Hereditary Angioedema (HAE)

  1. What Is Hereditary Angioedema?
  2. The Bradykinin Mechanism
  3. Three Types of HAE
  4. What HAE Attacks Feel Like
  5. What Triggers HAE Attacks
  6. Diagnosing HAE
  7. Treating an Acute Attack
  8. Preventing Attacks: Long-Term and Short-Term Prophylaxis
  9. Key Research Papers
  10. Connections
  11. Featured Videos

What Is Hereditary Angioedema?

Hereditary Angioedema (HAE) is a rare genetic disorder that causes sudden, unpredictable episodes of severe swelling deep in body tissues. Unlike a typical allergic reaction, HAE swelling is not caused by histamine — it is driven by a chemical called bradykinin. This single fact has enormous consequences: the antihistamines, steroids, and epinephrine that work for allergic reactions do not work for HAE, and patients who receive only those treatments can die of airway swelling while waiting for drugs that will not help them.

HAE is caused by a deficiency or malfunction of a protein called C1-esterase inhibitor (C1-INH), encoded by the SERPING1 gene. Without enough functional C1-INH, a cascade of enzymes runs unchecked and floods tissues with bradykinin, causing plasma to leak out of blood vessels into surrounding tissue. The swelling is non-pitting (it does not indent when pressed), is not red or itchy, and there are no hives — a key difference from allergic angioedema.

HAE is inherited in an autosomal dominant pattern: one mutated copy of the gene is enough to cause the disease. Each child of an affected parent has a 50% chance of inheriting HAE. However, approximately 25% of cases arise from de novo (new, spontaneous) mutations, so a negative family history does not rule out the diagnosis.

The condition affects approximately 1 in 10,000 to 1 in 50,000 people worldwide regardless of sex, race, or ethnicity. Historically, before effective treatments existed, untreated laryngeal (throat) attacks carried a mortality rate of 25–30%. A physician named Heinrich Quincke first described episodic angioedema in 1882; the hereditary pattern was characterized by William Osler in 1888; and the causative protein deficiency was identified by Fred Rosen in 1963 — opening the door to targeted treatment.

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The Bradykinin Mechanism

To understand why HAE treatments work the way they do, it helps to understand the underlying chemistry. C1-INH (C1-esterase inhibitor) is a serpin — a serine protease inhibitor — that acts as a brake on several enzyme systems simultaneously:

When C1-INH is absent or dysfunctional, plasma kallikrein operates without restraint. It cleaves HMWK in large amounts, releasing bradykinin. Bradykinin binds to bradykinin B2 receptors on vascular endothelial cells (the cells lining blood vessel walls). This binding opens gaps between cells, dramatically increasing vascular permeability. Plasma — the liquid part of blood — leaks out of capillaries and floods the surrounding tissue. The result is the swollen, fluid-filled tissue of an HAE attack.

Because histamine plays no role in this process, blocking histamine receptors (with antihistamines like diphenhydramine or cetirizine) cannot stop or reverse an HAE attack. Similarly, corticosteroids (which dampen histamine-related and arachidonic acid-related inflammation) and epinephrine (which counteracts histamine's effects on blood vessels) are ineffective at the root cause. Epinephrine may temporarily ease laryngeal symptoms by causing vasoconstriction, but it does not reduce bradykinin levels and provides no sustained benefit.

The practical implication is critical: every HAE patient should carry HAE-specific rescue medication, and every emergency provider treating a patient with known or suspected HAE should reach for bradykinin-targeting drugs first — not the allergy cart.

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Three Types of HAE

There are three recognized subtypes of HAE, distinguished by laboratory findings and genetics:

Type 1 HAE (approximately 85% of cases)

In Type 1, a mutation in the SERPING1 gene causes the body to produce too little C1-INH protein — a quantitative deficiency. Lab tests show both low C1-INH antigen (the protein itself is sparse) and low C1-INH functional activity (what little protein exists may also not work well). The complement protein C4 is chronically low even between attacks, because uninhibited C1 enzymes continuously consume it. C3 levels remain normal. The serum C4 level is the best and most practical screening test.

