FIGURE 2-5 Relationship between a family history of allergic disease and the time of onset of allergic disease in children. Family history is an extremely important determinant of the risk of allergic disease in children.

Source: Smith, 1988.

Asthma is a lung disease characterized by (1) airway obstruction that is reversible (but not completely so in some patients) either spontaneously or with treatment, (2) airway inflammation, and (3) increased airway responsiveness to a variety of stimuli (NHLBI, 1991). Lung inflammation is often present even when symptoms are absent. People with asthma often experience intermittent wheezing, coughing, chest tightness, and shortness of breath.

The diagnosis of asthma for an individual patient is usually made during a physician office visit, emergency room visit, or hospitalization. The diagnosis is based on one or more of the following: history of episodic symptoms, signs of asthma on physical examination (chiefly wheezing), or pulmonary function test results demonstrating reversible airflow obstruction and bronchial hyperreactivity (see Chapter 5). Bronchial hyperreactivity may be quantified by determining the concentration of methacholine or histamine required to induce a transient drop in lung function. In asthmatics, the concentration of methacholine required to induce a drop in lung function is usually lower than in nonasthmatics.