1.1 STROKE

Stroke is a form of cardiovascular disease affecting the blood supply to the brain. Also referred to as cerebrovascular disease or apoplexy, strokes actually represent a group of diseases that affect about one out of five people in the United States (Demarin et al., 2001). When physicians speak of stroke, they generally mean there has been a disturbance in brain function, often permanent, caused by either a blockage or a rupture in a vessel supplying blood to the brain.

In order to function properly, nerve cells within the brain must have a continuous supply of blood, oxygen, and glucose (blood sugar). If this supply is impaired, parts of the brain may stop functioning temporarily. If the impairment is severe, or lasts long enough, brain cells die and permanent damage follows (Kopito, 2001). Because the movement and functioning of various parts of the body are controlled by these cells, they are affected also. The symptoms experienced by the patient will depend on which part of the brain is affected.

While the incidence of stroke has decreased a great deal over the past few decades, there is evidence that this trend may be leveling off. Stroke is costly. The cost in human terms, to patients and their families, is impossible to estimate.  Although stroke is often viewed as a disease of the elderly, it sometimes affects younger individuals. The incidence of stroke does increase with age, but nearly a quarter of all strokes occur in people under the age of 60 (Demarin et al., 2001).

Stroke patients are often cared for by neurologists, because of the complex nature of the symptoms caused by damage to the brain. However, strokes are very closely related to heart disease. Heart attacks (myocardial infarctions) and stroke are both caused by diseases of the blood vessels. They share many of the same risk factors, and modifying these risk factors may reduce the possibility of stroke. Many of the therapies used for cardiac disease show promise for some types of stroke. People who already have coronary disease may be at greater risk for stroke, and vice versa.

1.2 Stroke and the human brain

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 (Safavi-Abbasi et al., 2006).  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 (Vahlgren et al., 2007).

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.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 (Safavi-Abbasi et al., 2006). 

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 (Alexandrov et al., 2004).