Definition of aneurysm A pathologically restricted dilatation of the arterial wall is called an aneurysm. Aneurysms can be classified according to the cause, shape and structure of the aneurysm wall. Aneurysms can be classified by etiology: atherosclerotic, syphilitic, bacterial, traumatic, and congenital aneurysms, as well as clotted aneurysms. Types of aneurysms 1. saccular aneurysm; 2. spindle aneurysm; 3. sinuous aneurysm; 4. navicular aneurysm; 5. intercalated aneurysm; 6. pseudoaneurysm Morphological types of aneurysms 1. saccular aneurysm: the wall of the involved vessel segment is spherically dilated, and the larger ones can be 15 to 20 cm in diameter. because of the vortex formed when blood flows through, this type of aneurysm is often complicated by thrombosis. 2. Shuttle-shaped aneurysm: The wall of the vessel is uniformly dilated and gradually and uniformly shrinks toward one end until it reaches the original vessel diameter, so it is shuttle-shaped. This type of artery is less likely to have wall thrombosis. 3.Cylindrical aneurysm: The vessel suddenly expands in the shape of a roller at the beginning, and then suddenly transitions to a normal vessel. Wall thrombosis can occur. 4.Boat-shaped aneurysm: The vessel wall is dilated laterally, while the opposite vessel wall is unchanged; it is common in the case of a coarctation aneurysm. 5. Meandering aneurysm: Asymmetrical dilatation of similar vascular segments one after another, therefore, the involved vessels are meandering and swollen. It is mostly seen in vessels where the direction of blood flow is repeatedly changed (e.g., arteries of the pelvis). Classification according to the structure of the aneurysm wall 1.True aneurysm: The wall consists of all three layers of vessel wall tissue, and most aneurysms belong to this type. 2.Pseudoaneurysm: Most of the aneurysms are caused by trauma to blood vessels and blood enters the surrounding tissues through the rupture and forms a hematoma, and then the inner surface of the hematoma is covered by endothelium after it is mechanized. Therefore, pseudoaneurysm is a kind of hematoma covered by endothelium. 3. Clamped aneurysm: It is mostly seen in the elderly (with the exception of patients with Marfan syndrome) and is more common in men than women. The blood flows from the strongest shear stress of the vascular tree and the most obvious changes in blood pressure (ascending aorta and aortic arch), and the blood flows from the endothelial rupture into the pathologically lax mesothelium (rarely from bleeding of the trophoblastic vessels) and splits the mesothelium longitudinally in the direction of blood flow, forming a pseudo-vascular lumen. This pseudo-vascular lumen can re-enter the true vessel lumen and the blood flow acts as a tortuous by-pass. The etiology and pathogenesis of this aneurysm are quite complex and can be seen in congenital vascular malformations, metabolic connective tissue diseases (e.g., Marfan syndrome, in which the elastic fibers of the aortic mesentery are broken and absent and collagen and proteoglycans are increased), increased proteoglycans in the vascular wall in hypothyroidism, syphilitic aortitis, and atherosclerosis.