Type 2 HAE (approximately 15% of cases)

In Type 2, the SERPING1 mutation produces C1-INH protein in normal or even elevated amounts — but the protein does not function correctly (a qualitative deficiency). The key laboratory finding is normal or elevated C1-INH antigen with low C1-INH functional activity. C4 is still chronically low (consumed by uninhibited complement enzymes), and C3 remains normal. If a patient has normal C1-INH antigen levels but low functional activity, Type 2 should be strongly considered. Genetic testing of SERPING1 confirms the diagnosis.

Type 3 HAE (rare; predominantly women)

Type 3 HAE is quite different from Types 1 and 2. C1-INH levels and C4 levels are both normal. The most common identified cause is a gain-of-function mutation in the F12 gene, which encodes Factor XII (Hageman factor). The mutant Factor XII is overactive, driving excessive kallikrein activity and bradykinin release even without any C1-INH deficiency. In some patients no causative mutation is found. Type 3 is strongly linked to estrogen: attacks are triggered or dramatically worsened by oral contraceptives, hormone replacement therapy, and pregnancy. Women of childbearing age using estrogen-containing contraception who develop unexplained recurrent angioedema should be evaluated for Type 3 HAE.

A note on C1-INH levels vs. C4

Although C1-INH levels are low in Types 1 and 2, the serum C4 level is a better screening test because C4 remains depressed even between attacks (not just during them). A normal C4 level effectively excludes Types 1 and 2 HAE. C4 is inexpensive, widely available, and part of routine complement panels.

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What HAE Attacks Feel Like

HAE attacks are episodic — they come and go, typically lasting 2 to 5 days without treatment and resolving spontaneously. Swelling builds slowly over hours to reach its peak, then recedes. Attacks can involve three main body regions, each with distinct symptoms:

Subcutaneous (under the skin) swelling

The most visually obvious form of HAE. Swelling commonly affects the extremities (hands, arms, legs, feet), face (lips, eyelids, cheeks), and genitalia. The swelling is characteristically non-pitting (it does not leave an indentation when pressed), cool to the touch, not red, and not itchy — and critically, there are no hives (urticaria). The absence of hives is one of the most important clinical clues that distinguishes HAE from allergic angioedema. The skin may have a slightly mottled or firm appearance but does not look like a typical allergic rash.

Gastrointestinal swelling

GI attacks are probably the most commonly misdiagnosed form of HAE. Swelling of the gut wall causes severe abdominal cramps, nausea, vomiting, diarrhea, and sometimes ileus (temporary intestinal paralysis). The pain can be excruciating and mimic a surgical emergency — many HAE patients have undergone unnecessary appendectomy or exploratory laparotomy before the diagnosis was established. Abdominal ultrasound during an attack may show free fluid (ascites) around the bowel loops, which can be a helpful diagnostic clue. If a patient has recurrent, unexplained abdominal crises with no clear surgical cause, HAE should be on the differential diagnosis list.

Laryngeal (throat) swelling

Laryngeal HAE is the most dangerous manifestation. Swelling in the throat and larynx causes hoarseness, stridor (a high-pitched breathing sound), difficulty swallowing, and a sensation of throat tightening. It can progress rapidly to complete airway obstruction and asphyxiation. This is why HAE historically carried a 25–30% mortality rate in undiagnosed or untreated patients — people died of laryngeal attacks that were mistaken for allergic reactions and treated with drugs that did not work. Today, with proper diagnosis and on-hand rescue medication, laryngeal attacks are survivable, but they remain a medical emergency requiring immediate treatment.

Prodromal erythema marginatum

About 50% of patients experience a warning rash before a major attack: erythema marginatum, a serpiginous (snake-like, winding-edged), mottled reddish rash that is not raised and not itchy. This is the opposite of hives. Recognizing this rash as a prodrome — a warning sign — gives patients time to take rescue medication early, before swelling becomes severe. Patients should be taught to recognize it.

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What Triggers HAE Attacks

Attacks can be spontaneous and unpredictable, but many have identifiable triggers. Knowing and managing triggers is an important part of living with HAE:

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Diagnosing HAE

HAE is notoriously underdiagnosed. The average time from symptom onset to correct diagnosis is 8 to 12 years. The condition is often mistaken for allergic angioedema, irritable bowel syndrome, or a surgical abdomen. Knowing what tests to order can save years of suffering.

