STROKE MECHANISMS

A stroke occurs when there is interruption of the blood supply to a particular area of the brain, ultimately leading to cell injury and cell death.

Strokes can be classified as either:   1) Ischemic   or   2) Hemorrhagic

1) Ischemic Strokes

Ischemic strokes are the most common, accounting for up to 80% of strokes, and occur when there is an occlusion of a blood vessel impairing the flow of blood to the brain. Ischemic strokes are divided into:

[A] thrombotic      [B] embolic      [C] systemic hypoprofusion

[A] THROMBOTIC STROKE

In thrombotic stroke, a thrombus or clot gradually builds up in the artery, eventually occluding it and disturbing blood flow to tissue downstream from the thrombus. Since blockage of the artery is gradual, onset of thrombotic strokes is generally slower than that of embolic strokes. Three primary influences predispose to the formation of thrombi, the so-called Virchow triad:
(1) Endothelial Injury
  • atherosclerosis
  • vasculitis
  • endocardial injury following MI
  • hypertension
  • bacterial endotoxins
  • (2) Stasis or Turbulent Blood Flow
  • turbulence over atherosclerotic plaques
  • stasis in aneurysms
  • stasis in noncontractile endocardium following MI
  • stasis due to atrial fibrillation
  • stasis due to vascular occlusion from sickle-cell anemia
  • (3) Hypercoagulability
  • genetic hypercoagulable states (factor V leiden, protein C deficiency, etc.)
  • acquired hypercoagulability (prolonged immobilization, cancer, prosthetic cardiac valves, heparin induced thrombocytopenia)

  • [B] EMBOLIC STROKE

    In an embolic stroke, a piece of material (or embolus) travels from a distant location and lodges in the blood vessel, occluding it. The most common type of embolus is a blood clot. Because the blockage arrives from another location, the onset of embolic strokes is usually quicker than that of thrombotic strokes. As well, because of this, treatment of the stroke must also include determining the source of the embolus so as to prevent further emboli. Because a blood clot is the most common type of embolus, all of the risk factors listed above for thrombotic stroke (Virchow Triad) also apply to embolic strokes.

    NOTE:  although a blood clot is the most common type of embolus, other types do exist. A few of the most common are listed below:
  • fat (from bone marrow in a broken bone)
  • air (from accidental injection)
  • cancer cells metastasizing from a distant site
  • bacteria (i.e. in bacterial endocarditis)
  • [C] SYSTEMIC HYPOPROFUSION

    In systemic hypoperfusion, blood flow is decreased to all areas of the body. This most commonly occurs due to failure of the heart to pump blood (such as in a cardiac arrest) or because of loss of blood through bleeding. Because the reduction in blood flow is global, all parts of the brain are affected, particularly watershed areas. Watershed areas are those areas of the brain that lie between the territories of the major arteries. These areas are supplied by the smallest vessels and therefore are most susceptible to damage if the blood supply is decreased.

    2) Hemorrhagic Strokes

    Although less common (~15% of strokes), hemorrhagic (or bleeding) strokes can be much more serious. This is because in addition to the interruption of blood supply to the target tissue, the hemorrhage (bleed) can also cause increasing intracranial pressure which can physically impinge on brain tissue, further impairing perfusion of the cerebrum.

    There are many causes of cerebral hemorrhage, the most common of which are listed below:
  • chronic arterial hypertension
  • aneurysm
  • vascular malformation (a-v malformation, cavernoma, telangectasias, venous angioma)
  • abnormally fragile vessels (amyloid angiopathy, vasculitis, sickle-cell anemia)
  • bleeding diatheses (anticoagulants, fibrinolytics, thromocytopenia, leukemia, hemophilia)
  • drug abuse
  • head trauma (primary hemorrhagic contusion, shear injury, vascular avulsion)
  • hemorrhage into preexisting lesions (primary brain tumors, metastases, granulomas)
  • secondary hemorrhage into pre-existing infarct