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| + | =From general pathology= |
| + | |
| + | ==Introduction== |
| + | |
| + | * A thrombus is a solid blood clot that forms within the vessels or heart during life. |
| + | ** The process of thrombus formation is known as thrombosis. |
| + | ** Thrombii may occur anywhere within the cardiovascular system. |
| + | |
| + | ===Thrombosis=== |
| + | |
| + | * The process usually begins with platelets becoming sticky. |
| + | ** Adhere to each other and to the endothelium. |
| + | * This increased "stickiness" is induced by contact with: |
| + | ** Collagen |
| + | ** Basement membrane |
| + | ** Damaged endothelial cells |
| + | ** Bacteria |
| + | |
| + | ===Appearance of a Thrombus=== |
| + | |
| + | * A thrombus is a layered mass which is attached to the vessel wall. |
| + | * The composition of a thrombus consists of red blood cells, [[[[Neutrophils|Neutrophils]] - WikiBlood|neutrophils]] and platelets bound together by fibrin. |
| + | * Thrombi take different appareance depending on whether they are arterial or venous. |
| + | ** Arterial thrombi |
| + | *** Tend to be pale and have a tail in the direction of blood flow. |
| + | **** The high rate of blood flow sweeps red cells away - the thrombus is composed of mainly white cells, platelets and fibrin which are left behind. |
| + | ** Venous thrombi |
| + | *** Tend to be a darker red. |
| + | **** The slow blood flow allows the clot to form quicker. |
| + | *** Loosely arranged. |
| + | *** Contains many red blood cells. |
| + | |
| + | ==Causes of Thrombosis== |
| + | |
| + | * Thrombosis is caused by any condition that allows platelets to adhere to the endothelial wall. |
| + | |
| + | ===Damage to the Vessel Wall=== |
| + | |
| + | * When there is damage of the overlying endothelium, collagen is exposed and platelets will attach to this. |
| + | ** The blood clotting process will occur, giving formation of an intraluminal blood clot. |
| + | *** The process of formation occurs in phases giving a cross section of the clot a laminated appearance. |
| + | * Vessel walls may be damaged in several ways: |
| + | ** Trauma |
| + | ** Localisation by colonies of bacteria |
| + | ** Viral infection of endothelial cells |
| + | ** Migration of parasites. |
| + | * Repeated puncture of the jugular vein is a common cause of serious or fatal thrombosis. |
| + | |
| + | ===Changes in Normal Blood Flow=== |
| + | |
| + | ====Stasis==== |
| + | |
| + | * Stasis prevents coaglation factors being cleared by the liver and reticuloendothelial system. |
| + | * Coagulation factors build up, leading to clot formation. |
| + | * When stasis occurs in a vein, the thrombus forms behind the valve cusps. |
| + | ** Quickly enlarges to fill the lumen. |
| + | |
| + | ====Turbulence==== |
| + | |
| + | * Turbulence disrupts the laminar flow. |
| + | ** Platelets are brought into contact with the endothelial wall and may even damage the endothelium. |
| + | * Thrombi tend to form at arterial branches and venous junctions where there are eddy currents and local stasis. |
| + | |
| + | ===Changes in Blood Composition=== |
| + | |
| + | * An increase in the number and adhesiveness of platelets, or in plasma fibrin and clotting factors results in a hypercoagulateable state |
| + | * There may also be a decrease in the activity of the fibrinolyltic system. |
| + | * This may be caused by surgical or traumatic shock, or inflammation with toxaemia. |
| + | |
| + | ==Types of Thrombosis== |
| + | |
| + | ===Arterial Thrombosis=== |
| + | |
| + | * Although this is common in man, it is uncommon in domestic animals. |
| + | ** A major problem in man associated with atherosclerotic vascular disease. |
| + | * When they do occur in animals, arterial thrombi are usually small lesions. |
| + | ** However, these may be sited strategically, thereby causing problems. |
| + | |
| + | ====Arteriosclerosis==== |
| + | |
| + | * Seen in aged dogs and horses. |
| + | * Affect the coronary artery and other major arteries. |
| + | |
| + | ====Verminous arteritis==== |
| + | |
| + | * May occur with or without aneurysm. |
| + | * Seen in horses as a result of ''Strongylus vulgaris'' infestation. |
| + | ** Affects the root of cranial mesenteric artery, renal artery and aorta. |
| + | ** Results from larval migration in vessel walls. |
| + | |
| + | ====Iliac thrombosis==== |
| + | |
| + | * Seen in cats in the UK. |
| + | * Often referred to as "saddle embolus". |
| + | * Associated with cardio-myopathy and atrial thrombosis. |
| + | |
| + | ===Cardiac Thrombosis=== |
| + | |
| + | * Cardiac thrombosis is usually valvular, but can occasionally be mural. |
| + | |
| + | ====Farm Species==== |
| + | |
| + | * In farm animals, and rarely in the horse, infective/ inflammatory thrombosis occurs. |
| + | ** Known as endocarditis. |
| + | |
| + | ====Dogs and Horse==== |
