Patient Education Blog


What is HHT?

HHT (Hereditary Hemorrhagic Teleangiectasia) is an inherited disorder in which small blood vessels develop abnormally [ref 1].  It is also called Osler-Weber-Rendu syndrome. It is estimated that 30,000 to 60,000 people (1 out every 5,000 to 10,000) in the United States have it. Individuals with HHT develop small abnormal blood vessels (teleangiectasias) under the skin (usually in the fingers and hands) and in the lining of the nose and mouth.  Similar abnormal blood vessels can also develop in internal organs like the lungs, brain, liver and intestines (called arterial-venous malformations or AVMs).  The most common symptom of HHT is frequent nose bleeds that can develop even during the childhood and teenage years.  AVMs in internal organs can lead to bleeding and infections.  Due to chronic bleeding affected individuals may develop low blood counts, i.e. anemia.


Why Do Blood Clots Occur in People with HHT?

Similar to individuals without HHT, patients with HHT may develop deep vein thrombosis (DVT) and pulmonary embolism (PE).  DVT and PE can be associated with the usual provoking factors (transient risk factors), e.g. hospitalization, immobility, major surgery, major trauma, hormone therapy (birth control pill, patch, or ring or hormone replacement therapy), pregnancy, or long-distance travel. Obesity and smoking are also risk factors, as are inherited or acquired clotting disorders (=thrombophilias), such as factor V Leiden, the prothrombin 20210 mutation, deficiency of protein S, protein C and antithrombin, and antiphospholipid antibodies. Furthermore, DVT and PE can occur without an obvious triggering factor, then referred to as “unprovoked clot” or “idiopathic clot”.

It is not known whether HHT patients are at higher than usual risk for DVT and PE.  Additional risk factors for blood clots in people with HHT include use of medications that may increase the risk for blood clots.

  • Estrogens, are used to decrease bleeding from the abnormal blood vessels. They are known risk factor for DVT and PE.
  • Amicar® (aminocaproic acid) and Lysteda® (tranexamic acid), so called anti-fibrinolytic drugs, are used for the treatment of HHT patients. While there have been concerns that these drugs may increase the risk for DVT and PE, it is reassuring that recent reports suggest this is not the case [ref 2,3].
  • There is increasing evidence to support the use of certain anti- cancer drugs (Thalomid® = Thalidomide and Avastin® = Bevacizumab) to treat bleeding in people with HHT in whom routine medications and interventions have not been effective [ref 4,5,6].
    • Avastin®: This drug has been associated with an increased risk for DVT and PE in cancer patients when taken at higher doses [ref 7]. There is no consensus on the best dose to use in HHT patients: lower or similar doses as in cancer patients have been used. It is not known whether Avastin® leads to an increased risk of blood clots in HHT patients at the higher (or lower) doses, but it may well not.
    • Thalidomide: The risk for blood clots in cancer patients is particularly pronounced if it is given together with another type of medication, steroids. Steroids are not given with Thalidomide in the treatment of HHT patients. Thus, blood clot risk in HHT patients treated with Thalidomide may not be an issue.
  • Anemia and the need for blood transfusions may require hospital admission and immobility, which are a risk factor for DVT and PE.
  • Finally, patients with HHT may have disturbances in their clotting system, such as elevated clotting factor VIII levels, that may increase the risk of DVT and PE [ref 2].

How Often do Blood Clots Occur in people with HHT?

Not a lot is known about how often people with HHT develop blood clots.  The only large published report on blood clots in HHT patients showed that 6.5 % of patients had a DVT or PE (20 of 309 patients) [ref 8]. This equals to one patient out of 15 patients. This is not unsubstantial.


How are Blood Clots Prevented

When a patient with HHT is admitted to the hospital and is immobile, leg “squeezers” (= intermittent compression devices = ICDs) should be considered. These are mechanical air pumps put on the legs that keep the blood circulating and, thus, prevent DVTs. They are routinely used for DVT prevention (prophylaxis) in patients in whom “blood thinner” drugs cannot be safely used to prevent DVT in the hospital. After major surgery (hip and knee replacement, abdominal or pelvic surgery) injection into the skin of a low dose of a blood thinner can be considered.

How Are Blood Clots Treated?

Management of HHT patients who have developed a DVT or PE can be very challenging, as bleeding from the abnormal HHT blood vessels in the nose and gastrointestinal tract may become much worse when “blood thinners” are given [ref 8,9]. Yet, “blood thinners” are needed in this situation to prevent the clots from getting bigger or new ones from forming.  A thorough assessment of all the patient’s risk factors for blood clots and those for bleeding is needed to make a solid decision on (a) what drug to use, (b) what intensity of treatment to choose (i.e. target INR in the patient on warfarin) and, importantly, (c) how long to treat with “blood thinners”. In the case of an unprovoked DVT or PE blood testing for a clotting disorder (thrombophilia) and a blood D-dimer test may be helpful to help make decisions on length of “blood thinner” treatment. Evaluation and management in a specialized HHT Center [ref 1] is probably the best option for these patients to best balance the risk of bleeding and re-clotting.

What’s Important?

  • Patients with HHT should know the symptoms of blood clots in legs (deep vein thrombosis = DVT) and lungs (pulmonary embolism = PE) and the risk factors for DVT and PE.
  • HHT patients should get good DVT prophylaxis when hospitalized, using at least intermittent compression devices (ICDs). In the case of major surgery, prophylaxis with “blood thinning” medications should be considered.
  • While all people with HHT should ideally be followed at an HHT Center, it is particularly important for a patient with HHT who develops a blood clot to be seen in an HHT Center.


