To understand the signs and symptoms of stroke and why they can differ from patient to patient, it is necessary to understand a little about the brain and how it functions. There are literally thousands of possible symptoms that can result from a stroke, depending on which blood vessels and parts of the brain are involved. It is also important to realize that except for a brief period after birth, brain cells are unable to divide and form new cells. When brain cells die, they are not replaced. This is part of the reason for the limited ability of the brain to repair itself after injury, and why recovery from stroke is only partial in many cases. While someone who suffers a heart attack, for example, can lose 10 percent of heart tissue and still run a marathon, losing 10 percent of the tissue in certain parts of the brain can result in a devastating disability.

The human brain is the most complex structure known. It is composed of 100 billion nerve cells, called neurons; each neuron may connect to thousands of other brain cells. The trillions of connections are necessary for the integrative power of the brain. They also control body movements, interpret all sensations (hearing, vision, touch, balance, pain, taste, and smell), and mediate thought and language. Different areas of the brain control different functions.

Although the brain represents only 2 percent of the body’s weight, it uses about 25 percent of the body’s oxygen supply and 70 percent of the glucose (sugar). Unlike muscles, the brain cannot store nutrients, and thus it requires a constant supply of glucose and oxygen. If the blood supply is interrupted for as little as 30 seconds, unconsciousness results;permanent brain damage may follow in as little as four minutes. The brain’s high metabolic rate, sensitivity to changes in blood flow, and  dependence on continuous blood flow are what can make strokes so  dangerous. Figure 18.2 shows the major arteries supplying the brain.

The brain can be divided into three areas: brain stem, cerebellum, and cerebrum. The brain stem controls many of the body’s basic functions, including breathing, chewing, swallowing, and eye movements.The major pathways from the cerebrum—the thinking part of the brain—also pass through the brain stem to the body. The cerebellum, attached to the back of the brain stem, coordinates movements and balance.

The cerebrum is divided into two hemispheres, left and right. In general, the left brain receives input (sensations) from the right side of the body and controls movement on the right side, so that a stroke in the right side of the brain will cause left-sided weakness. Conversely, the right brain controls the left side of the body.Each side of the cerebrum is further divided into four lobes. The frontal lobes control motor function, planning, and expression of language. The temporal lobes are involved with hearing, memory, and behavior. The parietal lobes interpret sensation and control understanding of language. The occipital lobes perceive and interpret vision. The right and the left sides of the cerebrum are not identical, but rather have specialized functions. In almost all right-handed people and most left-banders, the left brain is “dominant” and performs most language functions. The right side of the brain controls the abilities to understand spatial relations and recognize faces, as well as musical ability. It also helps focus attention.

Just as strokes area strong risk factor for heart disease,heart disease is a strong risk factor for stroke, although only for one type of stroke, ischemic strokes.

Heart disease is associated with stroke in two ways. First, damage to the heart (as, for example, from a heart attack) may make it more likely that clots  will form within the heart. These clots can break loose and travel to the brain, causing a cardioembolic stroke. Heart disease and stroke are also associated because they are both manifestations of atherosclerotic disease in the blood vessels. If the blood vessels feeding the heart (the coronary arteries) are diseased, it is likely that arteries to the brain are also affected.

Patients with evidence of coronary artery disease, congestive heart failure, left ventricular hypertrophy (enlargement of the left side of the heart), disease of the heart valves, or arrhythmias (irregular heart rhythms) have a several-fold increase in the risk of stroke.

Several recent studies suggest that people with atrial fibrillation who take daily doses of either aspirin or warfarin (Coumadin) have a reduction of up to 80percent in their risk of stroke. These findings suggest that an estimated 20,000 to 50,000 strokes might be prevented each year if all people with this condition had prophylactic drug treatment.

The role of oral contraceptives in stroke risk is still inconclusive, primarily because most research has looked at the effects of high-dose estrogen pills, and most women now use lower-dose preparations. Estrogen is believed to promote blood clotting; lowerdose estrogen preparations are thought to minimize this effect. Because studies have found no increase in current risk of stroke or heart attack in women who previously used oral contraceptives, it is believed that the pill does not promote atherosclerosis.

Several retrospective studies have suggested that oral contraceptive use is associated with an increase in stroke risk, while other studies have only found a significant risk of brain hemorrhage in women over age 35 who take the pill and smoke. Smokers who have migraine headaches and take oral contraceptives may be at a particularly high risk of stroke.

Experts usually advise women who smoke not to use oral contraceptives—or better, to quit smoking.In contrast, there is evidence to suggest that estrogen replacement therapy for postmenopausal women may slow the atherosclerotic process. In this group the use of estrogens may actually lower the risk of stroke (and heart disease).

Most treatment of stroke during the acute phase centers on maintaining fluids and electrolytes (chemical substances in the blood, such as sodium and potassium),  avoiding low blood pressure (hypotension), and avoiding the secondary complications of stroke and paralysis. The latter includes pneumonia, urinary tract infections, muscle contractures, and pressure breakdown of the skin (bedsores). The physician will also attempt to anticipate and avert deterioration after a stroke. This will require constant monitoring and evaluation and may necessitate a number of laboratory tests.

Anticoagulant medications such as heparin are sometimes used to treat an acute ischemic stroke. While heparin does not dissolve existing clots, it can prevent the formation of new ones. Thus it may help prevent subsequent strokes, which occur in up to 20 percent of ischemic stroke cases. Because heparin can increase some patients’ tendency to bleed, its use is often restricted to those with the highest risk of recurrent stroke: patients with a progressing stroke, more than one TIA, or a cardiac source of embolism (often seen with myocardial infarction, atrial fibrillation, or valvular diseases). Related drugs known as heparinoids are now being evaluated and appear to be at least as effective, with a lower risk of bleeding.

Surgery is usually not used to treat an acute stroke, although it maybe indicated for a hemorrhagic stroke (subarachnoid and brain hemorrhages) or a recent blockage of a carotid artery.

The networks of the cerebral cortex exhibit two main principles of structural and functional organization, segregation and integration . Anatomical and functional segregation refers to the existence of specialized neurons and brain areas, often organized into distinct neuronal populations (groups or columns) or cortical areas. These specialized and segregated sets of neurons selectively respond to specific input features (such as orientation, spatial frequency, or wavelength), or conjunctions of features (such as faces). They reside in cortical areas that process separate feature dimensions (such as color and motion) or sensory modalities.

However, segregated and specialized neuronal units do not operate in isolation. There is abundant evidence that coherent perceptual and cognitive states require the coordinated activation, that is, the functional integration, of very large numbers of neurons within the distributed system of the cerebral cortex . Electrophysiological studies have shown that perceptual or cognitive states are associated with specific and highly dynamic (short-lasting) patterns of temporal correlations (functional connectivity) between different regions of the thalamocortical system . Human neuroimaging experiments have revealed that virtually all perceptual or cognitive tasks, for example, object recognition, memory encoding
and retrieval, reading, working memory, attentional processing, motor planning, and awareness, are the result of activity within large-scale and distributed brain networks.