Asthma is a heterogeneous disease characterized by chronic inflammatory disease of the airways and notable for hyper-responsivity to multiple exacerbating stimuli. While a chronic condition, asthma is generally an intermittent disease with acute exacerbations interspersed with symptom-free periods. In severe cases, some patients may experience some symptoms of airway obstruction daily.

An estimated 26 million people in the United States have asthma (19 million adults and 7 million children), and the condition is a leading cause of absence from work and school.

Adults with asthma often experience accelerated loss of lung function compared to adults without asthma, but the clinical significance of this and the extent to which these declines contribute to the development of fixed airflow obstruction are unknown.

The condition has genetic as well as environmental determinants. Roughly half of cases are due to genetic susceptibility and half are related to environmental factors.

Immunohistopathologic features of asthma include inflammatory cell infiltration with neutrophils, eosinophils, lymphocytes, mast cell activation, and epithelial injury.

Asthma occurs at all ages but predominantly in early life. Half of cases develop in the first decade of life, and another third present in the first 4 decades of life. Asthma is generally divided into 2 types: allergic asthma (caused by exposure to an allergen) and nonallergic asthma.

The single largest risk factor for the development of asthma is atopy, and allergic asthma is associated with a familial or personal history of allergic diseases. These patients may have positive skin test reactions to intradermal antigen testing and/or elevated levels of immunoglobulin E (IgE). Nonallergic asthma patients do not have histories of allergy and have negative skin testing and normal IgE levels.  

Exacerbations are manifested by widespread narrowing of airways that may resolve spontaneously or with therapy. Narrowed airways result in the experience of dyspnea, cough, and wheezing. Triggers for such exacerbations include allergens, exercise, cold air, viral or bacterial infection, etc. These episodes last for minutes to hours, can be self-resolving, or may be severe enough to cause respiratory failure and death in the absence of emergency and intensive care.

The clinical examination is most notable for widespread, high-pitched wheezing generally noted on expiration. Wheeze may not be present if the patient is asymptomatic and in between exacerbations. It may also be absent in the setting of severe disease and limited air movement.

Other physical findings of severe obstruction include nasal flaring, subcostal retractions, use of accessory muscles such as the sternocleidomastoids, sitting in a tripod position, and a prolonged expiratory phase. Hypertension is frequently increased in patients with asthma.