Discrete subvalvular aortic stenosis: is the presence of upstream complex blood flow disturbances an important pathogenic factor?

Background. Discrete fixed subvalvular aortic stenosis can occur as an isolated abnormality or in association with other cardiac anomalies, especially ventricular septa] defect, persistent patent ductus arteriosus or coarctation of the aorta (1-4) . It most often presents as a crescent-shaped ridge of fibroelastic tissue protruding from the left septal surface into the subaortic region or as a ring with extensions to the septum and anterior leaflet of the Initial valve (4,5) . Although there is no evidence that these abnormalities are present at birth, they usually become evident during the Ist decade of life . This observation has led to the suggestion that discrete fixed subvalvular aortic stenosis is an acquired lesion whose hemodynamic severity can progress with time (1-4,6) . An alternative interpretation is that the development of subaortic stenosis requires the changes in pressure and flow associated with birth or growth, or both, to establish the conditions that would generate the septa] tissue reaction . By this view the lesion is "acquired," yet still "congenital," because the underlying predisposing abnormality is waiting for the necessary conditions to become manifest . Numerous studies (6,7) have shown that redevelopment of the obsttuc . lion is common even after successful surgical resection of the obstructing tissue and can occur as long as 17 years after the initial procedure (7) . There are many theories to explain the pathogenesis and natural history of discrete subvalvular aortic stenosis in humans including : t . Polygenic inheritance. Although an animal model for the congenital development of subvalvular aortic stenosis exists, no human equivalent or underlying predisposing structural variation has been described (8),