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Overview - Asthma
Asthma
The Gale Encyclopedia of Science, 2014
Asthma, which is derived from the Greek word aazein, meaning sharp breath, is a lung disease that
affects approximately 25 million people in the United States. In the U.S. in 2010, approximately 7 million
people 18 years of age or younger and 17.8 million adults had asthma. In urban areas, youth asthma
rates can be upwards of 40 percent.
In people with asthma, the airways of the lungs are hypersensitive to irritants such as cigarette smoke
or allergens (compounds that trigger a reaction by the immune system). When these irritants are
inhaled, the airways react by constricting, or narrowing. Some people with asthma have only mild,
intermittent symptoms that can be controlled without drugs. In others, the symptoms are chronic,
severe, and sometimes life threatening.
Asthma is sometimes referred to as a disease of "twitchy lungs," which means that the airways are
extremely sensitive to irritants. The airways are the tubes (bronchi) that bring air from the windpipe
(trachea) to the lungs. An individual bronchus, in turn, branches into smaller tubes called bronchioles.
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At the end of the bronchioles are small, balloon-like structures called alveoli. The alveoli are tiny sacs
that allow oxygen to diffuse into the blood and carbon dioxide to diffuse from body tissues into the
lungs to be exhaled.
During an asthma attack, the bronchi and bronchioles constrict and obstruct the passage of air into the
alveoli. Besides constricting, the airways may secrete a great deal of mucus in an effort to clear the
irritation from the lungs. The airway walls also swell, causing inflornination and further obstruction. As
the airways become increasingly obstructed, oxygen cannot reach the small airsacs; blood oxygen
levels drop; and the body's tissues and organs become oxygen-deprived. At the same time, carbon
dioxide cannot escape the small airsacs for exhalation; blood levels of carbon dioxide increase and
exert a toxic effect on the tissues and organs of the body.
Underlying the bronchial inflammation is an immune response in which white blood cells known as type
2 helper T (Th2) cells are prominent. Th2 cells secrete chemicals known as interleukins that promote
allergic inflammation and stimulate another set of cells known as B cells to produce IgE and other
antibodies. In contrast, type 1 helper T (Th1) cells produce specific types of interferon and interleukin,
which initiate the killing of viruses and other intracellular organisms by activating macrophages and
cytotoxic I cells. These two subgroups of helper T cells arise in response to different immunogenic
stimuli and cytokines, and they constitute an immunoregulatory loop: cytokines from Th1 cells inhibit
Th2 cells, and vice versa. An imbalance in this reciprocal arrangement may be the key to asthma; when
freed from the restraining influence of interferon, the Th2 cells can provoke airway inflammation.
One of the hallmarks of asthma is that the airway obstruction is reversible. This reversibility of the
airway swelling is used to definitively diagnose asthma. If the swelling and inflammation can be brought
under control with asthma drugs, the person has asthma and not some other upper respiratory tract
disease.
In addition to cigarette smoke and various allergens, other triggers can cause asthma attacks. A cold
or other upper respiratory infection may bring on an asthma attack. Strong emotions, such as
excitement, tension, or anxiety, may trigger asthma symptoms. Exercise can cause symptoms of
asthma. Weather conditions, such as extreme cold, heat, or humidity can cause an asthma attack.
Pollution and increasing ozone levels are also associated with episodes of asthma. Other
environmental factors include occupational exposure to certain substances like animal dander, wood
particles, dusts, various industrial chemicals, and metal salts.
The characteristic sign of asthma is wheezing, the noisy, whistling breathing that a person makes as he
or she tries to push air in and out of narrowed airways. Other symptoms of asthma include a tight
chest, shortness of breath, and a cough.
Currently, several drugs are used to treat asthma. Some patients with mild asthma only need to use
medication intermittently to control wheezing, while patients with more serious asthma need to take
medication at regular intervals to avoid life-threatening attacks. It is important for asthma patients to
see their doctors if the frequency or severity of their symptoms change. It has been suggested that
many of the life-threatening asthma attacks are in people who once had mild asthma—with symptoms
that could be treated as they occurred—which then progressed to a more severe case of the disease.
