Bronchiolitis describes a clinical syndrome in children younger than 2 years of age that initially manifests with upper respiratory symptoms followed by lower respiratory tract infection and inflammation. Viral infections such as respiratory syncytial virus (RSV), rhinovirus, human metapneumovirus, parainfluenza, and influenza are often the precipitating infection, although bacterial infections may also precipitate this syndrome.

Viruses infect the terminal bronchiolar epithelial cells causing edema, excessive mucus, and obstructed small airways. Signs and symptoms begin with rhinorrhea, cough, and fever and can progress to respiratory distress. Physical exam findings often include accessory muscle use, nasal flaring, chest retractions, wheezing, prolonged expiratory phase, and coarse or fine crackles.

Risk factors for severe disease include prematurity, age <12 weeks, and underlying chronic illness such as chronic pulmonary disease or congenital heart disease. Severity of typical illness peaks at 3-5 days of symptoms, with cough typically resolving by days 8-15. Complications include dehydration, apnea, respiratory failure, and secondary bacterial infections.