Dizziness and fainting

Anatomy or system affected: Blood vessels, brain, circulatory system, head, nervous system, psychic-emotional system

Definition: Dizziness is a feeling of light-headedness and unsteadiness, sometimes accompanied by a feeling of spinning or other spatial motion; fainting is a loss of consciousness as a result of insufficient amounts of blood reaching the brain. Both are symptoms of many conditions, which may be harmless or serious.

Causes and Symptoms

In humans, several mechanisms have evolved by which adequate blood flow to organs is maintained. Without a constant blood supply, the body’s tissues would die from a lack of essential nutrients and oxygen. In particular, the brain and heart are very sensitive to changes in their blood supply as they, more than any other organs, must receive oxygen and nutrients at all times. If they do not, their cells will die and cannot be replaced.

While the heart supplies most of the force needed to propel the blood throughout the body, tissues rely on changes in the size of arteries to redirect blood flow to where it is needed most. For example, after a large meal, the blood vessels that lead to the gastrointestinal tract enlarge (vasodilate) so that more blood can be present to collect the nutrients from the meal. At the same time, the blood vessels that supply muscles decrease in diameter (vasoconstrict) and effectively shunt the blood toward the stomach and intestines. During exercise, the blood vessels that supply the muscles dilate, and the ones leading to the intestinal tract vasoconstrict. This mechanism allows the cardiovascular system to supply the most blood to the most active tissues.

The brain is somewhat special in that the body tries to maintain a nearly constant blood flow to it. Located in the walls of the carotid arteries, which carry blood to the brain, are specialized sensory cells that have the ability to detect changes in blood pressure. These cells are known as baroreceptors. If the blood pressure going to the brain is too low, the baroreceptors send an impulse to the brain, which in turn speeds up the heart rate and causes a generalized vasoconstriction. This reflex response raises the body’s blood pressure, reestablishing adequate blood flow to the brain. If the baroreceptors detect too high a blood pressure, they send a signal to the brain, which in turn slows the heart rate and causes the arteries of the body to dilate. These reflexes prevent large fluctuations in blood flow to the brain and other tissues.

Most people have experienced a dizzy feeling or maybe even a fainting response when they have stood up too quickly from a prone position. The ability of the baroreceptors to maintain relatively constant arterial pressure is extremely important when a person stands after having been lying down. Immediately upon standing, the pressure in the carotid arteries falls, and a reduction of this pressure can cause dizziness or even fainting. Fortunately, the falling pressure at the baroreceptors elicits an immediate reflex, resulting in a more rapid heart rate and vasoconstriction, minimizing the decrease in blood flow to the brain.

Blood pressure is not the only factor that is essential in maintaining tissue viability. The accumulation of waste products and a lack of essential nutrients and gases can also have a profound effect on how much blood flows through a particular tissue and how quickly. In a region of the carotid arteries near the baroreceptors are chemoreceptors. Chemoreceptors detect the concentration of the essential gas oxygen and the concentration of the gaseous waste product carbon dioxide. When carbon dioxide concentrations increase, and oxygen concentrations decrease, the chemoreceptors stimulate regions in the brain to increase the heart rate and blood pressure in an attempt to supply the tissues with more oxygen and flush away the excess carbon dioxide. If the chemoreceptors detect high levels of oxygen and low levels of carbon dioxide, an impulse is transmitted to the brain, which in turn slows the heart rate and decreases the blood pressure.

Normally, most of the blood flow to the brain is controlled by the baroreceptor and chemoreceptor reflexes. However, the brain has a backup system. If blood flow decreases enough to cause a deficiency of nutrients and oxygen and an accumulation of waste products, special nerve cells respond directly to the lack of adequate energy sources and become strongly excited. When this occurs, the heart is stimulated, and blood pressure rises.

Dizziness is a sensation of light-headedness often accompanied by a sensation of spinning (vertigo). Occasionally, a person experiencing dizziness will feel nauseated and may even vomit. Most attacks of dizziness are harmless, resulting from a brief reduction in blood flow to the brain. There are several causes of dizziness, and each alters blood flow to the brain for a slightly different reason.

A person rising rapidly from a sitting or lying position may become dizzy. This is known as postural hypotension, which is caused by a relatively slow reflexive response to the reduced blood pressure in the arteries providing blood to the brain. Rising requires increased blood pressure to supply the brain with adequate amounts of blood. Postural hypotension is more common in the elderly and in individuals prescribed antihypertensive medicines (drugs used to lower high blood pressure).

