Pathology and pathophysiology of the aortic root.

Pathologies of the aortic root amenable to repair with valve preservation include aneurysm formation, development of aortic insufficiency (AI) and aortic dissection. In the normal aortic root, the walls are constructed of 50-70 layers of concentric lamellar units. These units consist of sheets of elastin sandwiching smooth muscle cells interspersed with collagen and glycosaminoglycans. Medial degeneration results in disruption of the extracellular matrix (ECM), loss of smooth muscle cells and pooling of proteoglycans/glycosaminoglycans. These structural changes are associated with aneurysm formation. Aortic root aneurysms are commonly linked to hereditary thoracic aortic diseases including Marfan syndrome and Loeys-Dietz syndrome. One important pathway for hereditary thoracic aortic diseases is the transforming growth factor-β (TGF-β) cell-signalling pathway. Pathogenic gene mutations affecting various levels of this pathway have been implicated in aortic root aneurysm formation. Secondary effects of aneurysm formation include AI. Severe chronic AI leads to a pressure and volume load on the heart. Once symptoms develop or significant left ventricular remodelling and dysfunction occurs, the patient's prognosis is poor without surgery. Another consequence of aneurysm formation and medial degeneration is the risk of aortic dissection. Aortic root surgery is performed in 34-41% of surgeries for type A aortic dissection. Predicting those who will experience aortic dissections remains a challenge. Finite element analysis, study of fluid-structure interactions and aortic wall biomechanics are important areas of ongoing research.