lung and airway disorders
Asthma
Causes of Asthma
The causes of asthma are unknown, but asthma likely results from complex interactions between many genes, environmental conditions, and nutrition. Environmental conditions and circumstances around pregnancy, birth, and infancy have been associated with the development of asthma in childhood and later in adulthood. Risk appears to be higher if a person's mother became pregnant at a young age or had poor nutrition during pregnancy. Risk may also be higher if someone is born prematurely, had a low weight at birth, or was not breastfed. Environmental conditions such as exposures to household allergens (such as dust mites, cockroaches, and pet dander) and other environmental allergens have also been associated with the development of asthma in older children and adults. Diets low in vitamins C and E and in omega–3 fatty acids have been also linked to asthma, as has obesity. There is no evidence that dietary supplements of these substances prevent the development of asthma; however, it has been shown that weight loss can reduce the risk and severity of asthma. Thus, obesity is an important modifiable risk factor for asthma. Having smaller families with fewer children, cleaner indoor environments, and use of vaccinations and antibiotics in early life may decrease the body's ability to develop resistance to allergens in the environment and may partly explain the increase in asthma in places where these conditions exist (the hygiene hypothesis). Narrowing of the airways is often caused by abnormal sensitivity of cholinergic receptors, which cause the muscles of the airways to contract when they should not. Certain cells in the airways, particularly Mast Cells , are thought to be responsible for initiating the response. Mast cells throughout the bronchi release substances such as histamine and leukotrienes, which cause the following: Smooth muscle to contract Mucus secretion to increase Certain white blood cells to move to the area White Blood Cells , a type of white blood cell found in the airways of people with asthma, release additional substances, contributing to airway narrowing. In anasthma attack(sometimes called a flare-up or an exacerbation), the smooth muscles of the bronchi contract, causing the bronchi to narrow (called bronchoconstriction). The tissues lining the airways swell due to inflammation and mucus secretion into the airways. The top layer of the airway lining can become damaged and shed cells, further narrowing the airway. A narrower airway requires the person to exert more effort to breathe. In asthma, the narrowing is reversible, meaning that with appropriate treatment or on their own, the muscular contractions of the airways stop, inflammation resolves so that the airways widen again, and airflow into and out of the lungs returns to normal. During an asthma attack, the smooth muscle layer goes into spasm, narrowing the airway. The middle layer swells because of inflammation, and excessive mucus is produced. In some segments of the airway, mucus forms plugs that nearly or completely block the airway. In people who have asthma, the airways narrow in response to stimuli (triggers) that usually do not affect the airways in people without asthma. Such triggers include Allergens Infections Irritants Exercise (called exercise-induced asthma) Stress and anxiety Aspirin Many inhaled allergens, including pollens, particles from dust mites, body secretions from cockroaches, particles from feathers, and animal dander, can trigger an asthma attack. These Overview of Allergic Reactions (IgE, a type of antibody) on the surface of mast cells to trigger the release of asthma-causing chemicals. (This type of asthma is called allergic asthma.) Although food allergies induce asthma only rarely, certain foods (such as shellfish and peanuts) can induce severe attacks in people who are sensitive to these foods. Infectious triggers are usually viral respiratory infections, such as Common Cold , ; fumes (such as from perfumes, cleaning products, or air pollution); cold air; and stomach acid in the airways caused by Gastroesophageal Reflux Disease (GERD) (GERD). Air pollution has been linked to asthma attacks. Some people who have asthma can develop airway narrowing when exercising. This type of airway narrowing may be due to breathing drier, colder air through the mouth while exercising. Stress and anxiety can trigger mast cells to release histamine and leukotrienes and stimulate the vagus nerve (which connects to the airway smooth muscle), which then contracts and narrows the bronchi. Crying or hearty laughing may trigger symptoms in some people. Aspirinand nonsteroidal anti-inflammatory drugs (NSAIDs) are triggers for almost 30% of people with severe asthma, but they are triggers in less than 10% of people with asthma overall. Eosinophilic asthma is a rare, severe subtype of asthma in which very high levels of eosinophils are present in the blood. The higher the level of eosinophils the more severe the person's symptoms. Reactive airways dysfunction syndrome (RADS) is a rapid onset and persistent asthma-like disorder that occurs in people with no history of asthma. It is a form of Overview of Environmental and Occupational Lung Disease caused by a single large exposure to nitrogen oxide or volatile organic compounds (such as those in certain bleaches and cleaning products). People have symptoms similar to those of asthma, including cough, wheezing, and shortness of breath. Treatment is similar to usual 