Bronchodilators open up (dilate) constricted lung airways by relaxing the muscles that line the
bronchial tubes. Oral bronchodilators include theophylline. A related compound, called aminophylline,
is given via injection for severe episodes of asthma. During severe, rapidly-appearing asthma attacks,
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injections of epinephrine are given just under the patient's skin. Epinephrine has a quick, but shortlasting, effect of bronchodilatation.
Most asthma patients are given bronchodilators such as albuterol that are used in a mist form that is
inhaled from either a special inhaler device or an aerosol machine. Some patients are instructed to use
their bronchodilator at regular intervals, while others may just be told to use the inhaler if they notice
the beginning of an asthma attack. The inhaled medications are quick-acting because they are directly
applied to the constricted airways.
Salmeterol combines the direct effects of inhaled bronchodilators with the long-lasting protection
afforded by oral bonchodilators and is popular for the control of mild and severe asthma.
Asthma also responds favorably to anti-inflammatory drugs, which reduce the swelling and
inflammation of the airways. These drugs can be inhaled or taken in pill form. Two types of antiinflammatory drugs are prescribed for asthma patients: chromolyn sodium and corticosteroids.
Chromolyn sodium is also prescribed for people with allergies, and it has few side effects. Oral
corticosteroids are very effective in treating asthma, but should be reserved for severe cases due to
their serious side effects. Short-term side effects include increased appetite, weight gain, hypertension,
and fluid retention. Over the long-term, corticosteroids may cause osteoporosis, cataracts, and
impaired immune response. These side effects usually preclude the use of corticosteroids for long
periods of time. In fact, short-courses of steroids are preferred. These "steroid bursts" are given as a
treatment for a sudden severe asthma attack, perhaps brought on by exposure to an allergen or a viral
infection, over about a week's time and then discontinued.
Inhaled corticosteroids have few side effects. These medications are also prescribed for allergy
patients. Unlike their oral counterparts, these drugs can be taken for much longer periods of time. They
are especially useful in controlling moderate asthma.
Another class of asthma medications are called leukotriene receptor antagonists. These drugs,
including an oral medication called zafirlukast, interfere with the actions of a class of chemicals called
leukotrienes. Leukotrienes help produce the symptoms of asthma. Interference with their actions
decreases asthma symptomatology. The receptor antagonists greatly reduce asthma severity when
taken daily and work especially well in conjunction with inhaled steroids and salmeterol inhalers. This
regimen (zafirlukast + inhaled steroids + inhaled salmeterol) seems to improve daily living for many
asthmatics. Other inhaled bronchodilators are then reserved for exacerbations, such as may occur
during a viral infection.
Bronchiole
The smallest diameter air tubes, branching off of the bronchi, and ending in the alveoli (air sacs).
Bronchodilator
A drug, either inhaled or taken orally, that dilates the lung airways by relaxing the chest muscles.
Bronchus (plural, bronchi)
One of the two main airway tubes that branch off from the windpipe and lead to each lung.
Wheezing
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The characteristic sound of asthma; it results when a person tries to push air in and out of narrowed
airways.
A key to lessening episodes of asthma is eliminating the irritant. If asthma is brought on by cigarette
smoke, the patient must avoid this irritant. If asthma is brought on by exercise, the person should try to
find a level of exertion that is comfortable. Using an inhaled bronchodilator before exercising may also
control asthma symptoms.
Further Readings
Books
Bellenir, Karen. Asthma Sourcebook: Basic Consumer Health Information about the Causes,
Symptoms, Diagnosis, and Treatment of Asthma in Infants, Children, Teenagers, and Adults. Holmes,
PA: Omnigraphics, 2006.
Web sites
Centers for Disease Control and Prevention (CDC). "Asthma." http://www.cdc.gov/asthma/ (accessed
October 1,2013).
National Heart, Lung, and Blood Institute. "What Is Asthma?." http://www.nhIbi.nih.gov/health/healthtopios/topics/asthrria/ (accessed October 1,2013).
Full Text: COPYRIGHT 2014 Gale, Cengage Learning.
Source Citation
Scogna, Kathleen. "Asthma." The Gale Encyclopedia of Science, edited by K. Lee
Lerner and Brenda Wilmoth Lerner, 5th ed., Gale, 2014. Science in Context,
link.galegroup.com/apps/doc/CV2644030187/SCIC?
u=birm45752&xid=6113c8d3. Accessed 8 Feb. 2017.
Gale Document Number: GALEICV2644030187
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