Hereditary angioedema with C1 esterase inhibitor deficiency is a rare genetic disorder that causes episodes of painful swelling in various parts of the body, including the face, limbs, abdomen, and potentially life-threatening areas like the throat and airways. This condition affects families across generations, bringing unpredictable and sometimes debilitating attacks that can last several days.

How Common Is This Condition

Hereditary angioedema caused by C1 esterase inhibitor deficiency is estimated to affect approximately 1 in 50,000 people worldwide^[3]. This makes it a rare disorder, though its actual prevalence may be higher because many cases go undiagnosed or are mistaken for other conditions. The disease affects people of all ethnic backgrounds equally, with no preference for any particular racial or geographic group^[10].

The condition typically begins showing symptoms during childhood or adolescence, with most people experiencing their first attack during the first two decades of life^[5]. Symptoms commonly first appear between 5 and 11 years of age, and approximately 40 percent of affected individuals experience their first episode by age 5^[2]. About 75 percent of patients have at least one episode by age 15^[4]. Early onset of symptoms may suggest a more severe course of disease throughout life^[2].

Both men and women are affected by this disorder, as it follows a pattern of inheritance that does not favor either gender. The frequency and severity of attacks can worsen during adolescence and typically continue throughout a person’s lifetime^[2].

What Causes This Disorder

Hereditary angioedema with C1 esterase inhibitor deficiency is caused by changes, called variants or mutations, in a specific gene. The gene involved is called SERPING1, which provides instructions for making a protein known as C1 inhibitor (or C1-INH)^[3]. This protein plays a crucial role in controlling inflammation and immune responses in the body.

More than 150 different mutations in the SERPING1 gene have been identified in people with this condition^[5]. These mutations can be of various types, including missense, nonsense, frameshift, deletion, and insertion mutations. Each of these types affects how the C1 inhibitor protein is made or how it functions once it is produced.

There are two main types of hereditary angioedema caused by C1 inhibitor problems. In Type I, which accounts for about 85 percent of all cases, the body does not produce enough C1 inhibitor protein^[2]. The genetic mutations in Type I lead to proteins that are either incomplete or misfolded, and therefore cannot be properly released from cells. Blood levels of C1 inhibitor in these patients are typically only 5 to 30 percent of normal, rather than the 50 percent that might be expected from having only one functioning gene^[10].

In Type II, which represents about 15 percent of cases, the C1 inhibitor protein is produced in normal or even elevated amounts, but the protein does not work correctly^[7]. These mutations typically occur at or near the active site of the protein where it performs its function. The defective protein can be released into the bloodstream, but it cannot do its job of controlling inflammation.

The disease is inherited in an autosomal dominant pattern^[3]. This means that having just one copy of the altered gene in each cell is enough to cause the disorder. A person with hereditary angioedema has a 50 percent chance of passing the condition to each of their children. However, up to 25 percent of cases occur as spontaneous mutations, meaning they happen for the first time in a person with no family history of the condition^[10].

Who Is at Higher Risk

The most significant risk factor for developing hereditary angioedema with C1 esterase inhibitor deficiency is having a family history of the condition. Because this disorder is genetic, children of affected parents have a 50 percent chance of inheriting the altered gene and developing the disease^[3]. About 75 percent of people with this condition have a family history of angioedema^[2].

However, having the genetic mutation does not mean a person is guaranteed to have frequent or severe attacks. The frequency and severity of attacks vary greatly, even among members of the same family^[3]. Some people may experience attacks weekly, while others may go months between episodes.

Certain life circumstances and exposures can increase the risk of having an attack in people who already have the condition. Physical trauma, even minor injuries such as those that occur during dental procedures or tongue piercings, can trigger swelling episodes^[4]. Surgery of any kind is a well-known trigger for attacks.

Infections, particularly viral illnesses, can precipitate attacks^[4]. Emotional stress and psychological factors also play a role in triggering episodes. Some people notice that cold exposure can bring on swelling^[4].

For women, hormonal influences significantly impact the condition. Pregnancy can trigger attacks, and the frequency of episodes may worsen during adolescence when hormonal changes are significant^[2]. Medications containing estrogen, including birth control pills and hormone replacement therapy, are known to aggravate the condition and increase attack frequency^[4]. Certain blood pressure medications, specifically angiotensin-converting enzyme (ACE) inhibitors, can also trigger episodes and should typically be avoided^[2].

Certain foods may trigger attacks in some individuals, though specific triggers vary from person to person. Vigorous exercise and menstruation have also been identified as potential triggers for swelling episodes^[2].

Recognizing the Symptoms

The hallmark symptom of hereditary angioedema is recurrent episodes of swelling, medically known as angioedema. This swelling is distinctly different from allergic reactions. It is non-pitting, meaning that when you press on the swollen area, it does not leave an indentation. Importantly, the swelling is not associated with hives, itching, or redness^[2]. This absence of allergic features helps distinguish hereditary angioedema from other causes of swelling.

The swelling can occur in several areas of the body. More than 90 percent of people with this condition develop swelling affecting the skin, particularly the face, lips, limbs (especially hands and feet), and genitals^[2]. Facial swelling can sometimes extend to involve the throat, which is particularly concerning. The swelling is often accompanied by pain and can temporarily affect the function of the swollen body part.

