Hereditary angioedema (HAE) is a rare genetic condition that causes recurring episodes of severe swelling in the skin, gut, and airways. It affects roughly 1 to 2 people per 100,000 worldwide. Unlike allergic reactions that cause swelling, HAE doesn’t respond to antihistamines, epinephrine, or steroids, which is why it’s frequently misdiagnosed for years before someone gets the right answer.
What Causes the Swelling
HAE traces back to a problem with a single protein called C1 inhibitor (C1-INH). This protein acts as a brake on several enzyme systems in the blood, including one called the contact system. Normally, C1-INH keeps this system in check by physically binding to enzymes and crushing them into an inactive shape. When C1-INH is missing or doesn’t work properly, those enzymes activate each other in a feedback loop, ultimately producing a small molecule called bradykinin.
Bradykinin is the direct cause of the swelling. It forces the walls of small blood vessels to become leaky, allowing fluid to pour into surrounding tissue. This is fundamentally different from the histamine-driven swelling in allergic reactions, which is why allergy medications have no effect on HAE attacks.
The Three Types of HAE
HAE is divided into types based on what’s going wrong with C1-INH:
- Type I (most common): The body produces too little C1-INH protein. The protein that does get made works normally, but there simply isn’t enough of it.
- Type II: The body produces normal or even elevated amounts of C1-INH, but the protein is structurally defective and doesn’t function. Blood tests can be misleading here because protein levels look fine on a standard measure.
- HAE with normal C1-INH (formerly Type III): An extremely rare subclass where C1-INH levels and function are both normal. Instead, mutations in other genes (affecting clotting factors, plasminogen, or blood vessel proteins) drive swelling through related pathways. This type is often estrogen-dependent, with attacks triggered or worsened by oral contraceptives or pregnancy.
Inheritance Pattern
Types I and II are caused by mutations in a gene called SERPING1, which carries the instructions for making C1-INH. The condition follows an autosomal dominant pattern, meaning you only need one copy of the mutated gene to have the disease. If one parent has HAE, each child has a 50% chance of inheriting it. That said, roughly 25% of cases arise from new, spontaneous mutations with no family history.
What an Attack Feels Like
Attacks typically begin with a prodrome: a tingling or tightness in the skin at the site that’s about to swell, usually 1 to 2 hours before visible swelling starts. Some people experience sudden mood changes, anxiety, or deep exhaustion several hours beforehand. Between 30% and 50% of people with HAE develop a distinctive flat, reddish rash called erythema marginatum that doesn’t itch, either just before or during an attack.
The swelling itself builds over 12 to 24 hours and typically resolves within 1 to 3 days without treatment. It can strike almost anywhere: the face, hands, feet, genitals, or the lining of the gut. Abdominal attacks cause intense cramping, nausea, vomiting, and sometimes diarrhea, and are frequently mistaken for appendicitis or other surgical emergencies. The most dangerous attacks involve the throat and airway, where swelling can obstruct breathing.
Common Attack Triggers
Many attacks seem to come out of nowhere, but several known triggers increase the likelihood:
- Physical trauma: Even minor injuries, bumps, or pressure on the skin.
- Dental and surgical procedures: Tooth extractions and even routine dental impressions can provoke swelling in the head and neck area. Without preventive medication, the risk of procedure-related swelling ranges from about 6% to 30%, and symptoms can appear as late as 24 to 48 hours afterward.
- Emotional stress: A consistently reported trigger across studies.
- Hormonal changes: Puberty, menstruation, and pregnancy can all increase attack frequency.
- Certain medications: ACE inhibitors (used for blood pressure) and estrogen-containing oral contraceptives are well-established triggers because they interfere with the same bradykinin pathway.
- Infections: Common illnesses can lower the threshold for an attack.
How HAE Is Diagnosed
Diagnosis relies on blood tests that measure three things: C4 levels, C1-INH protein quantity, and C1-INH function. C4 is a complement protein that runs low in HAE even between attacks, making it a useful screening marker.
In Type I, all three values come back low. In Type II, C1-INH protein quantity looks normal or high, but the functional assay reveals the protein isn’t working, with activity falling at or below 50% of normal. This is why measuring function is essential. Skipping that test is one reason Type II gets missed. For HAE with normal C1-INH, standard blood work comes back normal, and diagnosis requires genetic testing to identify the specific mutation involved.
Because HAE is rare and its symptoms overlap with allergic angioedema and abdominal conditions, the average time to diagnosis can stretch for years. Anyone with recurrent unexplained swelling that doesn’t respond to antihistamines, especially if there’s a family history, should be tested.
Treating Acute Attacks
Three classes of medication are approved to stop attacks once they start. Each one targets a different step in the bradykinin pathway. One type replaces the missing C1-INH protein directly, either from donated plasma or made synthetically. Another blocks the bradykinin receptor on blood vessel walls so that even when bradykinin is produced, it can’t trigger leaking. The third inhibits kallikrein, the enzyme that snips bradykinin free in the first place.
International guidelines from the World Allergy Organization recommend that every person with HAE have on-demand medication available at all times. The goal is to treat attacks as early as possible, ideally at the first sign of tingling or prodromal symptoms, because early treatment shortens both the severity and duration of swelling. All attacks, regardless of location, are recommended for treatment because it’s impossible to predict whether a mild-appearing episode will escalate.
Preventing Attacks Long-Term
For people who have frequent or severe attacks, long-term preventive therapy can dramatically reduce how often swelling occurs. Three first-line options are currently recommended:
- Plasma-derived C1-INH replacement: Given by injection under the skin every 3 to 4 days. This tops up the missing protein on an ongoing basis.
- Lanadelumab: An injection given every 2 to 4 weeks that blocks kallikrein, the enzyme responsible for generating bradykinin.
- Berotralstat: A daily pill that also inhibits kallikrein, making it the only oral preventive option currently available.
The overarching treatment goal, according to international consensus guidelines, is complete disease control and normalization of daily life. This represents a significant shift from earlier decades when people with HAE were simply told to avoid triggers and hope for the best. With modern therapies, many people achieve zero or near-zero attacks per year.
Living With HAE
Because attacks can involve the airway, people with HAE are advised to carry on-demand treatment with them at all times. Planning ahead for dental work and surgical procedures is important: short-term preventive medication given before the procedure significantly reduces the chance of post-procedure swelling. If you’re taking ACE inhibitors for blood pressure, switching to an alternative class of medication eliminates that trigger entirely.
HAE also affects life in less visible ways. The unpredictability of attacks can create anxiety around travel, social plans, and work. Abdominal attacks cause severe pain that’s frequently dismissed as psychosomatic before diagnosis. Getting a confirmed diagnosis and having a treatment plan in place makes a meaningful difference not just physically but in the sense of control people feel over their lives.