| + | |
| + | * Cardiac thrombosis is generally of degenerative/ non-infectious cause. |
| + | ** Endocarditis may occur, though uncommonly. |
| + | * There are important effects of cardiac thrombosis. |
| + | *# Distortion of valve cusps leads to mechanical effects. |
| + | *#* Ineffective closeure gives leakage (incompetence). |
| + | *#* Distortion and enlargement of the valves by the thrombus mas fuves narrowing of the valve outflow (stenosis). |
| + | *#* These effects lead to progressive inefficiency of cardiac pumping. |
| + | *#** Causes [[Venous Congestion and Hyperaemia - Pathology#Generalised (Systemic) Venous Congestion|general venous congestion]], [[Oedema - Pathology|oedema]] and circulatory failure. |
| + | *# Emboli are disseminated from the thrombus mass. |
| + | *#* If the lesion is on the left side of the heart, emboli are disseminated systemically. |
| + | *#** Become trapped in the capilarry beds, e.g. of the kidney and spleen. |
| + | *#*** Blockage leads to local ischaemic necrosis. |
| + | *#* If the lesion is on the right side of the heart, there is pulmonary dissemination. |
| + | |
| + | ====Atrial Thrombosis==== |
| + | |
| + | * Uncommon. |
| + | ** Seen rarely in cats and dogs. |
| + | * Results from stasis in atrial appendages. |
| + | |
| + | ===Venous Thrombosis=== |
| + | |
| + | * Venous thrombosis is a fairly common type of thrombus in the veterinary species. |
| + | ** This is because: |
| + | **# Veins are relatively thin-walled. |
| + | **#* Are therefore more susceptible to distortion, inflammatory damage and iatrogenic vene puncture damage. |
| + | **# Veins have relatively slower blood flow rates. |
| + | **#* Cell aggregates tend to persist more readily. |
| + | * Most venous thrombosis in domestic animals results from : |
| + | ** Extension of inflammatory reactions. |
| + | ** Erosion/disruption caused by malignant tumours. |
| + | ** Pressure from adjacent space-occupying masses. |
| + | ** Vene puncture damage. |
| + | |
| + | ===Capillary Thrombosis=== |
| + | |
| + | * These are microthrombi. |
| + | ** Only appreciable histologically. |
| + | * Formation may be: |
| + | ** Localised |
| + | *** Associated with acute local inflammation. |
| + | *** Not very significant unless strategically sited. |
| + | **Generalised |
| + | *** May be seen in terminal disease as a reflection of vascular failure. |
| + | *** Often associated with shock syndromes as part of [[Thrombosis - Pathology#Disseminated Intravascular Coagulation|disseminated intravascular coagulation]]. |
| + | *** Clinically very significant. |
| + | |
| + | ==Evolution of a Thrombus== |
| + | |
| + | * If the animal survives the immediate effects of a thrombus, the thrombus may evolve in one of the following ways: |
| + | # The thrombus may gradually '''enlarge''' and eventually cause total '''obstruction''' of a vessel. |
| + | # The thrombus may be completely '''removed by fibrinolytic activity'''. |
| + | #* Fibrinolysis is a very active process - clots are usually removed within a few days of formation. |
| + | #* Blood flow is crucial to the process of fibrinolysis. |
| + | #** An occlusive thrombus may prevent the necessary enzymes from reaching the clot. |
| + | #* To overcome occlusion, contraction of fresh clots under the influence of thrombasthenin (released by platelets) forms a slit-like channel beside the thrombus. |
| + | #** The blood that then flows past may completely dissolve the clot. |
| + | # '''Organisation''' |
| + | #* A thrombus acts as a foreign body, causing an inflammatory response in the underlying blood vessel or heart wall. |
| + | #* The external surface of the thrombus quickly becomes covered by endothelium. |
| + | #** Is excluded from the clotting mechanism. |
| + | #* [[Neutrophils|Neutrophils]] invade the mass and may digest the centre. |
| + | #** Occasionally, subsequent invasion by bacteria may lead to to purulent inflammation. |
| + | #** Normally, fibroblasts and capillary buds follow the [[[[Neutrophils|Neutrophils]] - WikiBlood|neutrophils]] into the thrombus and a fibrous vascularised connective tissue forms. |
| + | #*** Capillaries channels anastomose to produce vessels that traverse the thrombus and re-establish blood flow. |
| + | #**** This is known as canalisation of a thrombus. |
| + | #*** Fibrous tissue matures and contracts, eventually causing the thrombus to become incorporated into the vessel wall as a fibrous lump. |
| + | # A piece of the thrombus may '''break off and form an embolus'''. |
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| [[Category:Arterial_Pathology]][[Category:Cardiovascular_System_-_Vascular_Pathology]][[Category:To_Do_-_Cardiovascular]] | | [[Category:Arterial_Pathology]][[Category:Cardiovascular_System_-_Vascular_Pathology]][[Category:To_Do_-_Cardiovascular]] |
| [[Category:Venous_Pathology]] | | [[Category:Venous_Pathology]] |