Strokes can be classified into two major categories :
Ischemic stroke Ischemia is due to
interruption of the blood supply. In an ischemic stroke, blood supply to part
of the brain is decreased, leading to dysfunction and necrosis of the brain
tissue in that area. There are four reasons why this might happen :
1. Thrombosis (
obstruction of a blood vessel by a blood clot forming locally ) In
thrombotic stroke, a thrombus ( blood clot ) usually forms 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, if the thrombus breaks off, at
which point it is 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.
Small vessel disease involves the smaller arteries inside
the brain, branches of the circle of Willis, middle cerebral artery, stem, and
arteries arising from the distal vertebral and basilar artery.
2. Embolism (
idem due to a blood clot from elsewhere in the body ) Embolic
stroke refers to the blockage of an artery 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 number of other substances
including fat ( e.g. from bone marrow in a broken bone ), air, cancer cells or
clumps of bacteria ( usually from infectious endocarditis ). Emboli most
commonly arise from the heart ( especially in atrial fibrillation ) but may
originate from elsewhere in the arterial tree. In paradoxical embolism, a deep
vein thrombosis embolises through an atrial or ventricular septal defect in the
heart into the brain.
3. Systemic hypoperfusion ( general decrease in blood supply, e.g. in
shock ) 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.
This phenomenon is also referred to as "last
meadow" to point to the fact that in irrigation the last meadow receives
the least amount of water.
4. Venous thrombosis Cerebral venous sinus
thrombosis leads to stroke due to locally increased venous pressure, which
exceeds the pressure generated by the arteries. Infarcts are more likely to
undergo hemorrhagic transformation (leaking of blood into the damaged area)
than other types of ischemic strok.
Hemorrhagic stroke Most of the
hemorrhagic stroke syndromes have specific symptoms ( e.g. headache, previous
head injury ). Hemorrhage is due to rupture of a blood vessel or an abnormal
vascular structure. Intracranial hemorrhage is the accumulation of blood
anywhere within the skull vault. A distinction is made between intra-axial
hemorrhage ( blood inside the brain ) and extra-axial hemorrhage ( blood inside
the skull but outside the brain ). Intracerebral hemorrhage ( ICH ) is bleeding
directly into the brain tissue, forming a gradually enlarging hematoma (
pooling of blood ). The hematoma enlarges until pressure from surrounding
tissue limits its growth, or until it decompresses by emptying into the
ventricular system, CSF or the pial surface. A third of intracerebral bleed is
into the brain's ventricles. ICH has a mortality rate of 44 percent after 30
days, higher than ischemic stroke or even the very deadly subarachnoid
hemorrhage.
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