Hereditary hemorrhagic telangiectasia (HHT) is a condition in which blood vessels form abnormally, resulting in telangiectasias which are abnormal small capillaries and veins on the surface of the skin or in the mouth or nose. As the name implies, HHT is hereditary caused by mutations in at least two specific genes.

The degree to which each individual with Hereditary hemorrhagic telangiectasia is affected is quite variable. Most people with the condition will have telangiectasias on their skin, in their mouths, and nose. Nosebleeds occur frequently, especially as the individual ages. In the addition to telangiectasias, some people with HHT will have arteriovenous fistulae (AVF) or arteriovenous malformations or AVMs. The vascular malformations consist of direct communications between arteries and veins. They can occur anywhere, but in patients with HHT they are most commonly found in the lungs, liver, brain, and spine. AVM or AVF in the lung causes de-oxygenated blood returning from the veins of the body to shunt into the pulmonary veins, reducing the oxygen level in the arteries that supply the rest of the body. This typically causes the patient to feel tired or weak and to have a reduced exercise tolerance. In severe cases, the patient may have a blue color, especially in the lips, and "clubbing" of the fingernails. Measurement of blood oxygen level shows a decrease. Because the capillary bed of the lung serves as a filter for blood clots and other particles in the veins, AVM or AVF can lead to passage of such material into the arteries, causing a stroke or brain abscess. AVM can also burst and cause bleeding into the surrounding tissue. This is most common in the brain and bowel. AVM in the liver typically cause failure of the heart and theliver.

The diagnosis of HHT can usually be strongly suspected when a patient has telangiectasias involving the skin as well as an AVM or AVF in the lungs, liver, brain or spine. These findings plus a family history of the condition clinches the diagnosis. Occasionally, HHT will be present without a family history. The AVM is usually diagnosed by imaging procedures, such as magnetic resonance imaging (MRI) or computed tomography (CT). Bubble echocardiography is useful to detect the presence of an AVM in the lung. To confirm the diagnosis of HHT, a blood sample can be sent for genetic studies to identify certain specific mutations associated with HHT.

In the presence of a strong family history of Hereditary hemorrhagic telangiectasia, it is reasonable to screen the children for some of the most common and serious manifestations, especially AVM's in the lungs and brain. Screening typically consists of a bubble Echocardiogram and an MRI scan of the brain.

The AVM's and AV fistulae that involve the lungs or brain in patients with Hereditary hemorrhagic telangiectasia are usually treated by embolization.