The hallmark of croup is the inflammation and edema of the subglottic region of the larynx, trachea, and bronchi, leading to partial airway obstruction. The subglottic region, being narrow in children, becomes significantly constricted as the mucosa swells, which increases resistance to airflow. Because laminar resistance varies with the inverse fourth power of the radius (Poiseuille's law), 1 mm of circumferential edema in a 4 mm infant subglottis removes about 75% of the cross-sectional area and raises resistance roughly 16-fold, whereas the same 1 mm in an 8 mm adult airway removes about 44% of the area and raises resistance about 3-fold. As the airway narrows, turbulent airflow occurs, producing the characteristic stridor heard in patients with croup, especially during inspiration: the subglottis and trachea lie outside the thorax, so the negative intraluminal pressure generated during inspiration pulls the narrowed extrathoracic airway further inward, while expiration tends to splint it open. As obstruction worsens the stridor becomes biphasic, and stridor that grows quieter in a tiring child signals impending respiratory failure rather than improvement. The inflammation also irritates the vocal cords, resulting in hoarseness, and the increased effort required to move air past the narrowed airway leads to the distinctive "seal-like" barking cough.
The gradual onset of symptoms typically begins after one to two days of upper respiratory tract infection (URI) symptoms, such as runny nose, nasal congestion, and mild cough. As the infection progresses, the child may develop the more classic signs of croup, including stridor, worsening cough, and hoarseness. The symptoms often intensify at night; proposed explanations include the overnight nadir in endogenous cortisol, cooler and drier night air, and reduced clearance of secretions when the child lies supine, all of which favor more pronounced airway inflammation and swelling. In severe cases, the child may exhibit signs of increased respiratory effort, such as nasal flaring, chest retractions (when the skin between the ribs pulls inward during breathing), and cyanosis (bluish discoloration of the skin, indicating low oxygen levels).
