low cross-reactivity (< 2% to 5%); however, in patients with prior life-threatening ACEi angioedema, ARBs should be used with extreme caution or alternative antihypertensives (e.g., CCBs) should be chosen.
2. Hereditary Angioedema (HAE)
- Genetics: Autosomal dominant disorder caused by mutations in the SERPING1 gene on chromosome 11q, which encodes C1-esterase inhibitor (C1-INH). C1-INH is the primary physiological inhibitor of plasma kallikrein and activated factor XII (Hageman factor); deficiency leads to unchecked activation of the contact/kallikrein-kinin cascade and massive bradykinin generation.
- Classification & Diagnostic Complement Profile:
| HAE / AAE Subtype | Prevalence | C1-INH Protein (Antigen) Level | C1-INH Functional Activity | Serum C4 Level | Serum C1q Level |
|---|---|---|---|---|---|
| HAE Type 1 | ~85% of HAE | Markedly Low (< 50%) | Markedly Low (< 50%) | Persistently Low (< 50%) | Normal |
| HAE Type 2 | ~15% of HAE | Normal or Elevated | Markedly Low (< 50%) | Persistently Low (< 50%) | Normal |
| Acquired C1-INH Deficiency (AAE) | Rare (onset > 50y) | Low or Normal | Markedly Low (< 50%) | Markedly Low (< 50%) | UNIQUELY LOW (< 30%) |
- Screening Test of Choice: Serum C4 level is the premier screening test (persistently low in > 95% of patients with Type 1 and Type 2 HAE, even during asymptomatic baseline periods; drops to near undetectable during acute attacks). If C4 is low, order C1-INH antigenic level and C1-INH functional assay.
- Acquired C1-INH Deficiency (AAE): Late-onset angioedema in adults aged > 50 years associated with B-cell lymphoproliferative disorders (e.g., Chronic Lymphocytic Leukemia, Non-Hodgkin Lymphoma, Monoclonal Gammopathy of Undetermined Significance [MGUS]) or autoantibodies directed against C1-INH. Characterized by markedly low serum C1q levels (C1q is consumed by neoplastic/autoimmune immune complexes; C1q is completely normal in hereditary forms).
- Targeted Treatment of Acute HAE Attacks (On-Demand):
- C1-Esterase Inhibitor Concentrates: Plasma-derived human C1-INH (Berinert 20 U/kg IV) or recombinant human C1-INH (Ruconest 50 U/kg IV). Restores functional C1-INH to halt bradykinin generation.
- Bradykinin B2-Receptor Antagonist: Icatibant (30 mg subcutaneous injection in the abdominal wall; can be self-administered by the patient).
- Plasma Kallikrein Inhibitor: Ecallantide (30 mg subcutaneous injection in 3 divided doses; risk of hypersensitivity, administer under healthcare supervision).
- Long-Term HAE Prophylaxis:
- Lanadelumab: Fully human monoclonal antibody against active plasma kallikrein (300 mg SC every 2 to 4 weeks; first-line prophylactic agent).
- Berotralstat: Oral, once-daily plasma kallikrein inhibitor (150 mg PO daily).
- Subcutaneous C1-INH Concentrate: Purified plasma-derived C1-INH (Haegarda 60 IU/kg SC twice weekly).
Test Your Knowledge
A 64-year-old woman with a history of Stage I follicular lymphoma in remission presents to the outpatient clinic with recurrent episodes of painless, non-pruritic swelling of her lips, tongue, and soft palate over the past 4 months. She has never experienced hives, wheals, or cutaneous itching and has never taken an ACE inhibitor or NSAID. An episode 2 weeks ago was accompanied by severe, colicky lower abdominal pain and watery diarrhea that resolved spontaneously after 48 hours. Physical examination demonstrates marked macroglossia and uvular edema without urticaria or erythema. Laboratory evaluation reveals: Serum C4 is 6 mg/dL (normal 16–47 mg/dL), C1-inhibitor (C1-INH) antigenic protein level is 8 mg/dL (normal 19–37 mg/dL), C1-INH functional activity is 18% (normal > 68%), and serum C1q level is 14 mcg/mL (normal 50–250 mcg/mL). Which of the following is the most accurate diagnosis?
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B
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D