Hereditary Angioedema

• C1-INH Protein and Contact System Activation: C1-INH is a serine protease inhibitor that serves as the primary negative regulator of factor XIIa (activated factor XII), prekallikrein, and kallikrein in the contact (Hageman) system. In HAE, either reduced C1-INH protein levels (Type I HAE, ~80% of cases) or dysfunctional C1-INH protein (Type II HAE, ~20% of cases) fail to inhibit kallikrein appropriately. This allows unopposed activation of the contact system, leading to excessive conversion of high-molecular-weight kininogen (HMWK) to bradykinin, a potent vasoactive peptide. Bradykinin increases vascular permeability through activation of bradykinin B2 receptors on endothelial cells, promoting migration of fluid into interstitial spaces and producing the characteristic non-pitting, localized angioedema. Type III HAE (previously called "normal C1-INH HAE") results from gain-of-function mutations in factor XII or plasminogen, also leading to excessive bradykinin generation through alternative mechanisms.

• Complement Cascade Dysregulation: C1-INH also functions as a primary inhibitor of the first component of complement (C1q). In HAE, reduced C1-INH activity permits uncontrolled C1 activation, leading to consumption of C4 and C3 components. The characteristic finding of markedly depressed serum C4 levels (often <15 mg/dL during and between attacks) reflects ongoing complement consumption. While complement activation itself does not directly cause bradykinin production, complement degradation products (C3a, C5a) have weak anaphylatoxin effects, though this is not the primary driver of angioedema in HAE. Notably, C1-INH deficiency does not produce true urticaria because mast cell degranulation (which releases histamine) does not occur; this distinguishes HAE from allergic angioedema.

• Fibrinolytic Pathway Involvement: C1-INH also inhibits tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA), which are essential for fibrin degradation. Deficient C1-INH permits excessive activation of plasmin, intensifying fibrinolysis. This mechanism contributes to tissue edema and may explain why some patients exhibit heightened fibrinolytic activity during acute attacks. Additionally, kallikrein directly activates the fibrinolytic cascade by converting plasminogen to plasmin, creating a reciprocal amplification of both bradykinin and fibrinolytic pathway activation.

• Genetic Basis and Type Classification: HAE is transmitted in an autosomal-dominant pattern with mutations in the SERPING1 gene encoding C1-INH. Type I HAE results from loss-of-function mutations causing quantitative C1-INH deficiency (serum C1-INH levels typically 10-50% of normal). Type II HAE results from mutations producing dysfunctional C1-INH protein, with normal or elevated C1-INH antigen levels but markedly reduced functional activity. Approximately 25% of HAE cases arise from de novo mutations with negative family history. Type III HAE is associated with mutations in F12 (factor XII) or PLG (plasminogen) genes; these patients typically have normal C1-INH levels and function, making diagnosis more challenging.

• Bradykinin as the Central Mediator: Bradykinin is the primary pathophysiologic mediator in HAE, acting through B2 receptors present on endothelial cells, smooth muscle, and fibroblasts. B2 receptor activation increases expression of vascular endothelial growth factor (VEGF) and other permeability-enhancing factors, increases intracellular calcium, and disrupts vascular tight junctions through internalization of VE-cadherin. This explains why clinical manifestations are non-pruritic and non-erythematous (unlike mast cell-mediated angioedema), and why traditional antihistamines and corticosteroids are ineffective—these attacks are not driven by histamine or immune mechanisms but by a bradykinin-driven increase in vascular permeability.

• Type I Hereditary Angioedema (Quantitative C1-INH Deficiency): Comprises approximately 80% of HAE cases and results from haploinsufficiency due to loss-of-function mutations in SERPING1 (located on chromosome 11q12.1) that reduce C1-INH protein synthesis or stability. Serum C1-INH levels are typically 10-50% of normal, with correspondingly depressed functional C1-INH activity. These patients have markedly reduced C4 levels (characteristically <10 mg/dL), often with C3 levels also reduced. Type I HAE typically manifests in childhood or early adulthood and follows Mendelian autosomal-dominant inheritance; approximately 75% of affected individuals have an affected parent, while 25% represent de novo mutations.

