DESCRIPTION

The word asthma originates from an ancient Greek word meaning panting. Essentially, asthma is an inability to breathe properly. When any person inhales, the air passes into the lungs through progressively smaller airways called bronchioles. The lungs contain millions of bronchioles, all leading to alveoli -- microscopic sacs where oxygen and carbon dioxide are exchanged. Asthma is a chronic condition in which these airways undergo changes when stimulated by allergens or other environmental triggers that cause patients to cough, wheeze, and experience shortness of breath (dyspnea).

Asthma affects 5% to 10% of the world's population, and research indicates that, in the U.S., the prevalence of asthma is greatly increasing in all age groups except older men. More than 17 million Americans have asthma, and the number of cases increased by 75% between 1980 and 1994. Other respiratory diseases, sinusitis, and ear infections are also on the rise, suggesting that airborne or environmental factors may be at work affecting all of these conditions, including asthma. Asthma occurs before the age of 15 more often in males, but it may be more common in girls after puberty. White women are at much greater risk of death than white men and the disparity is increasing. About 90% of U.S. deaths from asthma occur among the elderly, the majority of who are women. Some encouraging news includes reports of a general leveling off of asthma deaths in America since 1989 and a decline in British asthma deaths over the past decade.

SYMPTOMS

Asthma appears to have two primary stages: hyperreactivity (also called hyperresponsiveness) and the inflammatory response.

Hyperreactive Response In the hyperreactive response, smooth muscles in the airways constrict and narrowing excessively in response to inhaled allergens or other irritants. It should be noted that the airways in everyone's lungs respond by constricting when exposed to allergens or irritants, but people without asthma are able to breathe in deeply to relax the airways and rid the lungs of the irritant. When asthmatics try to take those same deep breaths, their airways do not relax and the patients pant for breath. Smooth muscles in the airways of people with asthma may have a defect, perhaps a deficiency in a critical chemical that prevents the muscles from relaxing.

Inflammatory Response The hyperreactive stage is followed by the inflammatory response, in which the immune system responds to allergens or other environmental triggers by delivering white blood cells and other immune factors to the airways. These so-called inflammatory factors cause the airways to swell, to fill with fluid, and to produce thick sticky mucus. This combination of events results in wheezing, breathlessness, inability to exhale properly, and a phlegm-producing cough. Inflammation appears to be present in the lungs of all patients with asthma, even those with mild cases, and plays a key role in all forms of the disease.

CAUSE

The mechanisms that cause asthma are complex and vary among population groups and even individuals. Because many asthma sufferers also have allergies, researchers are investigating the events that occur in both these conditions. Not all people with allergies have asthma, however, and not all cases of asthma can be explained by allergic response. Other contributing causes are being investigated. Asthma is most likely a result of genetic susceptibility, which probably involves several genes and various environmental triggers.

The Allergic Response In people who have asthma caused by an allergic response, various airborne allergens or other triggers set off a cascade of events in the immune system that lead to inflammation and hyperreactivity in the airways. The process is not completely understood, but the conductor in this orchestra of immune factors appears to be subgroups of white blood cells known as helper T-cells. These subgroups, TH2-cells, and specifically those known as gamma delta T cells, overproduce interleukins (IL), a subgroup of immune factors known as cytokines, which are powerful inflammatory agents. Of special interest are IL 9, IL 5, and IL 13. Interleukin 5, for example, appears to attract eosinophil cells, which are important players in airway hyperreactivity. Interleukin 9 stimulates the release of antibodies known as immunoglobulin E (IgE). During an allergic attack, these antibodies can bind to various cells in the immune system, including eosinophils, basophils, and mast cells, which are generally concentrated in the lungs, skin, and mucous membranes. Once IgE binds to mast cells, these cells are programmed to release a number of chemicals, particularly those known as leukotrienes, that cause inflammatory changes in the airways of the lungs, including narrowing of the airways, mucus production, and stimulation of nerve endings in the airway lining.

Common Allergens. Direct irritants to the lung such as animal dander, pollen, molds, and fungi can induce an asthma attack. The primary allergens that trigger asthma in the home are dust mites, specifically mite feces, which are coated with enzymes that contain a powerful allergen. Cockroaches are also major asthma triggers and may reduce lung function even in people without a history of asthma. The connection between asthma and nonseasonal or seasonal allergic rhinitis (hay fever or rose fever caused by pollen allergies) is unclear. They often coexist together, and although most people with asthma have a history of allergic rhinitis, only 1% to 20% of children with allergic rhinitis actually develop asthma. It is likely that the two conditions have a common cause rather than one causing the other.

Pollution and Cigarette Smoking. Air pollution has been associated with the development of asthma and asthma-related hospitalization. Specific pollutants targeted for their role in triggering asthma include diesel fumes, sulfur dioxide from power and paper industries, and nitrogen dioxide from exhaust and gas ovens. There are conflicting reports on the effects of ozone levels; recent studies indicate that high levels do not appear to increase the risk for hospitalization from asthma attacks. Cigarette smoke can accelerate the decline in lung function related to asthma. Studies also show that even exposure to secondhand smoke can double the risk of asthma-related emergency room visits.

