Aetiology

Croup is a generic term that covers a heterogeneous group of diseases that affects the larynx, trachea, and bronchi (1):

• components of the croup syndrome include (1)
  • laryngotracheitis
  • laryngotracheobronchitis
  • laryngotracheobronchopneumonitis
  • spasmodic croup
• some define croup as two types
  • infectious croup:
    • commonly caused by virus
      • viral croup
    • rarely caused by bacterium
      • Mycoplasma pneumoniae is a rare cause of mild croup in children (1)
      • rarely caused by Staphylococcus aureus
  • spasmodic croup
    
    
• viral croup (1)
  • is an upper respiratory tract infection (acute laryngotracheobronchitis)
  • affects children 6 months to 12 years of age
  • has a peak incidence at two years of age
  • is the commonnest cause of airway obstruction in children 6 months to 6 years of age (1)
  • boys are affected more often than girls
  • is commonly seen in the fall and winter months
  • parainfluenza viruses (type1, 2, 3) account for the majority of cases
    • most common cause is Human parainfluenza virus 1 (HPIV-1)
    • rarely, other viruses are responsible, for example
      • influenza A and B viruses
      • adenovirus
      • measles
      • RSV (Respiratory syncytial virus)
  • the onset is slower and is usually preceded by a coryzal prodrome (with fever) (1)
    
    
• spasmodic croup (1)
  • associated with the viruses that cause viral croup
  • onset is sudden, without a viral prodrome
  • histology of the subglottic tissues shows noninflammatory oedema
  • tends to recur
  • may represent an allergic reaction to viral antigens
• often the distinction of the two types is impossible
  • hence sometimes spasmodic croup is considered part of the spectrum of viral croup(1)

Reference:

• (1) Doug Knutson. Viral Croup. Am Fam Physician 2004;69:535-42