Introduction: Pulmonary hypertension of unknown origin, also called primary pulmonary hypertension (i.e. there is no associated heart or lung disease), comprises three different pathological entities: (i) pulmonary veno-occlusive disease, (ii) recurrent thromboembolic or thrombotic pulmonary disease and (iii) plexogenic pulmonary hypertension (PPH) [1, 2]. The last-named is characterized by predominant involvement of small pulmonary arteries which has three distinctive characteristics: (i) concentric ‘onion-like’ intimal fibrosis, (ii) fibrinoid necrosis of the wall of some arteries occasionally associated with an organized or organizing thrombus, and (iii) dilated side-branches whose wall is often reduced to a single elastic lamina and is lined interiorly by cellular proliferation showing a ‘plexiform’ arrangement. This review is concerned with primary PPH and drug-induced PPH [3] as opposed to PPH complicating post-tricuspid cardiac shunts, although the actual sequence of events leading to remodelling of pulmonary vessels is better known in the latter than in the former. Since a recent exhaustive review is available [3], we shall concentrate on the role of vasodilatator treatment in PPH and the present understanding of cause(s) of PPH.

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