bronchiolitis

Bronchiolitis is caused by viral infection (most commonly RSV, but also parainfluenza, human metapneumovirus, adenovirus, and rhinovirus) leading to inflammation, edema, and necrosis of the epithelial lining of the bronchioles, with subsequent mucus production and cellular debris causing small airway obstruction. This is particularly problematic in infants due to their naturally smaller airway diameter, making them more susceptible to obstruction and increased airway resistance (Poiseuille's law - resistance inversely proportional to radius^4). Peak incidence occurs in infants 2-6 months old, typically during fall/winter months. Clinical presentation includes rhinorrhea and mild fever progressing to tachypnea, wheezing, crackles, retractions, and nasal flaring over 2-3 days. Severe cases may show hypoxia and apnea (especially in premature infants). Diagnosis is primarily clinical; RSV antigen testing or PCR can confirm etiology but is not required for management. Chest X-ray may show hyperinflation, atelectasis, or peribronchial thickening but is not routinely needed. Management is supportive: nasal suctioning, oxygen supplementation if hypoxic, and IV fluids for dehydration. Bronchodilators, corticosteroids, and antibiotics are NOT recommended as first-line therapy per AAP guidelines since bronchiolitis is viral and small airway inflammation doesn't respond well to bronchodilators. Ribavirin may be used in severe immunocompromised cases. Palivizumab (RSV monoclonal antibody) is used for prophylaxis in high-risk infants (premature, chronic lung disease, congenital heart disease). Complications include respiratory failure, apnea in young infants, and secondary bacterial infection. There is an associated increased risk of subsequent asthma diagnosis in children who had severe RSV bronchiolitis, though causation vs. association remains debated.