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Prevalence of PAH, in all of its forms, is estimated at around 1/67,000. IPAH is one of the most commonly diagnosed forms.

Clinical manifestations of IPAH are similar to other forms of PAH regardless of its etiology. IPAH develop in adults and in rare cases in children; women are twice as likely as men to be affected. The diagnosis is often made in patients in their mid-forties. Initial symptoms include dyspnea, fatigue, syncope, chest pain, palpitations and pedal edema. Precordial signs include loud and palpable second heart sound, right ventricular heave, pulmonary ejection click and murmurs of pulmonary and tricuspid regurgitation. 70% of patients present heart failure (classed as New York heart association functional classification (NYHA FC) III or IV). More rarely, clubbing of digits and Raynaud phenomenon may be observed. Hemoptysis has also been reported.

IPAH is caused by vascular remodeling of small pulmonary arteries due to unknown causes. Mutations in PAH predisposing genes are identified in about 15% to 20% of PAH patients initially considered to have an idiopathic form of the disease. IPAH patients carriers of a mutation in PAH predisposing gene have to be reclassified in ''heritable PAH'' (see this term). The main genetic risk factor of PAH is mutations in BMPR2gene (2q33). Mutations in ACVRL1>(12q13), ENG>(9q34), KCNK3 >(2p23),CAV1 (7q31), TBX4(17q21) have been identified in few cases.

Genetic counseling has to been proposed to all IPAH patients and mutations in PAH predisposing genes have to be searched. All PAH predisposing gene are transmitted in an autosomal dominant manner with an incomplete penetrance. In the cases of BMPR2 mutations, the penetrance is estimated to be 42% in female mutation carriers and 14% in male mutation carriers.