Swelling of the upper airway is one of the most feared complications of hereditary angioedema. This can affect the tongue, soft palate, or voice box (larynx). When the airway swells, a person may experience stridor, which is a gasping or high-pitched sound when breathing in^[2]. Other warning signs include hoarseness, drooling, and changes in voice. Swelling behind the teeth is more likely to progress to dangerous airway obstruction than swelling in front of the teeth^[2]. About 50 percent of people with hereditary angioedema experience at least one episode involving the throat during their lifetime^[7]. In the past, when treatments were not available, such attacks resulted in death rates up to 30 percent^[7].

Abdominal attacks are another common manifestation of this condition. Swelling can affect the lining of the digestive tract, causing severe abdominal pain, nausea, vomiting, and diarrhea^[2]. The pain can be intense enough that people sometimes undergo unnecessary surgical procedures, including appendix removal, because the symptoms are mistaken for other abdominal emergencies. These abdominal episodes can lead to dehydration and collapse if severe^[2].

About one-third of people experience warning signs before an attack begins. These prodromal symptoms may include fatigue, flu-like symptoms, indigestion, or a tingling sensation in the area that will swell^[2]. Some people develop a distinctive non-itchy rash called erythema marginatum either before or during an attack^[3].

Without treatment, attacks typically reach their peak intensity around 24 hours after onset and then gradually resolve over 48 to 72 hours^[2]. On average, untreated individuals have swelling episodes every one to two weeks, and most episodes last for about three to four days^[3]. However, the frequency and duration vary considerably between individuals.

Preventing Attacks

While hereditary angioedema cannot be cured, several strategies can help reduce the frequency and severity of attacks. Because the condition is genetic, it cannot be prevented in children born to affected parents, but awareness of the condition allows for earlier diagnosis and treatment.

Avoiding known triggers is an important preventive strategy for people who have been diagnosed. Identifying personal triggers through careful tracking of attacks can help individuals avoid situations or exposures that tend to provoke swelling. This might include avoiding certain medications, particularly those containing estrogen or ACE inhibitors^[2].

Managing stress through relaxation techniques, counseling, or other stress-reduction methods may help some people reduce attack frequency. Being cautious about physical trauma and taking special precautions before dental work or surgical procedures is essential. Healthcare providers can provide preventive treatment before planned procedures that might trigger attacks^[10].

Prophylactic medications are available for people who experience frequent attacks. These are medications taken regularly to prevent episodes from occurring. Some of these are attenuated androgens, which are synthetic hormones that can increase C1 inhibitor production. Another option is regular replacement therapy with C1 inhibitor concentrates^[10]. Newer prophylactic medications that work by different mechanisms have also become available in recent years.

Education is a crucial component of prevention. People with hereditary angioedema should be taught to recognize early warning signs of attacks, particularly those involving the airway. They should understand when to seek emergency care and have an individualized management plan. Because 75 percent of cases have a family history, screening of family members can lead to early diagnosis and prompt treatment, potentially preventing serious complications^[2].

How the Body Is Affected

Understanding what happens in the body during hereditary angioedema helps explain why symptoms occur. The C1 inhibitor protein normally acts as a brake on several systems in the body that promote inflammation. It blocks the activity of certain proteins that would otherwise cause excessive inflammation^[3].

When there is not enough functioning C1 inhibitor, several cascade systems in the body become overactive. One of these is the complement system, which is part of the immune defense. Without adequate C1 inhibitor to regulate it, the complement system becomes inappropriately activated. Another system affected is called the contact system, which involves a protein called plasma kallikrein^[6].

The overactivity of plasma kallikrein is particularly important in hereditary angioedema. When C1 inhibitor levels are low or the protein is not functioning properly, plasma kallikrein activity increases. This increased kallikrein activity triggers the release of excessive amounts of bradykinin^[6].

Bradykinin is a powerful substance that affects blood vessels. It causes the walls of blood vessels to become more permeable, meaning they become leaky. When blood vessel walls become leaky, fluid that normally stays inside the vessels escapes into surrounding tissues. This fluid accumulation in tissues is what causes the visible swelling during an attack^[3].

The location where bradykinin is released determines where swelling occurs. When it affects blood vessels in the skin, visible swelling of the face, hands, or other body parts results. When it affects blood vessels in the lining of the intestines, fluid leaks into the intestinal wall and abdominal cavity, causing the severe abdominal pain characteristic of abdominal attacks. When it affects the throat or airway, the life-threatening swelling of these structures occurs.

Because hereditary angioedema is caused by a deficiency in a regulatory protein rather than an allergic reaction, it does not involve immunoglobulin E (IgE), which is the antibody responsible for allergic reactions^[8]. This is why people do not develop hives or itching, and why allergy medications do not work for this condition.

The unpredictability of attacks relates to the fact that various triggers can activate the contact system and complement pathway. However, many attacks occur without any identifiable trigger, making the condition particularly challenging to live with. The severity of attacks can vary based on how much bradykinin is released and where in the body the release occurs.