Angioedema

01Pathophysiology

Angioedema results from a transient increase in vascular permeability in the deep dermis, subcutaneous tissue, and submucosal layers, leading to plasma extravasation and localized non-pitting edema. Understanding the mediator involved is the single most important concept for answering exam questions correctly, because the mediator determines whether standard allergic treatments will work.

Histamine-mediated angioedema occurs when mast cells degranulate in response to an allergen (IgE-mediated) or a direct trigger (complement activation, physical stimuli). Released histamine causes vasodilation and increased capillary permeability. Because the same mast cells reside in the superficial dermis, patients almost always develop concurrent urticaria (wheals and flare). This is why the presence of hives is a reliable clue that the mechanism is histamine-driven, and the patient will respond to epinephrine, antihistamines, and glucocorticoids.

Bradykinin-mediated angioedema operates through an entirely different pathway. Bradykinin is a potent vasodilator peptide that is normally degraded by angiotensin-converting enzyme (ACE, also known as kininase II). When bradykinin accumulates, either because its degradation is blocked (ACE inhibitors) or because it is overproduced (C1-INH deficiency leading to unchecked kallikrein activity), the result is vascular leak in the deeper tissues. Critically, bradykinin does not activate mast cells, which explains why these patients do not develop urticaria and why epinephrine and antihistamines are ineffective. This distinction is the most commonly tested concept.

In hereditary angioedema (HAE), deficiency or dysfunction of C1 esterase inhibitor (C1-INH) removes the normal brake on the contact activation (kallikrein-kinin) pathway. C1-INH normally inhibits kallikrein, factor XIIa, and activated C1. Without this inhibition, kallikrein cleaves high-molecular-weight kininogen to produce excess bradykinin. The complement pathway is also dysregulated, which is why C4 is consumed and remains low even between attacks, making it a useful screening marker.

The abdominal pain seen in HAE attacks occurs because bradykinin-driven edema affects the bowel wall, causing intestinal wall swelling that mimics a surgical abdomen. This can lead to unnecessary laparotomies if the diagnosis is not recognized.

02Classification and Clinical Manifestation

Classification of Angioedema by Mechanism

Clinical Manifestation by Subtype

03Diagnostic Workup

When a patient presents with angioedema, the first clinical decision is whether the swelling is histamine-mediated or bradykinin-mediated. The most reliable bedside discriminator is the presence or absence of urticaria. If urticaria is present, the process is histamine-driven, and you treat accordingly with no further complement workup needed.

If urticaria is absent, the next question is whether the patient is on an ACE inhibitor. If yes, the diagnosis is ACE inhibitor-induced angioedema, and the treatment is drug discontinuation. No complement testing is required for this diagnosis.

If urticaria is absent AND the patient is not on an ACE inhibitor, you must evaluate for hereditary or acquired angioedema. The best initial test is the C4 level. C4 is consumed in all forms of C1-INH deficiency and remains low even between attacks, making it an excellent screening tool. A normal C4 level between attacks has a strong negative predictive value and effectively rules out HAE Types I and II.

If C4 is low, proceed with C1-INH quantitative level and C1-INH functional assay. A low C1-INH level with low function confirms Type I. A normal or elevated C1-INH level with low function confirms Type II (the protein is produced but is dysfunctional, so the functional assay is what catches it).

To differentiate hereditary from acquired C1-INH deficiency, order a C1q level. C1q is normal in HAE but low in acquired angioedema, because in the acquired form, autoantibodies or lymphoproliferative consumption affect the classical complement pathway upstream. An older patient with new-onset angioedema, no family history, and a low C1q should prompt evaluation for an underlying lymphoproliferative disorder.

04Management and Treatment

For any patient presenting with angioedema, airway assessment and protection is the first and most critical step. Laryngeal involvement can progress rapidly to complete obstruction. If stridor, voice changes, or tongue swelling are present, prepare for intubation. Surgical airway (cricothyrotomy) should be available as a backup.

