Wheezing is a continuous musical sound produced by airflow through a narrowed or compressed airway. Wheezing can originate from both small and large airways. Ten to 15% of infants wheeze during the first year of life, and as many as one fourth of children younger than 5 years of age present to a physician with wheezing. Although asthma is the most common cause of recurrent wheezing, the differential diagnosis of wheezing in children includes foreign body aspiration, infections, vascular rings and slings, tracheobronchomalacia, tracheoesophageal fistula (TEF), mediastinal masses, gastroesophageal reflux (GER), dysfunctional swallow, immunodeficiency syndromes, cystic fibrosis (CF), and vocal cord dysfunction (VCD).

Conceptually, it is helpful to distinguish between acute-onset wheezing versus chronic or recurrent wheezing. Common causes of acute-onset wheezing in children are foreign body aspiration and infections. Foreign body aspiration occurs more in children younger than 3 years of age but less in infants. Classically, patients present with the history of a choking episode, but acute-onset wheezing alone warrants suspicion for foreign body aspiration. Exam may reveal unilateral wheezing or unequal breath sounds. Chest radiograph (CXR) may show unilateral hyperinflation or atelectasis or may be normal. Patients are usually unresponsive to bronchodilators. Definitive diagnosis is made by bronchoscopy. Common infectious agents that cause wheezing in children include respiratory syncytial virus, parainfluenza virus, metapneumovirus, rhinovirus, and Mycoplasma pneumoniae. Patients typically present with other signs of infection such as nasal congestion, rhinorrhea, cough, and fever. CXR findings and response to bronchodilator are variable.

Etiologies for recurrent wheezing can be categorized as anatomic or structural versus functional. Vascular rings and slings, tracheobronchomalacia, TEF, and mediastinal masses are some anatomic or structural causes of recurrent wheezing in children. Vascular rings and slings (i.e., double aortic arch, right-sided aortic arch, pulmonary artery sling) can compress large airways, such as the trachea and mainstem bronchi, thereby leading to stridor or wheezing. Patients generally present early in life, and CXR is almost always abnormal. Bronchodilator may actually exacerbate wheezing
due to associated tracheobronchomalacia. Further workup involves barium esophagram, echocardiogram, and cardiac magnetic resonance imaging. In addition to vascular rings and slings, cardiac lesions that lead to pulmonary artery dilation, left atrial enlargement, left ventricular failure, or pulmonary venous outflow obstruction can lead to wheezing. Tracheobronchomalacia also presents early in life with stridor or wheezing but generally becomes more pronounced at 2 to 3 months of age. Stridor and wheezing worsen with infections and activity. CXR is often nondiagnostic, and bronchodilator may exacerbate wheezing due to the increasing airway obstruction caused by smooth muscle relaxation. Definitive diagnosis is made by bronchoscopy.