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Introduction

Fatigue may be defined as a subjective state in which one feels tired or exhausted, and in which the capacity for normal work or activity is reduced. There is, however, no commonly accepted definition of fatigue when it is considered in the context of health and illness. This lack of definition results from the fact that a person's experience of fatigue depends on a variety of factors. These factors include culture; personality; the physical environment (light, noise, vibration); availability of social support through networks of family members and friends; the nature of a particular fatiguing disease or disorder; and the type and duration of work or exercise. For example, the experience of fatigue associated with disease will be different for someone who is clinically depressed, is socially isolated, and is out of shape, as compared to another person who is not depressed, has many friends, and is aerobically fit.

Fatigue is sometimes characterized as normal or abnormal. For example, the feeling of tiredness or even exhaustion after exercising is a normal response and is relieved by resting; many people report that the experience of ordinary tiredness after exercise is pleasant. Moreover, this type of fatigue is called acutesince the onset is sudden and the desired activity level returns after resting. On the other hand, there is a kind of fatigue that is not perceived as ordinary; that may develop insidiously over time; is unpleasant or seriously distressing; and is not resolved by rest. This kind of fatigue is abnormal and is called chronic.

Some researchers regard fatigue as a defense mechanism that promotes the effective regulation of energy expenditures. According to this theory, when people feel tired they take steps to avoid further stress(physical or emotional) by resting or by avoiding the stressor. They are then conserving energy. Since chronic fatigue is not normal, however, it is an important symptom of some mental disorders; of a variety of physical diseases with known etiologies (causes); and of medical conditions that have no biological markers although they have recognizable syndromes (patterns of symptoms and signs).

Fatigue is sometimes described as being primary or secondary. Primary fatigue is a symptom of a disease or mental disorder, and may be part of a cluster of such symptoms as pain, fever, or nausea. As the disease or disorder progresses, however, the fatigue may be intensified by the patient's worsening condition, by the other disease symptoms, or by the surgical or medical treatment given to the patient. This subsequent fatigue is called secondary.

Risk factors

Fatigue is a common experience. It is one of the top ten symptoms that people mention when they visit the doctor. Some people, however, are at higher risk for developing fatigue. For example, the risk for women is about 1.5 times the risk for men, and the risk for people who don't exercise is twice that of active people. Some researchers question whether women really are at higher risk, since they are more likely than men to go to the doctor with health problems; also, men are less likely to admit they feel fatigued. Other risk factors include obesity; smoking; use of alcohol; high stress levels; depression; anxiety; and low blood pressure. Having low blood pressure is usually considered desirable in the United States, but is regarded as a treatable condition in other countries. Low blood pressure or postural hypotension (sudden lowering of blood pressure caused by standing up) may cause fatigue, dizziness, or fainting.

Disease

There are many diseases and disorders in which fatigue is a major symptom— for example, cancer, cardiovascular disease, emphysema, multiple sclerosis, rheumatic arthritis, systemic lupus erythematosus, HIV/AIDS, infectious mononucleosis, chronic fatigue syndrome, and fibromyalgia. The reasons for the fatigue, however, vary according to the organ system or body function affected by the disease. Physical reasons for fatigue include:

Circulatory and respiratory impairment. When the patient's breathing and blood circulation are impaired, or when the patient has anemia (low levels of red blood cells), body tissues do not receive as much oxygen and energy. Hence, the patient experiences a general sense of fatigue. Fatigue is also an important warning sign of heart trouble because it precedes 30%–55% of myocardial infarctions (heart attacks) and sudden cardiac deaths.
Infection. Microorganisms that disturb body metabolism and produce toxic wastes cause disease and lead to fatigue. Fatigue is an early primary symptom of chronic, nonlocalized infections found in such diseases as acquired immune deficiency syndrome (AIDS), Lyme disease, and tuberculosis.
Nutritional disorders or imbalances. Malnutrition is a disorder that promotes disease. It is caused by insufficient intake of important nutrients, vitamins, and minerals; by problems with absorption of food through the digestive system; or by inadequate calorie consumption. Protein-energy malnutrition (PEM) occurs when people do not consume enough protein or calories; this condition leads to wasting of muscles and commonly occurs in developing countries. In particular, young children who are starving are at risk of PEM, as are people recovering from major illness. In general, malnutrition damages the body's immune function and thereby encourages disease and fatigue. Taking in too many calories for the body's needs, on the other hand, results in obesity, which is a predictor of many diseases related to fatigue.
Dehydration. Dehydration results from water and sodium imbalances in body tissues. The loss of total body water and sodium may be caused by diarrhea, vomiting, bed rest, exposure to heat, or exercise. Dehydration contributes to muscle weakness and mental confusion; it is a common and overlooked source of fatigue. Once fatigued, people are less likely to consume enough fluids and nutrients, which makes the fatigue and confusion worse.
Deconditioning. This term refers to generalized organ system deterioration resulting from bed rest and lack of exercise. In the 1950s and 1970s, the National Aeronautics and Space Administration (NASA) studied the effects of bed rest on healthy athletes. The researchers found that deconditioning set in quite rapidly (within 24 hours) and led to depression and weakness. Even mild exercise can counteract deconditioning, however, and has become an important means of minimizing depression and fatigue resulting from disease and hospitalization.

Pain. When pain is severe enough, it may disrupt sleep and lead to the development of such sleep disorders as insomnia or hypersomnia. Insomnia is the term for having difficulty falling and/or staying asleep. Hypersomnia refers to excessive sleeping. In general, disrupted sleep is not restorative; people wake up feeling tired, and as a result their pain is worsened and they may become depressed. Furthermore, pain may interfere with movement or lead to too much bed rest, which results in deconditioning. Sometimes pain leads to social isolation because the person cannot cope with the physical effort involved in maintaining social relationships, or because family members are unsympathetic or resentful of the ill or injured person's reduced capacity for work or participation in family life. All of these factors worsen pain, contributing to further sleep disruption, fatigue, and depression.

Stress. When someone experiences ongoing pain and stress, organ systems and functional processes eventually break down. These include cardiovascular, digestive, and respiratory systems, as well as the efficient elimination of body wastes. According to the American Psychiatric Association, various chronic diseases are related to stress, including regional enteritis (intestinal inflammation); ulcerative colitis (a disease of the colon); gastric ulcers; rheumatoid arthritis; cardiac angina, and dysmenorrhea (painful menstruation). These diseases deplete the body's levels of serotonin (a neurotransmitter important in the regulation of sleep and wakefulness, as well as depression), and endorphins (opiate-like substances that moderate pain). Depletion of these body chemicals leads to insomnia and chronic fatigue.

Sleep disorders. There are a variety of sleep disorders that cause fatigue, including insomnia, hypersomnia, sleep apnea, and restless legs syndrome. For example, hypersomnia may be the result of brainabnormalities caused by viral infections. Researchers studying the aftermath of infectious mononucleosis proposed that exposure to viral infections might change brain function with the effect of minimizing restorative sleep; hence, some people developed hypersomnia. Another common disorder is sleep apnea, in which the patient's breathing stops for at least ten seconds, usually more than 20 times per hour. Snoring is common. People may experience choking and then wake up gasping for air; they may develop daytime hypersomnia (daytime sleepiness) to compensate. Sleep apnea is associated with aging, weight gain, and depression. It is also a risk factor for strokeand myocardial infarctions. Restless legs syndrome is a condition in which very uncomfortable sensations in the patient's legs cause them to move and wake up from sleep, or keep them from falling asleep. All of these disorders reduce the quality of a person's sleep and are associated with fatigue.

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