If the patient experiences vertigo with dizziness, the condition is usually caused by a disorder of the inner ear equilibrium system. Two disorders of the inner ear that can cause dizziness are labyrinthitis and Ménière’s disease. Labyrinthitis, inflammation of the fluid-filled canals of the inner ear, is usually caused by a virus. Since these canals are involved in maintaining equilibrium, when they become infected and inflamed, one experiences the symptom of dizziness. Ménière’s disease is a degenerative disorder of the ear in which the patient experiences not only dizziness but also progressive hearing loss.

Some brainstem disorders also cause dizziness. The brain stem houses the vestibulocochlear nerve, which transmits messages from the ear to several other parts of the nervous system. Any disorder that alters the functions of this nerve will result in dizziness and vertigo. Meningitis (inflammation of the coverings of the brain and spinal cord), brain tumors, and blood-flow deficiency disorders such as atherosclerosis may affect the function of the vestibulocochlear nerve.

Syncope (fainting) is often preceded by dizziness. Syncope is the temporary loss of consciousness as a result of an inadequate blood flow to the brain. In addition to losing consciousness, the patient may be pale and sweaty. The most common cause of syncope is a vasovagal attack, in which an overstimulation of the vagus nerve slows the heart. Often vasovagal syncope results from severe pain, stress, or fear. For example, people may faint when hearing bad news or at the sight of blood. More commonly, individuals who have received a painful injury will faint. Rarely, vasovagal syncope may be caused by prolonged coughing, straining to defecate or urinate, pregnancy, or forcing expiration. Standing still for long periods of time or standing up rapidly after lying or sitting can cause fainting. With the exception of vasovagal syncope, all the causes of syncope are attributable to inadequate blood returning to the heart. If blood pools in the lower extremities, there is a reduced amount available for the heart to pump to the brain. In vasovagal syncope and some disorders of heart rhythm, such as Adams-Stokes syndrome, it is the heart itself that does not force enough blood toward the brain.

Treatment and Therapy

Short periods of dizziness usually subside after a few minutes. Deep breathing and rest will usually help relieve the symptom. Prolonged episodes of dizziness and vertigo should be brought to the attention of a physician.

Recovery from fainting likewise will occur when adequate blood flow to the brain is reestablished. This happens within minutes because falling to the ground places the head at the same level as the heart and helps return the blood from the legs. If a person does not regain consciousness within a few minutes, a physician or emergency medical team should be notified.

The most common cause of syncope is decreased cerebral blood flow resulting from the limitation of cardiac output. When the heart rate falls below its normal seventy-five beats per minute to approximately thirty-five beats per minute, the patient usually becomes dizzy and faints. Although slow heart rates can occur in any age group, they are most often found in elderly people who have other heart conditions. Drug-induced syncope can also occur. Drugs for congestive heart failure (digoxin) or antihypertensive medications that slow the heart rate (propranolol, metoprolol) may reduce blood flow to the brain sufficiently to cause dizziness and fainting.

Exertional syncope occurs when individuals perform some physical activity to which they are not accustomed. These physical efforts demand more work from the cardiovascular system and in patients with some obstruction of the arteries that leave the heart, the cardiovascular system is overstressed. This defect, combined with the vasodilation in the blood vessels that provide blood to the working muscles, reduces the amount of blood available for use by the brain. If the person also hyperventilates during exercise, he or she will effectively reduce the amount of carbon dioxide in the blood and rid the cardiovascular system of this normal stimulus for increasing heart rate and blood flow to the brain. Some persons also hold their breath during periods of high exertion. For example, people attempting to lift something very heavy often take a deep breath just prior to exerting and then hold their breath when they lift the object. This practice, known as the Valsalva maneuver, increases the pressure within the chest cavity, which in turn reduces the amount of blood returning to the heart. A decrease in blood returning to the heart (venous return) causes a decrease in the availability of blood to be pumped out of the heart and reduces cardiac output. The reduction in cardiac output decreases the amount of blood flowing to the brain and initiates a fainting response. It is interesting to note that humans also use the Valsalva maneuver when defecating or urinating, particularly when they strain. These acts can also lead to exertional syncope.

