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Asthma is a clinical syndrome that affects 25 million Americans and accounts for 12.7 million medical visits yearly. One-third of those afflicted with asthma are children under the age of 18 years. It is estimated that roughly half of these children received their diagnosis prior to the age of 6 years. As a result, the origins of asthma are believed to have a clear genomic component that is often manifested in early childhood. The clinical course of this illness is influenced greatly by exposures, including respiratory viruses, indoor allergens, maternal tobacco smoke, and other physical and social aspects of the environment. Thus, this clinical disease has important consequences in childhood and may have important consequences for adult obstructive lung disease.

Asthma is an extremely common clinical problem and the most common cause of hospitalization for children in the United States. The estimated total annual costs of asthma care is rising dramatically and totaled approximately $56 billion in 2007 in the United States,1 representing a $3 billion increase since 2002. These costs include $50.1 billion per year in medical expenses, $3.8 billion per year in missed school or lost work days, and $2.1 billion per year in premature deaths. The paradox of this illness is that despite important strides in understanding etiologic environmental factors and mechanisms of airway inflammation characteristic of the syndrome, its prevalence and morbidity remain unacceptably high. Although asthma morbidity and mortality rates have been steady over the last few years, the rates are dramatically higher than 25 years ago and continue to be very significant, particularly for urban minority groups, low-income populations, and children.

The purpose of this chapter is to describe trends in asthma epidemiology, specifically prevalence, hospitalization, and mortality. In so doing, we examine potential reasons for these trends, and the recent research on the interactions of genes and environment. We also examine the relationship of the intermediate phenotypes of airway hyperresponsiveness and allergy to the asthma syndrome and consider a variety of risk factors for asthma occurrence. We conclude with a review of asthma natural history and the implications of the current trends.

Definitions and Prevalence

In 2007, the National Asthma Education and Prevention Program Expert Panel Report 3 (NAEPPR3)2 defined asthma as

a chronic inflammatory disorder of the airways in which many cells and cellular elements play a role: in particular, mast cells, eosinophils, neutrophils (especially in sudden onset, fatal exacerbations, occupational asthma, and patients who smoke), T lymphocytes, macrophages, and epithelial cells. In susceptible individuals, this inflammation causes recurrent episodes of coughing (particularly at night or early in the morning), wheezing, breathlessness, and chest tightness. These episodes are usually associated with widespread but variable airflow obstruction that is often reversible either spontaneously or with treatment.

Because asthma is a clinical syndrome, there is no gold standard for its diagnosis. As such, physicians ...

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