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Biology of Stroke
Disruption of the blood supply to the brain has serious, potentially drastic, consequences. Control of activities such as movement, perception, speech, various mental and bodily functions may be significantly impaired. Consciousness may be lost. Disruptions of blood circulation to the brain are known as stroke. Stroke occurs in two basic forms, both potentially life threatening.
Ischemic Stroke
About 75% of all strokes are due to blockage of the oxygen-enriched blood flowing to the brain. These blockages are triggered either by a thrombus (a stationary clot that forms in a blood vessel) or an embolus (a clot that travels through the bloodstream and becomes lodged in a vessel). Referred to as thrombolytic, or ischemic, strokes, they are often preceded by transient ischemic attacks, or TIAs, which are episodes of inadequate blood flow that may produce sudden physical weakness, an inability to talk, double vision, or dizziness.
With a TIA, periods of reduced circulation with lowered vital oxygen supply are brief and transitory. Restoration of blood flow occurs quickly and lasting neurological damage is avoided.
With stroke, however, the interruption of blood flow lasts long enough to kill brain cells, producing irreversible neurological damage.
| Figure 1. Ischemic Stroke |
Hemorrhagic Stroke
A second type of stroke due to cerebral hemorrhage is called bleeding, or hemorrhagic, stroke. This occurs when a brain aneurysm ruptures or when a weakened or inflamed blood vessel in the brain starts to leak. As blood flows into the brain, the buildup of pressure may either kill the tissue directly or destroy cells by impeding normal circulation to the affected region. This typically produces an excruciating headache, sometime followed by loss of consciousness. In contrast to thrombolytic strokes, which are generally survived, massive hemorrhagic strokes are fatal about 80% of the time.
Stroke Etiology
Stroke occurs when blood flow to the brain is interrupted by either a blocked or burst artery, resulting in a sudden decrease in the blood flow to an area of the brain, depriving brain cells of oxygen and other nutrients. Ischemia develops within minutes, forming two zones around the site of thrombosis or embolism. Brain cells within the red zone die quickly, with complete loss of functionality. However, cells in the green zone, the penumbra region, do not suffer immediate and permanent damage (see Figure 2 below). Restoration of cerebral blood flow, even to a sub-optimal level, provides an opportunity for those brain cells to recover and regain functionality.
Roll cursor over GREEN ZONE button to show green zone
Figure 2. Brain cells in the red zone die quickly, with complete loss of functionality. Brain cells in the green zone, the penumbra region, do not suffer immediate and permanent damage.
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Stroke Recovery
Recovery from stroke depends on the extent and location of brain damage. Some stroke victims recover fully, but in the vast majority of cases, there is lasting physical or mental disability.
Weakened stroke victims are also more vulnerable to infectious diseases such as pneumonia.
In addition, patients recovering from stroke commonly suffer from depression. Existence of this depression can significantly hinder recovery.
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