• Type II Hereditary Angioedema (Dysfunctional C1-INH): Accounts for approximately 20% of symptomatic HAE cases and results from missense mutations in SERPING1 that produce structurally abnormal C1-INH with reduced functional capacity despite normal or elevated antigen levels. The hallmark finding is normal or elevated C1-INH antigen concentration (by ELISA or radial immunodiffusion) with markedly reduced functional activity (by functional assay). Serum C4 levels are typically depressed similar to Type I HAE. Clinical presentation and severity are indistinguishable from Type I HAE, but diagnosis requires functional assessment rather than relying solely on antigen quantification.

• Type III Hereditary Angioedema (Normal C1-INH HAE): A genetically heterogeneous group (5-10% of clinical HAE) characterized by normal C1-INH antigen levels, normal C1-INH function, and normal or near-normal C4 levels. Most Type III cases result from gain-of-function mutations in the F12 gene (encoding factor XII), leading to spontaneous or contact-induced activation of factor XII and excessive bradykinin generation independent of C1-INH. Approximately 20-30% of Type III HAE cases remain genetically uncharacterized (designated HAE-nU). Type III HAE may manifest later in life (often in the third or fourth decade) and shows increased female predominance with possible estrogen-dependent acceleration of symptoms.

• Acquired Angioedema (AAE): A distinct entity accounting for 5-10% of angioedema cases, typically manifesting in patients >40 years old with no family history. Type 1 AAE results from lymphoproliferative disorders (chronic lymphocytic leukemia, lymphoma, Waldenstrom macroglobulinemia) that produce monoclonal antibodies against C1-INH or anti-idiotypic antibodies, causing immune complex-mediated C1-INH consumption. Type 2 AAE results from autoantibodies against C1q, often associated with systemic lupus erythematosus or other connective tissue diseases. AAE presents with identical clinical manifestations to HAE but typically with later age of onset, absence of family history, and evidence of underlying lymphoproliferative or autoimmune disease.

• Environmental and Physiologic Triggers: Attacks in HAE patients are often precipitated by specific triggers including trauma (dental procedures, surgery, minor injuries), infections (particularly upper respiratory infections), emotional stress, and estrogen exposure (oral contraceptives, hormone replacement therapy in women). Notably, approximately 30% of patients with HAE report no identifiable trigger, and attacks may occur spontaneously. ACE inhibitors and NSAIDs may theoretically exacerbate HAE by reducing bradykinin degradation, though clinical evidence is limited.

• Subcutaneous and Deep Tissue Angioedema (Most Common Manifestation): Patients experience recurrent episodes of localized, non-pitting, well-demarcated swelling typically affecting the extremities (hands, feet, forearms, legs), face, lips, tongue, and occasionally genitalia. Affected areas are characteristically non-erythematous, non-pruritic, and non-urticarial, distinguishing HAE from allergic angioedema mediated by mast cell degranulation and histamine release. Individual attacks typically last 24-72 hours and may range from mild (affecting limited areas) to severe (causing significant functional impairment). The skin appears normal or slightly pale, without the wheal-and-flare response characteristic of allergic reactions. Deep tissue involvement may produce dramatic swelling of lips (sometimes causing severe dysarthria) or extremities that grossly distort anatomy.

• Gastrointestinal Angioedema (Visceral Attacks): Approximately 70-80% of HAE patients experience abdominal manifestations during their disease course, ranging from acute abdominal pain to life-threatening intestinal obstruction. Angioedema affecting the gastrointestinal mucosa and submucosa produces severe colicky pain, often accompanied by nausea, vomiting, and diarrhea. Severe cases manifest with acute abdomen, including acute bowel obstruction or apparent peritonitis. Unlike skin manifestations, GI angioedema may be difficult to recognize clinically and radiologically, often leading to unnecessary surgical intervention (up to 20% of HAE patients undergo one or more inappropriate abdominal surgeries before diagnosis). Attacks typically involve the small intestine or colon and may produce transient areas of bowel wall edema visible on CT or ultrasound. GI symptoms can be severely debilitating and occasionally life-threatening due to fluid sequestration into the bowel.

