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National Heart, Lung, and Blood Institute. National Institutes of Health. Childhood Asthma Management Program: Asthma Feelings. AVA19988VNB1, 1996. The Asthma Feeling video explores the way asthmatic children feel about their disease and ways to cope with being ‘different’. The video is designed to help children feel more comfortable with having asthma and not allow it to cause undue interference in the lifestyle of the person. Producer: National Institutes of Health. Creative Commons license: Public Domain

Asthma is a chronic medical condition. It has been defined by the National Heart, Lung and Blood Institute as a common chronic disorder of the airways that is complex and characterized by variable and recurring symptoms, airflow obstruction, bronchial hyperresponsiveness (bronchospasm), and an underlying inflammation. The interaction of these features of asthma determines the clinical manifestations and severity of asthma and the response to treatment.

Public attention in the developed world has recently focused on asthma because of its rapidly increasing prevalence, affecting up to one in four urban children.

Asthma is caused by a complex interaction of environmental and genetic factors that researchers do not yet fully understand.[55] These factors can also influence how severe a persons asthma is and how well they respond to medication. As with other complex diseases, many environmental and genetic factors have been suggested as causes of asthma, but not all studies posing such claims have been verified by further studies. In addition, as researchers detangle the complex causes of asthma, it is becoming more evident that certain environmental and genetic factors may affect asthma only when combined.

Asthma is an airway disease that can be classified physiologically as a variable and partially reversible obstruction to air flow, and pathologically with overdeveloped mucus glands, airway thickening due to scarring and inflammation, and bronchoconstriction, the narrowing of the airways in the lungs due to the tightening of surrounding smooth muscle. Bronchial inflammation also causes narrowing due to edema and swelling caused by an immune response to allergens.

During an asthma episode, inflamed airways react to environmental triggers such as smoke, dust, or pollen. The airways narrow and produce excess mucus, making it difficult to breathe. In essence, asthma is the result of an immune response in the bronchial airways.

The airways of asthmatics are “hypersensitive” to certain triggers, also known as stimuli (see below). (It is usually classified as type I hypersensitivity.) In response to exposure to these triggers, the bronchi (large airways) contract into spasm (an “asthma attack”). Inflammation soon follows, leading to a further narrowing of the airways and excessive mucus production, which leads to coughing and other breathing difficulties. Bronchospasm may resolve spontaneously in 12 hours, or in about 50% of subjects, may become part of a ‘late’ response, where this initial insult is followed 312 hours later with further bronchoconstriction and inflammation.

The normal caliber of the bronchus is maintained by a balanced functioning of these systems, which both operate reflexively. The parasympathetic reflex loop consists of afferent nerve endings which originate under the inner lining of the bronchus. Whenever these afferent nerve endings are stimulated (for example, by dust, cold air or fumes) impulses travel to the brain-stem vagal center, then down the vagal efferent pathway to again reach the bronchial small airways. Acetylcholine is released from the efferent nerve endings. This acetylcholine results in the excessive formation of inositol 1,4,5-trisphosphate (IP3) in bronchial smooth muscle cells which leads to muscle shortening and this initiates bronchoconstriction.

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