For histamine-mediated angioedema, treatment follows the anaphylaxis algorithm. Administer intramuscular epinephrine (0.3 to 0.5 mg of 1:1,000 solution into the anterolateral thigh), an H1-receptor antagonist (diphenhydramine 25 to 50 mg IV), and intravenous corticosteroids (methylprednisolone 125 mg IV or hydrocortisone 200 mg IV). An H2-blocker (famotidine 20 mg IV) can be added. Corticosteroids prevent the late-phase reaction but take 4 to 6 hours to work. Patients with anaphylaxis should be observed for at least 6 hours and discharged with an epinephrine auto-injector.

For acute HAE attacks, standard anaphylaxis medications do not work. The treatment targets the bradykinin pathway directly. First-line options include:

• C1-INH concentrate (Berinert) at 20 IU/kg IV, which replaces the deficient protein

• Icatibant at 30 mg SC, a bradykinin B2 receptor antagonist that blocks the downstream effect

• Ecallantide at 30 mg SC (given as three 10 mg injections), a kallikrein inhibitor that prevents bradykinin generation

If none of these are available, fresh frozen plasma (2 to 4 units) can be used because it contains C1-INH. However, there is a theoretical concern that FFP also contains substrates like kininogen and prekallikrein, which could transiently worsen symptoms, though clinical experience generally supports its use.

For long-term prophylaxis in HAE, the current preferred agents are lanadelumab (a monoclonal antibody against kallikrein, given 300 mg SC every 2 weeks), berotralstat (an oral kallikrein inhibitor, 150 mg daily), or subcutaneous C1-INH concentrate (Haegarda, 60 IU/kg SC twice weekly). Danazol (an attenuated androgen at 200 mg PO two to three times daily) was historically used to upregulate hepatic C1-INH synthesis but has fallen out of favor due to significant adverse effects including hepatotoxicity, virilization, and lipid derangements. It is contraindicated in pregnancy.

For ACE inhibitor-induced angioedema, the drug must be permanently discontinued. Do not rechallenge with the same or a different ACE inhibitor. An ARB is an acceptable antihypertensive alternative. In refractory or severe presentations, icatibant or C1-INH concentrate may be considered, though evidence is still evolving.

Short-term prophylaxis is required for HAE patients undergoing dental procedures, intubation, or surgery. Administer C1-INH concentrate (1000 IU IV) 1 to 6 hours before the procedure. Have on-demand acute treatment readily available during the procedure as well.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students miss angioedema questions is by failing to recognize bradykinin-mediated angioedema as a separate entity from allergic angioedema. A classic trap vignette describes a patient on an ACE inhibitor who develops lip and tongue swelling without hives. The student selects epinephrine or antihistamines because "angioedema" sounds allergic, but the correct answer is to discontinue the ACE inhibitor and, if the attack is severe, administer icatibant or C1-INH concentrate. The absence of urticaria is the critical clue that the mechanism is bradykinin-driven, and standard allergy treatments will not work.

A second common trap involves timing. Students often assume ACE inhibitor angioedema only occurs shortly after starting the medication. In reality, it can develop months to years into therapy. If the vignette describes a patient who has been on lisinopril for three years and now presents with tongue swelling, the ACE inhibitor is still the culprit.

Another frequently tested concept is the recurrent abdominal pain presentation of HAE. A young patient with episodic severe abdominal pain, nausea, and vomiting who has a family history of similar episodes and a low C4 should raise immediate suspicion for HAE. The trap is ordering unnecessary surgical exploration. The CT showing bowel wall edema without a surgical lesion is the discriminating detail.

Finally, remember the lab pattern. C4 is the screening test; it is low in all forms of C1-INH deficiency and remains low between attacks. The C1-INH functional assay is the confirmatory test and catches both Type I and Type II. C1q distinguishes hereditary (normal C1q) from acquired (low C1q). Students who jump straight to C1-INH levels without first checking C4 are choosing the wrong "best initial test." The core competency being tested across all angioedema questions is your ability to classify the mechanism (histamine vs. bradykinin), select mechanism-appropriate treatment, and use the correct sequence of complement studies to arrive at the diagnosis.