Best initial screening test: Serum C4

The serum C4 level is the single most useful and cost-effective first test. C4 is chronically low in Types 1 and 2 HAE — not just during attacks, but between attacks too, because uninhibited C1 continuously consumes it. A normal C4 level effectively excludes Types 1 and 2 HAE. C4 can be ordered as part of a complement panel at any commercial lab. In Type 3 HAE, C4 is normal.

Confirmatory tests

Genetic testing

SERPING1 gene sequencing confirms Types 1 and 2 at the molecular level and is particularly useful when laboratory results are borderline, when testing children of an affected parent (to identify presymptomatic cases), and for family planning. For suspected Type 3, F12 gene sequencing can identify the gain-of-function mutation. However, genetic testing is not required for clinical diagnosis — the combination of history, family history, and complement labs is usually sufficient.

Family history

A positive family history strongly supports the diagnosis. However, approximately 25% of HAE cases result from de novo mutations — brand-new mutations that did not come from either parent. A negative family history therefore does not exclude HAE. Children of patients with Types 1 or 2 HAE should be tested early, as symptoms typically begin in childhood or adolescence and can be severe and even fatal without treatment.

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Treating an Acute Attack

The most important principle of treating an acute HAE attack is to act early. Waiting to see if an attack resolves on its own — or wasting time on ineffective antihistamines and steroids — can be fatal in laryngeal attacks. Every patient with HAE should have on-hand rescue medication at all times. All of the following treatments target the bradykinin pathway, not histamine.

Critical warning: Antihistamines, corticosteroids, and epinephrine are not effective treatments for HAE. Epinephrine may provide brief temporary relief of laryngeal symptoms through vasoconstriction, but it does not lower bradykinin levels and should never be used as a substitute for or delay in administering HAE-specific medication. Do not wait for these to work.

Plasma-derived C1-INH concentrate (pdC1-INH)

Berinert (CSL Behring): 20 IU/kg given intravenously. This replaces the missing or deficient C1-INH directly, restoring normal inhibition of kallikrein and the contact pathway. It has the most extensive clinical evidence base, is approved for all attack locations including laryngeal, and can be self-administered at home after training. Another product, Cinryze, is also available. For laryngeal attacks, pdC1-INH is the preferred first-line agent.

Icatibant (Firazyr)

Icatibant is a bradykinin B2 receptor antagonist — it blocks bradykinin from binding to its receptor on blood vessel walls, preventing the increase in vascular permeability. It is given as a 30 mg subcutaneous injection (just under the skin), with onset of action within 20–30 minutes. Patients can self-inject at home. A second dose can be given after 6 hours if the response is incomplete. It does not replace C1-INH or reduce bradykinin production, but by blocking the receptor, it prevents the downstream effect.

Ecallantide (Kalbitor)

Ecallantide is a plasma kallikrein inhibitor — it stops kallikrein from cleaving HMWK into bradykinin. It is given as three 10 mg subcutaneous injections (30 mg total). It is approved in the United States only and must be administered in a healthcare setting because it carries an approximately 3% risk of serious allergic reactions including anaphylaxis. It is not available in Europe.

Recombinant C1-INH (Ruconest)

Ruconest is a recombinant form of human C1-INH derived from transgenic rabbits. It is given intravenously and works like plasma-derived C1-INH. It is an option for patients who prefer a non-plasma-derived product. Patients with rabbit allergy should not use it.

Fresh Frozen Plasma (FFP)

FFP contains C1-INH along with all other plasma proteins. It can be life-saving when no HAE-specific drugs are available — for example, in a hospital without HAE medications in stock. FFP is less predictable and slower than specific HAE drugs, and some theoretical concern exists that the kallikrein substrates in FFP could transiently worsen swelling. Nevertheless, FFP is a last resort that is far better than nothing in a laryngeal emergency.

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Preventing Attacks: Long-Term and Short-Term Prophylaxis

Prevention is as important as acute treatment in HAE management. There are two categories: short-term prophylaxis before a known trigger, and long-term prophylaxis (LTP) to reduce the overall frequency and severity of attacks.

Short-Term Prophylaxis (before procedures)

Any invasive procedure — dental work, surgery, intubation, endoscopy — poses a risk of triggering an acute attack, sometimes with laryngeal involvement. Short-term prophylaxis should be planned for all HAE patients undergoing such procedures, regardless of how well-controlled they are:

Long-Term Prophylaxis (LTP)

LTP is appropriate for patients with frequent attacks (roughly one or more per month), a history of laryngeal attacks, attacks that severely impair quality of life, or inadequate access to acute rescue medication. Multiple options are now available:

The landscape of HAE treatment has been transformed in the past 15 years. Patients who previously suffered weekly attacks and lived in fear of laryngeal emergencies can now achieve dramatic reduction in attack frequency, and some achieve complete attack freedom on prophylaxis.