• Laryngeal Angioedema (Medical Emergency): Involvement of the larynx and upper airway occurs in 25-60% of HAE patients and represents the most life-threatening manifestation. Attacks present with progressive hoarseness, stridor, dysphagia, throat tightness, and sensation of impending airway closure. Laryngeal edema may develop rapidly over minutes to hours and can progress to complete airway obstruction, asphyxiation, and death. Importantly, laryngeal edema in HAE does not typically produce the visible oropharyngeal edema or angioedema of lips/tongue that might be expected; the primary site of obstruction is at the glottis or subglottic larynx, below the visible pharynx. This insidious presentation may cause delayed recognition of the threat until airway compromise is profound. Historical asphyxiation deaths in untreated HAE patients underscore the critical importance of rapid diagnosis and aggressive management of laryngeal involvement.

• Prodromal Symptoms: Many patients report nonspecific prodromal symptoms preceding angioedema attacks by hours to days, including erythema marginatum (a macular erythematous rash with raised, serpiginous borders resembling erythema multiforme), fatigue, mood changes, or abdominal discomfort. Erythema marginatum may appear on the trunk or extremities and is thought to reflect early-stage edema in dermal layers. Recognition of prodromal symptoms may allow patients to seek preventive therapy or medical attention before severe attacks develop.

• Physical Examination Findings: During acute attacks, affected areas demonstrate localized, non-pitting edema with preserved skin color (absence of erythema or urticaria). Mucous membranes may appear pale or slightly cyanotic if severely swollen. Extremities may show massive edema with skin tightness and potential compromise of distal perfusion in severe cases. The abdomen may exhibit distension, pain with palpation, and visible peristaltic waves if bowel obstruction is present. Laryngeal examination during acute attack may reveal glottic or subglottic edema with variable visualization depending on the extent of swelling (severe edema may prevent visualization due to obstruction). Between attacks, physical examination findings are typically normal.

• Clinical History and Recognition: The diagnostic approach begins with high clinical suspicion based on recurrent episodes of angioedema with the following key distinguishing features: (1) absence of urticaria or pruritus (unlike allergic angioedema), (2) involvement of deep subcutaneous tissues with non-pitting character, (3) typical distribution affecting extremities, face, and GI tract, (4) family history of similar attacks or sudden death (suggesting prior unrecognized laryngeal edema), and (5) ineffectiveness of antihistamines and corticosteroids. The historical presence of abdominal attacks misdiagnosed as acute abdomen or surgical emergencies should raise high suspicion for HAE. Critical distinguishing feature: patients with HAE typically have normal serum tryptase levels and no evidence of mast cell activation.

• Serum C4 Level (Initial Screening Test): Measurement of serum C4 concentration is the most practical initial screening test, with depressed C4 levels (<15 mg/dL) occurring in Types I and II HAE and correlating with disease activity. C4 levels are often strikingly depressed (5-10 mg/dL) and serve as a biochemical marker of ongoing complement activation. However, C4 levels may normalize between attacks in some patients (approximately 10-20% of Type I and II HAE patients have normal C4 during quiescent periods), so a normal C4 does not definitively exclude HAE. Type III HAE characteristically has normal C4 levels, making this test less useful for Type III diagnosis. The C4 level is valuable as part of a screening panel: a depressed C4 in a patient with recurrent angioedema without urticaria strongly suggests HAE.

• C1 Esterase Inhibitor Antigen and Function Assays: Quantitative C1-INH antigen measurement (by ELISA or radial immunodiffusion) and functional C1-INH assays (most commonly using spectrophotometric measurement of factor XIIa inhibition or C1-INH esterase activity) are essential for definitive diagnosis and classification. Type I HAE: Low C1-INH antigen (<50% normal) with correspondingly low functional activity. Type II HAE: Normal or elevated C1-INH antigen with markedly reduced functional activity (functional/antigen ratio typically <0.70). Normal values vary by laboratory but typically C1-INH antigen is 21-39 mg/dL (or 100% = patient/reference serum ratio) and functional activity is 70-130% of normal. The critical distinction is that functional C1-INH activity is reduced in both Type I and II HAE, while Type III HAE shows normal C1-INH antigen and normal functional activity.