  1. – the website of “HHT Foundation International”
  2. Kagoma YK et al. Use of antifibrinolytic therapy to reduce transfusion in patients undergoing orthopedic surgery: A systematic review of randomized trials. Thromb Res 2009;123:687-696.
  3. CRASH-2 trial collaborators. Effects of tranexamic acid on death, vascular occlusive events, and blood transfusion in trauma patients with significant haemorrhage (CRASH-2): a randomized, placebo-controlled trial. Lancet 2010;376:23-32.
  4. Lebrin, F et al. Thalidomide stimulates vessel maturation and reduces epistaxis in individuals with hereditary hemorrhagic telangiectasia. Nature Medicine;16:420-428.
  5. Brinkerhoff, BT et al. Long-term therapy with bevacizumab in hereditary hemorrhagic teleangiectasia. N Engl J Med 2011;364:688-689.
  6. Chen S et al. Safety of intranasal Bevacizumab (avastin) treatment in patients with hereditary hemorrhagic teleangiectasia-associated epistaxis. Laryngoscope. 2011;121:644-646.
  7. Nalluri SR et al. Risk of venous thromboembolism with the angiogenesis inhibitor bevacizumab in cancer patients. A meta-analysis. JAMA 2008;300:2277-2285.
  8. Shovlin CL et al. Elevated factor VIII in hereditary haemorrhagic teleangiectasia (HHT): Association with venous thromboembolism. Thromb Haemost 2007;98:1031-1039.
  9. Ghosh K et al. Hereditary haemorrhagic teleangiectasia (HHT): Negotiating between the Scylla of bleeding and Charybdis of thrombosis. Thromb Haemost 2008;100:162-164.

For Health Care Professionals: ThIS same education blog, written for health care professionals, can be found here.

Authors: This blog entry was written by Dr. Stephan Moll and Dr. Raj Kasthuri, both at the Hemostasis and Thrombosis Center, University of North Carolina, Chapel Hill, NC.  Dr. Kasthuri is also the Director of the UNC HHT Center of Excellence.


Disclosure: The authors have no financial conflict of interest with this education blog.

Last updated: March 1st, 2011

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3 Responses to “HHT”

  1. Joella Johnson says:

    My daughter, age 25, had a massive bi-lateral PE and then found to have another clot. IVC filter inserted. Has been on coumadin and still clots! Filter removed 03/09/11. Factor V Leiden positive. Question is, she had hemangiomas when a baby, and 1 cavernous hemangioma in chest. Should she have further testing?

    • Stephan Moll says:

      Dr. Raj Kasthuri, UNC Chapel Hill, responds:

      In direct response to the question
      There is insufficient information in the medical literature on the risk of venous blood clots in HHT. HHT-associated arterio-venous malformations (AVMs) develop in very specific internal organs. These include the liver (generally with advancing age and not usually seen in young adults), lungs, and brain. Having an AVM does not in itself increase the risk of DVT in patients with HHT. However, an AVM in the lung provides a conduit by which a clot in the legs can bypass the filtering effect of the lungs and cross to the arterial side, resulting in a stoke or arterial clots elsewhere (referred to as paradoxical embolism).

      In regards to your daughter’s case, does she have frequent nose bleeds? Does she have red spots around her mouth or in her lips, tongue or finger tips? These are the most common features of HHT. If any of these are present, then she needs further evaluation. If there were AVMs in her lungs they would have been detected on the CT scan that diagnosed her PEs. We are assuming that the PEs were associated with DVTs in the lower extremities. Assuming this is true, it is unlikely that HHT-related AVMs were the reason for her VTE. You mention that she had hemangiomas as a baby – have they resolved spontaneously with time? What happened to the cavernous hemangioma in the lung? Is it still present?

      In summary (given the limited information available to us)
      I suspect this is a patient with thrombophilia who developed recurrent venous clots. It is likely that the hemangioma in the lung was discovered incidentally and has not changed in size. The hemangiomas the patient had – if they were in the skin, are quite common. A number of them regress with age and the patient may not even have them any more. In the absence of other features of HHT, such as nose bleeds or mucocutaneous telangiestasias, the likelihood of the patient having HHT is very, very low. Given the absence of other features of KT syndrome, the likelihood of this being KTS is similarly low.

      Additional comments• In general, HHT associated AVMs tend to be relatively small – sub-centimeter to 1-2 cms. However, liver AVMs in HHT can get quite large, as we do not embolize them like lung and brain AVMs.
      • Hemangiomas tend to be larger and can be either cutaneous or visceral. Hemangiomas do not need to be treated unless patients are symptomatic.
      • It is unlikely that a patient will have Klippel-Trenaunay (KT) syndrome (will be discussed on Clot Connect in a future blog entry) and have no other manifestation of it except for DVT/PE. Patients tend to have skin involvement. The extent of venous malformations in KT syndrome is quite marked and patients can also have a lymphatic component. So, typically there is also asymmetry noted in the involved extremity.
      • On radiological imaging, a cavernous hemangioma is a well circumscribed, usually solitary, malformation. KT syndrome has numerous venous malformations that will ‘light up’ the scan. So, differentiating these 2 should not be difficult for the radiologist.

      1. Blei F. Basic science and clinical aspects of vascular anomalies. Curr Opinion Pediatrics 2005;17:501-509.
      2. Byung-Boong L et al. Management of arteriovenous malformations: A multidisciplinary approach. J Vasc Surg 2004;39:590-600.