For a physician to diagnose and treat dizziness and fainting accurately, he or she must take an accurate medical history, paying particular attention to cardiovascular and neurological problems. In addition to experiencing episodes of dizziness and fainting, patients often have a weak pulse, low blood pressure (hypotension), sweating, and shallow breathing. Heart rate and blood pressure are monitored while the patient assumes different positions. The clinician also listens to the heart and carotid arteries to determine whether there are any problems with these tissues, such as a heart valve problem or atherosclerosis of the carotid arteries. An electrocardiogram (ECG or EKG) can detect abnormal heart rates and rhythms that may reduce cardiac output. Laboratory tests are used to determine whether the patient has low blood sugar (hypoglycemia), too little blood volume (hypovolemia), too few red blood cells (anemia), or abnormal blood gases suggesting a lung disorder. Finally, if the physician suspects a neurological problem such as a seizure disorder, he or she may run an electroencephalogram (EEG) to record brain activity.

Treatment for any of these underlying disorders may cure the dizziness and fainting episodes. In patients with postural hypotension, merely being aware of the condition will allow them to change their behavior to lessen the chances of becoming dizzy and fainting. These patients should not make any sudden changes in posture that could precipitate an attack. Often, this means simply slowing down their movements and learning to assume a horizontal position if they feel dizzy. Patients also can learn to contract their leg muscles and not hold their breath when rising. This increases the amount of blood available for the heart to pump toward the brain. If these techniques do not provide an adequate solution for postural hypotension, then a physician can prescribe drugs to increase blood pressure.

Heart rhythm disturbances (arrhythmias) that cause an abnormally fast or slow heart rate can be corrected with drug therapy such as quinidine or disopyramide (if the rate is too rapid) or a pacemaker (if the rate is too slow). It is interesting to note that even too fast a heart rate can cause dizziness and fainting. In patients with this type of arrhythmia, the heart beats at such a rapid rate that it cannot efficiently fill with blood before the next contraction. Therefore, less blood is pumped with each beat.

Other treatments for dizziness and fainting may include correcting the levels of certain blood elements. Patients with hypoglycemia often feel dizzy. The brain and spinal cord require glucose as their energy source. In fact, the brain and spinal cord have a very limited ability to utilize other substrates, such as fat or protein, for energy. Because of this, patients often feel light-headed when there are inadequate levels of glucose in the blood. Patients can correct this condition by eating more frequent meals, and if necessary, physicians can administer drugs such as epinephrine or glucagon. These agents liberate glucose from storage sites in the liver.

Individuals with a low blood volume are often dehydrated, and upon becoming rehydrated, no longer have dizziness or fainting episodes. If dehydration is not corrected and becomes worse, the patient can go into shock, a state of inadequate blood flow to tissues that will result in death if left untreated. In addition to being dizzy or fainting, the patient is often cold to the touch and has a rapid heart rate, low blood pressure, bluish skin, and rapid breathing. These patients are treated by emergency medical personnel, who keep the individual warm, elevate the legs, and infuse fluid into a vein. Drugs may be used to help bring blood pressure back to normal. The cause of the shock should be identified and corrected.

Perspective and Prospects

As humans evolved, they assumed an upright posture. This is advantageous because it allows for the use of the front limbs for other things besides locomotion. Unlike most four-legged animals, however, humans have their brains above their hearts and must continually force blood upwards to reach this vital tissue. This adaptation to the upright posture is a continuing physiological problem because the cardiovascular system must counteract the forces of gravity to provide the brain with blood. If this does not occur, the individual becomes dizzy and faints.

Another significant problem that humans face is adaptation to brain blood flow during exercise. The amount of blood flowing to a tissue is usually proportional to the metabolic demand of the tissue. At rest, various organs throughout the body receive a certain amount of the cardiac output. For example, blood flow to abdominal organs such as the spleen and the kidneys requires about 43 percent of the total blood volume. The total flow to the brain is estimated to be 13 percent, and the skin and skeletal muscles require 21 percent and 9 percent, respectively. Other areas, such as the gastrointestinal tract and heart, receive the remaining 14 percent. During exercise, the skeletal muscles may receive up to 80 percent of the cardiac output while the rest of the organs are perfused at a much-reduced rate.

Most data indicates that the brain receives only 3 percent of the total cardiac output during heavy exercise. Even though there is a large change in the redistribution of cardiac output, physiologists do not know the absolute amount of blood reaching the brain or the mechanism for the change in the perfusion rate.

With strenuous aerobic exercise such as jogging, there is an increase in cardiac output. During strenuous anaerobic exercise such as weight lifting, however, there may be a decrease in cardiac output attributable to the Valsalva maneuver. Therefore, it has been difficult to predict accurately, using available techniques, the volume of blood reaching this critical tissue.

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