Incomplete obstruction to airflow within the upper respiratory tract leads to turbulence and the intense musical sound of stridor. It may be difficult to distinguish from a monophonic wheeze.
The larynx is particularly predisposed to obstruction because it is the only part of the respiratory tract that has an entire circle of non-compliant cartilage - the cricoid cartilage. In addition, the subglottis - the area just below the vocal cords - is also the narrowest part of the upper respiratory tract. Hence, pathology in the larynx, e.g. inflammation or a foreign body, may lead to airflow obstruction.
Stridor may be of three types according to the location of the airway obstruction:
- inspirational: most common; obstruction at, or above, the level of the cords
- biphasic: inspirational and expirational; obstruction in the subglottis or trachea
- expirational - obstruction in the small airways, e.g. asthma
With advancing obstruction, inspirational stridor may develop into the biphasic form.