Food Allergies. Although 67% of asthmatics believe their symptoms are aggravated by food allergies, studies indicate that this belief may be true in only 5% of cases. The primary suspects are monosodium glutamate, or MSG, (found in some canned soups, cheese, and certain vegetables) and sulfites (preservatives in wine and foods that include processed frozen potatoes and tuna). Contrary to what many believe, dairy products do not appear to exacerbate asthma symptoms in people who are not already allergic to them.

Occupational Triggers. An estimated 15% to 20% of adult-onset asthma cases are caused by occupational exposure to chemical triggers. Occupational asthma may occur after a prolonged period of exposure or it may develop suddenly following intense exposure to chemicals, often chlorine and ammonia.

Other Triggers. Cold air, thunderstorms, and extreme emotion are also known to aggravate asthma symptoms. Certain medications can trigger asthma attacks, such as aspirin and beta-blockers.

Genetic Factors About one third of all persons with asthma share the problem with another member of their immediate family. Genetic factors appear to play a more important role than common environmental factors in such families, and the condition may be more likely to be passed to children from the mother than from the father. One study also reported that the risk of having an asthmatic child was six times higher if both parents had a history of asthma than if just one had the disease. One study found that specific genetic regions increase the risks for asthma in different ethnic populations, including African Americans, Hispanics, and Caucasians.

Exercise-Induced Asthma In 40% to 90% of people with asthma, exercise will trigger coughing, wheezing, or shortness of breath. Exercise-induced asthma (EIA), however, is triggered only by exercise and is distinct from ordinary allergic asthma; some people have EIA alone, while others have both types. It occurs most often in children and young adults with asthma and is distinct from ordinary allergic asthma. People can have one or both types. Although EIA has the same symptoms as allergic asthma, it is not dangerous and does not require hospitalization. EIA occurs most often during intense exercise in cold dry air. There is some evidence that patients with EIA may also experience an asthmatic response hours after physical activity but more research is needed to confirm this.

Hormones

Hormones or changes in hormone levels appear to play a role in the severity of asthma in women. Between 30% and 40% of women with asthma experience fluctuations in severity that are associated with their menstrual cycle. One study indicated that such women tend to be older, have had asthma longer, and have more severe asthma than those whose asthma is not related to their periods. Their severe asthma attacks are likely to occur three days before and four days into the menstrual period. Oral contraceptives may help such asthma sufferers by leveling out hormonal changes. However, in postmenopausal women, hormone-replacement therapy, both with and without progesterone, poses twice the risk for late-onset asthma. During pregnancy of asthmatic women, one-third suffers more from the condition, one-third suffers less, and the other third experience no difference in severity. One interesting recent study suggests that expectant asthmatic mothers carrying a female baby tend to have more severe symptoms than do those who are bearing a male.

Viruses and Infections

The organisms Chlamydia pneumoniae, Mycoplasma pneumoniae, adenovirus, and respiratory syncytial virus are major causes of respiratory infections and are becoming important suspects in many cases of severe adult-onset asthma. In one study, patients whose asthma was initiated after infections had a more severe condition than those whose asthma was due to other causes, but asthma caused by infections did not last as long (5.6 years compared to 13.3 years).

Viruses and Infections in Children The organisms above are also major causes in children. According to one study, 11% of children with asthma showed signs of Chlamydia pneumoniae infection. Recently, a study reported that antibiotic treatment in childhood was highly associated with later asthma and hay fever. Such a finding may support the role of infections in later asthma or it may mean that antibiotics themselves directly affect the immune system. Some experts note that more young children are in day care, which increases the risk for wheezing and respiratory infections. A few studies indicate that the risk for asthma is high in children under five who wheeze, have frequent chest colds, or have a chronic cough, although some experts believe that such infections may actually protect against later asthma. (In young children, wheezing itself does not necessarily predict subsequent asthma.)

One theory for the dramatic increase in childhood asthma, in fact, blames the higher rate of childhood immunization against certain infectious diseases, including measles, and whooping cough. When children develop these infections, the immune system releases helper T-1 (TH1) white blood cells that stimulate the body's infection-fighters, but they also suppress the helper T-2 (TH2) cells, which are believed to trigger the asthmatic inflammatory response. Experts postulate that in some children who are vaccinated against these diseases, TH2 cells remain active and go on to stimulate asthma. Of some support for this theory are studies reporting that being a part of a large family reduces the risk of childhood asthma; children with many siblings are exposed to infections, which might increase immune factors that impede allergies.

Contributing Medical Conditions GERD (Gastroesophageal reflux disease). GERD, the cause of heartburn, is common in many asthmatic patients and is widespread among children with hard-to-control asthma. A constellation of irritable bowel syndrome, asthma, and gastroesophageal reflux disease also occurs in some people. GERD may cause asthma by spilling acid into the airways, which triggers a hyperreactive response. GERD may be suspected in patients who do not respond to asthma treatments, whose asthma attacks follow episodes of heartburn, or whose attacks are worse after eating or exercise. In such cases, treating the heartburn may also resolve asthma. People with asthma associated with GERD may be at risk for long-term erosion of the esophagus.