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Key Research Papers

  1. Rosen FS, Pensky J, Donaldson V, Charache P. Hereditary deficiency of a complement component (C'1 INH) in two families with hereditary angioneurotic edema. Science. 1963;139(3554):476–7. PMID: 14018001 — The landmark 1963 paper identifying C1-INH deficiency as the cause of hereditary angioedema.
  2. Cicardi M, et al. Angioedema due to acquired C1-inhibitor deficiency: a bridging condition between autoimmunity and lymphoproliferation. Allergy. 2006;61(4):422–33. PMID: 16512810 — Characterizes acquired vs. hereditary angioedema and the role of autoantibodies.
  3. Bork K, et al. Recurrent episodes of skin angioedema and severe attacks of abdominal pain induced by oral contraceptives or pregnancy. Scand J Gastroenterol. 2003;38(6):694–8. PMID: 12825880 — Demonstrates estrogen as a major trigger for angioedema in susceptible women.
  4. Craig TJ, et al. Efficacy of human C1 esterase inhibitor concentrate compared with placebo in acute hereditary angioedema attacks. J Allergy Clin Immunol. 2009;124(4):801–8. PMID: 19800676 — Pivotal trial establishing pdC1-INH (Berinert) as effective acute HAE treatment.
  5. Cicardi M, et al. Icatibant, a new bradykinin-receptor antagonist, in hereditary angioedema. N Engl J Med. 2010;363(6):532–41. PMID: 20818887 — Phase 3 trial demonstrating icatibant's efficacy as an acute HAE treatment.
  6. Cicardi M, et al. Lanadelumab for prophylaxis of hereditary angioedema attacks: a phase 3, randomized, double-blind, placebo-controlled trial (HELP study). N Engl J Med. 2018;379(26):2489–99. PMID: 30586691 — HELP trial: lanadelumab reduced HAE attacks by 87% vs. placebo; led to FDA approval.
  7. Zuraw BL, et al. Subcutaneous icatibant for the treatment of hereditary angioedema attacks. N Engl J Med. 2010;363(6):523–31. PMID: 20818888 — Companion FAST-3 trial supporting subcutaneous self-injection of icatibant.
  8. Bernstein JA, et al. Subcutaneous C1-inhibitor (Haegarda) for the prevention of hereditary angioedema (PREVENT study). Lancet. 2017;389(10082):1899–907. PMID: 28366545 — PREVENT trial: twice-weekly subcutaneous C1-INH reduced attacks by 84%; established Haegarda as first-line LTP.
  9. Farkas H, et al. International consensus on the diagnosis and management of pediatric patients with hereditary angioedema with C1 inhibitor deficiency. Allergy. 2017;72(2):300–13. PMID: 27545567 — International expert consensus on pediatric HAE, addressing special considerations for children.
  10. Riedl MA, et al. Efficacy and safety of plasma-derived C1-esterase inhibitor for acute hereditary angioedema attacks during 2 open-label studies. Ann Allergy Asthma Immunol. 2012;108(1):49–55. PMID: 22153576 — Open-label confirmation of pdC1-INH safety and effectiveness across attack locations.
  11. Zuraw BL, et al. Berotralstat (BCX7353) as a preventive treatment for hereditary angioedema: A randomized, placebo-controlled phase 3 trial. J Allergy Clin Immunol Pract. 2021;9(6):2305–14. PMID: 33601079 — ZENITH-1 trial establishing berotralstat as the first daily oral HAE prophylaxis option.
  12. Bork K. Hereditary angioedema with normal C1 inhibitor: clinical symptoms, triggered by oral contraceptives and during pregnancy. Allergy. 2003;58(4):382–3. PMID: 12708997 — Early characterization of Type 3 HAE (normal C1-INH) with estrogen as trigger.

Search PubMed: Hereditary Angioedema & C1 Inhibitor
Search PubMed: Bradykinin Angioedema Treatment
Search PubMed: Lanadelumab / Icatibant HAE Prophylaxis

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Connections

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