http://emedicine.medscape.com/article/353436-overview

Peter G Canaday, MD ; Jannette Collins, MD, MEd, FCCP

Asthma is a common disorder that primarily involves the airways. Traditionally, asthma has been considered a disorder of airway smooth muscle mediators and anatomic elements of the airway mucosa. Although the causes of asthma are separated into allergic and nonallergic ones, considerable crossover is observed in the features of both types of asthma, and treatment varies little between them. While asthma has been considered a disorder of the airways, a number of conditions have a presentation similar to that of bronchial asthma. These conditions originate, often silently, in organ systems other than the lungs, and they either provoke airway responses equivalent to those found in asthma or mimic the clinical findings of asthma.