What is HHT and AVMs?

I have done some research for everyone. I hope this answers some questions everyone might have.





What is hereditary hemorrhagic telangiectasia (HHT)?

HHT is a genetic disorder that causes abnormalities of blood vessels. Most blood vessels in the body of someone with HHT are normal. However, a small percentage of the blood vessels in a person with HHT have a specific type of abnormality.
Blood vessels are the tubes that carry blood around our bodies. There are two types of blood vessels: arteries and veins. Arteries carry blood under high pressure out to all areas of the body after being pumped by the heart. Veins carry blood that should be under low pressure, back to the heart. An artery does not usually connect directly to a vein. Usually there are very small blood vessels called capillaries that connect an artery to a vein.
A person with HHT has a tendency to form blood vessels that lack the capillaries between an artery and vein. This means that arterial blood under high pressure flows directly into a vein without first having to squeeze through the very small capillaries. This place where an artery is connected directly to a vein tends to be a fragile site that can rupture and result in bleeding. We usually call a blood vessel that is abnormal in this way a telangiectasia (tel-AN-jee-eck-TAZE-ee-ya), if it involves small blood vessels. We tend to call it an arteriovenous malformation (AVM) if involves larger blood vessels. So, an AVM might be thought of as a big telangiectasia. The basic abnormality in the blood vessel is the same.
Telangiectases tend to occur at the surface of the body such as the skin and the mucous membrane that lines the nose. AVMs tend to occur in the internal organs of the body. The telangiectases (plural for telangiectasia) and AVMs of HHT occur primarily in the nose, skin of the face, hands, and mouth and the lining of the stomach and intestines (GI tract), lungs, liver and brain. It is not currently known why these abnormal blood vessels tend to occur in certain parts of the body and not others.

What is an arteriovenous malformation (AVM)?
Normally blood flows from the heart through large arteries to all areas of the body. The arteries branch and get smaller until they become a capillary, which is just a single cell thick. The capillary bed is where the blood exchanges oxygen and nutrients with the body tissues and picks up waste. The blood travels from the capillary bed back to the heart through veins. In an AVM, arteries connect directly to veins without a capillary bed in between. This creates a problem called a high-pressure shunt or fistula. Veins are not able to handle the pressure of the blood coming directly from the arteries. The veins stretch and enlarge as they try to accept the extra blood. The weakened blood vessels can rupture and bleed and are also more likely to develop aneurysms. The surrounding normal tissues may be damaged as the AVM “steals” blood from those areas.
There are four types of AVMs:
1. Arteriovenous malformation – abnormal tangle of blood vessels where arteries shunt directly into veins with no intervening capillary bed; high pressure.
2. Cavernoma – abnormal cluster of enlarged capillaries with no significant feeding arteries or veins; low pressure.
3. Venous malformation – abnormal cluster of enlarged veins resembling the spokes of a wheel with no feeding arteries; low pressure, rarely bleed and usually not treated.
4. Capillary telangiectasia – abnormal capillaries with enlarged areas (similar to cavernoma); very low pressure.
AVMs can form anywhere in the body and cause symptoms based on their location.

Lung AVM
Approximately 30-50% of people with HHT have one or more AVM in the lungs (pulmonary AVM or PAVM). AVMs in the lung are at risk of rupture, particularly during pregnancy when blood pressure and blood volume tend to increase. This can lead to life threatening bleeding. In addition people with lung AVMs are at risk for stroke (a clot in the brain blocking off blood flow) or brain abscess (a bacterial brain infection). Stroke and brain abscess can be life threatening or disabling. In the normal lung, the capillaries between an artery and vein act as a filter for impurities (clots, bacteria, air bubbles) in the blood, before the blood circulates to other parts of the body, including the brain. When lung AVMs are present, these particles can pass through the AVM, go to the left side of the heart and then on to the brain or other organs. Fortunately, lung AVMs are usually easily treatable.

Brain AVM
Brain AVMs are found in about 5-20% of people with HHT and can also be successfully treated in most cases. They can be life threatening or disabling if they bleed. Since they often do not cause warning symptoms prior to causing a brain hemorrhage, we recommend screening for them in all people with HHT. Spinal AVMs are more rare and can also be removed. They can cause pain in the back over the spine or loss of feeling or function in an arm or leg.

Liver AVM
Liver AVMs can also occur, are relatively common, but rarely cause sudden, severe medical complications like lung and brain AVMs can. Liver AVMs almost never bleed and most are not currently treated. Large AVMs in the liver occasionally cause heart and liver failure, usually later in life. Heart failure can occur if the heart has been overworked for years, pumping extra blood through the low resistance pathway of an AVM (in this context an AVM is sometimes called a “Shunt”.