• Complement C3 Levels: Serum C3 concentration may be moderately depressed in some Type I and II HAE patients due to consumption via C1 activation (typical range 50-90 mg/dL in affected patients vs. normal 80-150 mg/dL), though C3 is less consistently affected than C4. C3 levels are normal in Type III HAE. C3 measurement provides additional biochemical confirmation of complement consumption but is less specific than C4.

• Diagnosis of Type III HAE: Type III HAE is more challenging to diagnose because C1-INH levels and function are normal, and C4 levels are typically normal or only mildly depressed. Diagnosis relies on (1) clinical presentation with recurrent angioedema, (2) normal C1-INH antigen and function, (3) genetic testing for F12 or PLG mutations (F12 mutations account for ~50% of genetically identified Type III cases), or (4) specialized tests of contact system activation such as factor XIIa activity. Some laboratories perform Factor XII mutation analysis (sequencing of F12 gene exon 2, where most mutations cluster). When Type III is suspected but F12 mutation testing is negative, diagnosis remains provisional.

• Diagnostic Criteria (Established by International Classification): Definitive diagnosis of HAE requires:
• Type I or II HAE: Low C1-

Immediate stabilisation (laryngeal attack = airway emergency)

• Airway first: assess for hoarseness/stridor and secure the airway early — awake fiberoptic intubation or surgical airway (cricothyrotomy) if progression is rapid, since edema at the glottis worsens over minutes to hours and late intubation may be impossible.
• Give a targeted agent simultaneously, not sequentially: the WAO/EAACI hereditary angioedema guideline and the US HAEA Medical Advisory Board recommend treating every laryngeal attack with an on-demand agent regardless of severity, and observing the patient afterward for rebound.

First-line on-demand (acute attack) therapy — any of

• Plasma-derived C1-INH concentrate (e.g., Berinert, Cinryze): replaces the missing inhibitor, restoring control of kallikrein and factor XIIa; dosed by weight (commonly 20 U/kg IV for plasma-derived concentrate). Preferred agent in pregnancy and children per WAO/EAACI.
• Icatibant, a bradykinin B2 receptor antagonist: 30 mg subcutaneously, self-administrable, blocks the terminal mediator.
• Ecallantide, a plasma kallikrein inhibitor: given by a health professional because of anaphylaxis risk.
• Recombinant human C1-INH (produced in transgenic rabbit milk): avoid with rabbit allergy.

If no targeted agent is available

• Solvent/detergent-treated plasma or FFP as a source of C1-INH — second-line only, since plasma also supplies HMWK and prekallikrein substrate and may transiently worsen swelling.

Prophylaxis (definitive long-term control)

• Long-term prophylaxis for frequent or severe attacks: subcutaneous C1-INH, lanadelumab (monoclonal anti-kallikrein antibody), or berotralstat (oral kallikrein inhibitor) are first-line in current WAO/EAACI and US HAEA recommendations.
• Attenuated androgens (danazol): now second-line/legacy therapy; increases hepatic C1-INH synthesis.
• Antifibrinolytics (tranexamic acid): weak, largely reserved when nothing else is available.
• Short-term prophylaxis before dental work, intubation, or surgery: C1-INH concentrate shortly before the procedure, with on-demand therapy on hand.

Contraindicated / ineffective

• Antihistamines, glucocorticoids, and epinephrine do not work — the attack is bradykinin-mediated, not mast cell–mediated.
• Avoid ACE inhibitors (block bradykinin degradation) and estrogen-containing contraceptives/HRT.
• Avoid danazol in pregnancy, lactation, and growing children.

