 A very small proportion ( those affected by SMAD4 ( MADH4 ) mutations , see below ) have multiple benign polyps in the large intestine , which may bleed or transform into colorectal cancer . A similarly small proportion experiences pulmonary hypertension , a state in which the pressure in the lung arteries is increased , exerting pressure on the right side of the heart and causing peripheral edema ( swelling of the legs ) , fainting and attacks of chest pain . It has been observed that the risk of thrombosis ( particularly venous thrombosis , in the form of deep vein thrombosis or pulmonary embolism ) may be increased . There is a suspicion that those with HHT may have a mild immunodeficiency and are therefore at a slightly increased risk from infections . 
