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Strokes can be classified into two major categories: ischemic and hemorrhagic. Ischemia can be due to thrombosis, embolism, or systemic hypoperfusion. Hemorrhage can be due to intracerebral hemorrhage, subarachnoid hemorrhage subdural hemorrhageand epidural hemorrhage. ~80% of strokes are due to ischemia.
Ischemic stroke
In an ischemic stroke, which is the cause of approximately 80% of strokes, a blood vessel becomes occluded and the blood supply to part of the brain is totally or partially blocked. Ischemic stroke is commonly divided into thrombotic stroke, embolic stroke, systemic hypoperfusion (Watershed or Border Zone stroke), or venous thrombosis. Cocaine abuse doubles the risk of ischemic strokes.
Thrombotic stroke
In thrombotic stroke, a thrombus-forming process develops in the affected artery. The thrombus — a built up clot — gradually narrows the lumen of the artery and impedes blood flow to distal tissue. These clots usually form around atherosclerotic plaques. Since blockage of the artery is gradual, onset of symptomatic thrombotic strokes is slower. A thrombus itself (even if non-occluding) can lead to an embolic stroke (see below) if the thrombus breaks off—at which point it is then called an "embolus." Thrombotic stroke can be divided into two types depending on the type of vessel the thrombus is formed on:
* Large vessel disease involves the common and internal carotids, vertebral, and the Circle of Willis. Diseases that may form thrombi in the large vessels include (in descending incidence):
o Atherosclerosis
o Vasoconstriction
o Dissection
o Takayasu arteritis
o Giant cell arteritis
o Arteritis/vasculitis
o Noninflammatory vasculopathy
o Moyamoya syndrome
o Fibromuscular dysplasia
* Small vessel disease involves the intracerebral arteries, branches of the Circle of Willis, middle cerebral artery, stem, and arteries arising from the distal vertebral and basilar artery. Diseases that may form thrombi in the small vessels include (in descending incidence):
o Lipohyalinosis (lipid hyaline build-up secondary to hypertension and aging) and fibrinoid degeneration (stroke involving these vessels are known as lacunar infarcts)
o Microatheromas from larger arteries that extend into the smaller arteries (atheromatous branch disease)
Embolic stroke
Embolic stroke refers to the blockage of arterial access to a part of the brain by an embolus—a traveling particle or debris in the arterial bloodstream originating from elsewhere. An embolus is most frequently a blood clot, but it can also be a plaque broken off from an atherosclerotic blood vessel or a number of other substances including fat (e.g., from bone marrow in a broken bone), air, and even cancerous cells. Another cause is bacterial emboli released in infectious endocarditis.
Because an embolus arises from elsewhere, local therapy only solves the problem temporarily. Thus, the source of the embolus must be identified. Because the embolic blockage is sudden in onset, symptoms usually are maximal at start. Also, symptoms may be transient as the embolus lyses and moves to a different location or dissipates altogether. Embolic stroke can be divided into four categories:
* those with known cardiac source
* those with potential cardiac or aortic source (from transthoracic or transesophageal echocardiogram)
* those with an arterial source
* those with unknown source
High risk cardiac causes include:
* Atrial fibrillation and paroxysmal atrial fibrillation
* Rheumatic mitral or aortic valve disease
* Bioprosthetic and mechanical heart valves
* Atrial or ventricular thrombus
* Sick sinus syndrome
* Sustained atrial flutter
* Recent myocardial infarction (within one month)
* Chronic myocardial infarction together with ejection fraction <28 percent
* Symptomatic congestive heart failure with ejection fraction <30 percent
* Dilated cardiomyopathy
* Libman-Sacks endocarditis
* Antiphospholipid syndrome
* Marantic endocarditis from cancer
* Infective endocarditis
* Papillary fibroelastoma
* Left atrial myxoma
* Coronary artery bypass graft (CABG) surgery
Potential cardiac causes include:
* Mitral annular calcification
* Patent foramen ovale
* Atrial septal aneurysm
* Atrial septal aneurysm with patent foramen ovale
* Left ventricular aneurysm without thrombus
* Isolated left atrial smoke on echocardiography (no mitral stenosis or atrial fibrillation)
* Complex atheroma in the ascending aorta or proximal arch
Systemic hypoperfusion (Watershed stroke)
Systemic hypoperfusion is the reduction of blood flow to all parts of the body. It is most commonly due to cardiac pump failure from cardiac arrest or arrhythmias, or from reduced cardiac output as a result of myocardial infarction, pulmonary embolism, pericardial effusion, or bleeding. Hypoxemia (low blood oxygen content) may precipitate the hypoperfusion. Because the reduction in blood flow is global, all parts of the brain may be affected, especially "watershed" areas --- border zone regions supplied by the major cerebral arteries. Blood flow to these areas does not necessarily stop, but instead it may lessen to the point where brain damage can occur.
Hemorrhagic stroke
A hemorrhagic stroke, or cerebral hemorrhage, is a form of stroke that occurs when a blood vessel in the brain ruptures or bleeds. Like ischemic strokes, hemorrhagic strokes interrupt the brain's blood supply because the bleeding vessel can no longer carry the blood to its target tissue. In addition, blood irritates brain tissue, disrupting the delicate chemical balance, and, if the bleeding continues, it can cause increased intracranial pressure which physically impinges on brain tissue and restricts blood flow into the brain. In this respect, hemorrhagic strokes are more dangerous than their more common counterpart, ischemic strokes. There are two types of hemorrhagic stroke: intracerebral hemorrhage, and subarachnoid hemorrhage. Amphetamine abuse quintuples, and cocaine abuse doubles, the risk of hemorrhagic strokes.
Subarachnoid hemorrhage
Main article: subarachnoid hemorrhage
Subarachnoid hemorrhage (SAH) is bleeding into the cerebrospinal fluid (CSF) of the subarachnoid space surrounding the brain. The two most common causes of SAH are rupture of aneurysms from the base of the brain and bleeding from vascular malformations near the pial surface. Bleeding into the CSF from a ruptured aneurysm occurs very quickly, causing rapidly increased intracranial pressure. The bleeding usually only lasts a few seconds but rebleeding is common. Death or deep coma ensues if the bleeding continues. Hemorrhage from other sources is less abrupt and may continue for a longer period of time. SAH has a 40% mortality over 30 day period.
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