Croup is an illness, usually caused by a viral infection, characterized by inflammation of the upper airway that causes stridor and barking cough.

The airway structures that are inflamed vary from case to case (patients may have laryngotracheobronchitis, laryngotracheitis, or laryngotracheobronchopneumonitis).

The most common virus causing croup is parainfluenza virus type 1. Other viruses known to cause croup include parainfluenza virus types 2 and 3, respiratory syncytial virus, human metapneumovirus, rhinovirus, coronavirus, and adenovirus, among others. Influenza virus is a rare cause of croup. Severe laryngotracheobronchitis can also be seen with measles.

Most cases occur in children between 3 months and 3 years old. Patients with anatomic abnormalities leading to a narrowed upper airway are at increased risk of croup.

Croup occurs throughout the year. The illness typically begins with rhinorrhea, cough, and occasionally fever. Within hours to 2 days, the patient develops a dry, barking cough, hoarseness, and stridor. Some patients will have expiratory wheezing on auscultation. Airway obstruction and respiratory distress requiring intubation are seen in a minority of cases.

The Westley Croup Score is frequently used to classify croup severity as mild (no stridor and minimal chest wall retractions at rest), moderate (stridor and chest wall retractions at rest), or severe (agitated or fatigued child with stridor and sternal contractions at rest).

In most patients, symptoms improve or resolve after about 4 days.