Hereditary angioedema (HAE) is a condition that affects about one in every 10,000 to 50,000 people. Since it’s so rare, most doctors don’t treat many people with the condition, if any. This can lead to many people with HAE having to wait for a long time before reaching a correct diagnosis.
Common symptoms of HAE include swelling (edema) in parts of the body, such as the hands, feet, face, and throat. This is particularly serious when it affects the throat because it can obstruct breathing and, in some cases, lead to asphyxiation. When the swelling affects the intestinal wall it can cause excruciating abdominal pain, vomiting, and nausea.
In almost all cases, HAE occurs because of changes to a specific gene called the C1 inhibitor gene, which codes for a C1 inhibitor blood protein. When the gene is altered, as in HAE, it can affect the reactions that allow fluid to leave capillaries and cause swelling. This is why Types I and II of HAE can also be called C1 Inhibitor Deficiency.
Genetic forms of HAE are hereditary. About 75% of people with HAE are estimated to inherit the condition from a family member; if a parent has the condition then each of their children has a 50% chance of also developing it. However, the other ~25% of people affected by HAE don’t have a family history of it, and instead had a spontaneous alteration to the C1-inhibitor gene when they were conceived.
Types of HAE
- Type I
Type I is the most common form of HAE and accounts for an estimated 80-85% of HAE patients. People with this form of the condition have a gene alteration that causes reduced C1-inhibitor. It’s usually, but not always, inherited.
People affected tend to develop symptoms of swelling at a young age (generally before 13 years old), which can worsen with puberty. Episodes of swelling can occur after emotional or physical stress, or for no obvious reason. Doctors may offer patients anabolic steroids (a.k.a. androgens), or one of the recently approved treatments for HAE, such as inhibitor medicines or a receptor antagonist.
- Type II
About 15 to 20% of people with HAE have Type II, which is also caused by a genetic alteration. However, in this form, the C1 inhibitor is dysfunctional, rather than being at a low level. This means that C1 inhibitor levels can be normal or elevated, but don’t work properly. The symptoms and treatment options are very similar to those for Type I HAE.
- Type III
Type III HAE is less well understood, and it is unclear what the underlying cause is. However, unlike Types I and II, it isn’t caused by C1 deficiency. The symptoms of Type III are largely similar to those of Types I and II. Researchers are working on treatment options for this form of HAE, and some people have reported improvement from existing HAE medicines.
The number of people with this form of HAE isn’t known, but it’s believed to affect females more often than males. The swelling has been linked to high levels of oestrogen, such as during pregnancy and when some types of contraceptives are taken.