The role of radiology in the evaluation of stridor.

Stridor is a medium pitched respiratory noise caused by partial obstruction of the large airways at the level of the pharynx, larynx, and/or trachea (usually the extrathoracic trachea). Stridor is characteristically heard on inspiration, but can be expiratory or biphasic if the obstructive lesion is more distal. Understanding its causes and being aware of the indications and limitations of the various imaging modalities available for its assessment are vital if the condition is to be investigated appropriately. Children and neonates are particularly susceptible to stridor because their immature airways are much more compliant than those of an adult and, therefore, are more prone to collapse. Furthermore, the diameter of a child’s airway is so small that only a slight reduction in diameter causes a considerable reduction in cross sectional area. For example, in a typical 1–2 year old child with a tracheal diameter of 6.5 mm, a circumferential increase in wall thickness of only 1 mm will reduce the cross sectional area by more than half. Stridor can be divided into an acute or subacute/chronic type of presentation. Acute stridor develops over hours and is a critical condition requiring immediate assessment and treatment. In cases of this type, time consuming and complex imaging investigations are unnecessary because the causes of this condition are limited, 1 and can usually be diagnosed on clinical grounds alone. However, imaging can play a valuable role in accurately determining the underlying causes of subacute or chronic stridor that develop over days or weeks. Table 1 shows the diVerent types of stridor presenting at diVerent ages. The various imaging modalities available for the assessment of stridor will be described and their merits discussed.