Asthma
Asthma is a disease of airways that is characterized by increased responsiveness of the tracheobronchial tree to a multiplicity of stimuli. Asthma is manifested physiologically by a widespread narrowing of the air passages which may be relieved spontaneously or as a result of therapy and clinically by paroxysms of dyspnea, cough, and wheezing. It is an episodic disease, acute exacerbations being interspersed with symptom-free periods. Typically, most attacks are short-lived, lasting minutes to hours, and after them the patient seems to recover completely clinically. However, there can be a phase in which the patient experiences some degree of airway obstruction daily. This phase can be mild, with or without superimposed severe episodes, or much more serious, with severe obstruction persisting for days or weeks, a condition known as status asthmaticus.
The prevalence and incidence of asthma is difficult to assess with certainty because of the lack of reliable population-based figures which have used uniform diagnostic criteria. However, it has been suggested that approximately 5 percent of adults and 7 to 10 percent of children in the United States and Australia have the disorder. Bronchial asthma occurs at all ages but predominantly in early life. About one-half of the cases develop before age 10 and another third occur before age 40. In childhood, there is a 2:1 male/female preponderance which equalizes by age 30.
From an etiologic standpoint, asthma is a heterogeneous disease, and attempts to define it in etiologic or pathologic terms have proved difficult. It is useful for epidemiologic and clinical purposes to classify the forms of this disease by the principal stimuli that incite or are associated with acute episodes. However, it is important to emphasize that the distinction between various types of asthma may often be artificial, and the response of a given subclassification may be initiated by more than one type of stimulus. With this reservation in mind, one can describe two broad groups: allergic and idiosyncratic.
Allergic asthma is often associated with a personal and/or family history of allergic diseases such as rhinitis, urticaria, and eczema; positive wheal-and-flare skin reactions to intradermal injection of extracts of airborne antigens; increased levels of IgE in the serum; and/or positive response to provocation tests involving the inhalation of specific antigen.
A significant segment of the asthmatic population will present with negative family or personal histories of allergy, negative skin tests, and normal serum levels of IgE, and therefore cannot be classified on the basis of defined immunologic mechanisms. These we term idiosyncratic. Many of these will develop a typical symptom complex upon contracting an upper respiratory illness. The initial insult may be little more than a common cold, but after several days the patient begins to develop paroxysms of wheezing and dyspnea that can last for days to months. These individuals should not be confused with the so-called infective asthmatics or with persons in whom the symptoms of bronchospasm are superimposed upon chronic bronchitis.
Unfortunately, many patients will not clearly fit into either of the above categories but will fall into a mixed group with features of each. In general, those patients whose onset of disease is in early life will tend to have a strong allergic component to their illness, while those who develop their asthma late tend to be nonallergic or to have mixed etiologies.