Clinical meaning
Croup (acute laryngotracheobronchitis) is a viral infection causing inflammation and edema of the subglottic airway, primarily affecting children 6 months to 3 years (peak incidence 1-2 years). Parainfluenza virus types 1 and 3 account for ~75% of cases; other causes include RSV, influenza, adenovirus, and human metapneumovirus. The subglottic trachea is the narrowest part of the pediatric airway (cricoid cartilage is a complete ring that cannot expand), making children uniquely susceptible to symptomatic obstruction from even mild edema. One millimeter of circumferential edema reduces the cross-sectional area of the infant subglottic airway by ~60% (Poiseuille law: airflow resistance is inversely proportional to radius to the fourth power). This produces the characteristic barking/seal-like cough, inspiratory stridor, and hoarseness.
Diagnosis & workup
Diagnostics & workup: - Clinical diagnosis based on characteristic presentation: barking cough, hoarseness, inspiratory stridor, low-grade fever; history of URI prodrome (1-3 days of rhinorrhea, cough) followed by sudden onset of stridor (often worse at night) - Westley Croup Score: assesses severity (stridor, retractions, air entry, cyanosis, level of consciousness); mild ≤2, moderate 3-7, severe ≥8 - AP neck X-ray (if diagnosis uncertain): 'steeple sign' -- subglottic narrowing of the tracheal air column (not routinely needed in classic presentation) - Differentiate from epiglottitis (EMERGENCY): epiglottitis has rapid onset, high fever, drooling, tripod positioning, muffled 'hot potato' voice, NO barking cough; lateral neck X-ray shows 'thumbprint sign' (swollen epiglottis) - Differentiate from foreign body aspiration: sudden onset without prodrome, unilateral wheezing, choking history - Differentiate from bacterial tracheitis: high fever, toxic appearance, poor response to croup treatment, purulent secretions