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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'''.
[[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]]