Epidemiology, Etiology, And Pathogenesis
The term croup encompasses a spectrum of disease involving the upper airway and sometimes (although rarely) extending into the lung parenchyma (laryngotracheitis, laryngotracheobronchitis, laryngotracheobronchopneumonitis). Acute laryngotracheitis is the most common presentation of these illnesses and is usually infectious in nature. In medical textbooks and review articles, differentiation is typically made between acute laryngotracheitis and recurrent croup that is associated with airway hyperreactivity (spasmodic croup).14-15 Both of these entities are usually caused by the same pathogens, and their clinical management is similar. Acute bacterial laryngotracheobronchitis is bacterial tracheitis, and it is commonly due to infection with Staphylococcus aureus or H influenzae.16
Males have a predominance over female patients on both visit and admission rates by a ratio of 3:2.17 Children aged 1 year old usually have the highest rate of visits to the ED and have the highest rate of admission to the hospital.17-18 Mortality quoted in a study from 1991 prior to the widespread use of corticosteroids was estimated to be less than 0.5%.19 More-recent extrapolations estimate the mortality rate now to be about 1 in 30,000 cases, which makes it an even more unlikely event.13,33
According to a large Canadian population-based study, viral croup accounts for 3% to 5% of total visits to EDs.17 This study demonstrates that although the number of cases presenting to the ED increased over the 6 years studied (from April 1999 to May 2005), there has been a steady decrease in the number of patients needing admission to hospitals and ICUs. The authors hypothesize that this is due to the introduction of evidence-based treatment, such as corticosteroids.
Croup is most often caused by a variety of viruses. Human parainfluenza viruses, especially type I and III, are the most common infectious agents, accounting for close to 80% of cases.20 In the Northern Hemisphere, croup follows a clear seasonal pattern, following the epidemics of human parainfluenza virus, and has biennial peaks in rates in November during odd years and February in even years.17-18 Although less frequent, croup may also occur with infection by influenza A and B, respiratory syncytial virus, adenovirus, coronavirus, human metapneumovirus, and Mycoplasma pneumoniae.21,22 Sporadic cases have also been identified with infectious agents such as enterovirus and mumps, and croup is a known complication of measles.23 Fungi and mycobacteria are extremely rare primary causes of laryngotracheobronchitis, and, when identified, should raise the suspicion of underlying immunodeficiency and prompt appropriate investigations. Immunosuppression may also occur with repeated treatments of corticosteroids or prophylaxis with antibiotics and may lead to opportunistic infections such as Candida or herpes simplex virus.24
The symptoms of croup are caused by infiltration of the subglottic region of the larynx by an infectious pathogen. This infiltration causes erythema, edema, and glandular hypersecretion of the subglottic mucosa. Because this narrowest part of the pediatric airway is bound by a complete ring of cartilage and cannot expand outward to accommodate for narrowing of the airway, obstruction may occur quickly with even the smallest amount of swelling. Poiseuille’s law states that the increase in airway resistance is inversely proportional to the fourth power of the radius. This phenomenon explains why even a minimal amount of airway edema may lead to an exponential rise in airway obstruction, especially in the young infant or toddler. In bacterial croup, pseudomembranes and fibrinous exudates may accumulate within the airway and cause further obstruction. The characteristic stridor of croup is caused by the passage of turbulent air through the narrowed airway at the level of the supraglottis, glottis, subglottis, or trachea and—depending on its timing with the respiratory cycle—may be inspiratory, expiratory, or biphasic.25