Disease complications

• Asphyxiation from laryngeal edema (EMERGENCY): bradykinin-driven submucosal edema at the glottis/subglottis obstructs airflow; signalled by hoarseness, throat tightness, dysphagia, then stridor. Historically the leading cause of death in undiagnosed HAE; any voice change in a known HAE patient mandates treatment and airway assessment.
• Hypovolemia and shock from a gastrointestinal attack (EMERGENCY): bowel wall and mesenteric edema sequesters plasma volume into the gut and peritoneum; signalled by severe colicky pain with vomiting, hemoconcentration, tachycardia, hypotension, and ascites or thickened bowel loops on imaging.
• Unnecessary laparotomy: visceral attacks mimic appendicitis, obstruction, or perforation; the giveaway is repeated "acute abdomen" episodes that resolve spontaneously with a normal white count and no surgical pathology.
• Bowel obstruction/intussusception from focal mucosal edema — transient and resolves with treatment of the attack rather than resection.
• Compartment-type compromise with massive extremity swelling; also disability, missed school/work, and high rates of anxiety and depression from unpredictable attacks.
• Attack escalation in pregnancy and after dental or airway instrumentation — trauma-triggered laryngeal attacks are the classic peri-procedural disaster.

Treatment-related complications

• Attenuated androgens (danazol): hepatotoxicity, hepatic adenoma and rarely hepatocellular carcinoma, dyslipidemia (low HDL), virilization/menstrual irregularity, weight gain, and growth suppression in children — signalled by rising transaminases or a hepatic lesion on surveillance imaging; hence periodic liver enzymes, lipids, and hepatic ultrasound.
• Thromboembolism with C1-INH concentrate: reported chiefly with high doses or central venous access; suspect with new limb swelling or hypoxemia (distinguish from an attack).
• Anaphylaxis (EMERGENCY): with ecallantide (hence administration by a health professional) and with recombinant C1-INH in rabbit-sensitized patients.
• Plasma-derived products carry a theoretical blood-borne pathogen risk; FFP may paradoxically worsen an attack by supplying kininogen substrate.
• Injection-site pain/erythema is the usual limitation of icatibant and subcutaneous prophylaxis.

• Angioedema without urticaria or pruritus that ignores antihistamines, steroids, and epinephrine is the single most tested pattern — think bradykinin, not histamine. If a stem says "no response to epinephrine and diphenhydramine," the answer is not "give more epinephrine."
• Best initial test = serum C4 (low in Types I and II, even between attacks). Confirm with C1-INH antigen level plus C1-INH functional assay: low antigen = Type I; normal or high antigen with low function = Type II. Function is low in both.
• The association examiners love: C1q. Normal C1q → hereditary; low C1q in an adult over 40 with no family history → acquired angioedema, prompting a search for a lymphoproliferative disorder (CLL, lymphoma, Waldenström) or anti-C1-INH autoantibody.
• Erythema marginatum is the classic prodromal buzzword — serpiginous, non-pruritic, non-urticarial rash preceding an attack.
• Best next step in a laryngeal attack: secure the airway and give a targeted agent — C1-INH concentrate, icatibant (B2 receptor antagonist, 30 mg SC), or ecallantide (kallikrein inhibitor), per WAO/EAACI and US HAEA recommendations. Never rely on steroids to "buy time."
• Recurrent unexplained abdominal pain plus a positive family history plus prior negative laparotomies is HAE presenting as a surgical mimic; up to a fifth of patients undergo needless abdominal surgery before diagnosis.
• Triggers to name: dental work/surgical trauma, infection, stress, ACE inhibitors (block bradykinin breakdown), and estrogen-containing contraceptives — the last two are contraindicated, and estrogen sensitivity is especially characteristic of factor XII (Type III) HAE.
• Common distractors: tryptase is normal (rules out mast cell activation); C4 is low while C1q is normal; danazol is no longer first-line prophylaxis — modern first-line prophylaxis is subcutaneous C1-INH, lanadelumab, or berotralstat, and danazol is contraindicated in pregnancy and children.