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Heyde's syndrome is a syndrome of gastrointestinal bleeding from angiodysplasia in the presence of aortic stenosis.〔〔 It is named after Dr. Edward C. Heyde, who first noted the association in 1958.〔 It is caused by the induction of Von Willebrand disease type IIA (vWD-2A) by a depletion of Von Willebrand factor (vWF) in blood flowing through the narrowed valvular stenosis.〔〔 == Pathophysiology == Von Willebrand factor is synthesized in the walls of the blood vessels and circulates freely in the blood in a folded form. When it encounters damage to the wall of a blood vessel, particularly in situations of high velocity blood flow, it binds to the collagen beneath the damaged epithelium and uncoils into its active form. Platelets are attracted to this activated form of von Willebrand factor and they accumulate and block the damaged area, preventing bleeding (see von Willebrand factor). In people with aortic valve stenosis, the stenotic aortic valve becomes increasingly narrowed resulting in an increase the speed of the blood through the valve in order to maintain cardiac output. This combination of a narrow opening and a higher flow rate results in an increased shear stress on the blood. This higher stress causes von Willebrand factor to unravel in the same way it would on encountering an injury site.〔〔〔 As part of the normal homeostasis of the blood, when von Willebrand factor changes conformation into its active state, it is degraded by its natural catabolic enzyme ADAMTS13, rendering it incapable of binding the collagen at an injury site.〔〔 As the quantity of von Willebrand factor in the blood decreases, the rate of bleeding dramatically increases.〔 The unraveling of high molecular weight von Willebrand factor in conditions of high shear stress is essential in the prevention of bleeding in the vasculature of the gastrointestinal system where small arterioles are common, as platelets cannot bind to damaged blood vessel walls well in such conditions.〔 This is particularly true in the presence of intestinal angiodysplasia, where arteriovenous malformations lead to very high blood flow, and so the loss of von Willebrand factor can lead to much more extensive bleeding from these lesions.〔〔 When people with aortic stenosis also have gastrointestinal bleeding, it is invariably from angiodysplasia.〔〔 It has been hypothesized that defects in high molecular weight von Willebrand factor could actually be the cause the arteriovenus malformations in intestinal angiodysplasia, rather than just making existing angiodysplasic lesions bleed. This hypothesis is complicated by the extremely high rates of intestinal angiodysplasia in older people (who also have the highest rate of aortic stenosis), and thus requires further research for confirmation.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heyde's syndrome」の詳細全文を読む スポンサード リンク
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