Aetiology

The illness is due to viral infection (typically parainfluenza virus types 1 or 3).[3] Several other viral pathogens have been recognised, including influenza A and B, adenovirus, respiratory syncytial virus, metapneumovirus, coronavirus HCoV-NL63, and rarely measles.[1][4][5][6][7][8] Distinctions have been made between viral croup and spasmodic croup. However, it remains unclear as to whether these entities represent different diseases or are merely a spectrum of the same disease. Clinically, it is difficult to distinguish between the two, and is likely to be unnecessary as treatment decisions are based upon history and clinical severity of the airway obstruction. Historically, laryngeal diphtheria was well known as a cause of croup, but this is now rare in immunised populations. Reports of diphtheric croup have been published in case series from India and Russia.[9][10][11][12] A weak link between a history of previous intubation and croup has been indicated.[13]

Pathophysiology

The symptoms result from upper-airway obstruction due to generalised inflammation and oedema of the airways. At the cellular level this progresses to necrosis and shedding of the epithelium. The narrowed subglottic region is responsible for the symptoms of seal-like barky cough, stridor (from increased airflow turbulence), and sternal/intercostal indrawing. If the upper-airway obstruction worsens, respiratory failure can result, leading to asynchronous chest and abdominal wall motion, fatigue, hypoxia, and hypercapnia.[14][15][16]

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