Viral croup.

Definition and Terminology The term “croup syndrome” refers to a group of diseases that varies in anatomic involvement and etiologic agents and includes laryngotracheitis, spasmodic croup, bacterial tracheitis, laryngotracheobronchitis, and laryngotracheobronchopneumonitis. Although the terms “laryngotracheitis” and “laryngotracheobronchitis” frequently are used interchangeably in the literature, they represent two different disease states. The most common and most typical form of the viral croup syndrome is acute laryngotracheitis, which involves obstruction of the upper airway in the area of the larynx, infraglottic tissues, and trachea and is due to an infectious agent. The lung parenchyma is involved occasionally. Among the noninfectious etiologies of this syndrome are foreign body aspiration, trauma (eg, due to intubation), and allergic reaction (eg, acute angioneurotic edema). Acute viral infection is the most common cause of croup, but bacterial and atypical agents also have been identified. It generally is accepted that acute laryngotracheitis and spasmodic croup are caused by viral agents alone, whereas both bacterial and viral agents may be responsible for causing disease further down the respiratory tract, such as laryngotracheobronchitis and laryngotracheobronchopneumonitis. Bacterial tracheitis, also known as membranous or bacterial croup, involves infection with bacteria such as Staphylococcus aureus, Haemophilus influenzae, and Corynebacterium diphtheriae and will not be discussed further in this review.