The larynx being exceedingly sensitive, is somewhat readily brought into a state of spasm. Thus the mere introduction of a brush with a stimulating solution into the larynx usually produces a spasm which may be sufficient almost to close the glottis for some seconds. A foreign body which has lodged in the larynx usually produces violent spasm, and the obstruction in these cases may be much more the result of spasm than of the foreign body. Spasm is also produced in some cases by inflammations of thy larynx.

Irritation of the nerves of the larynx or of the trachea sometimes leads to spasm of the glottis. Aneurysms by causing irritation of the recurrent laryngeal nerve may produce this effect, but they may do so by pressure directly on the trachea, the effect in this latter case being reflex.

A more independent spasm of the glottis is of somewhat frequent occurrence in children, and is known by the name of Laryngismus stridulus or False croup. There may be no appreciable disease of the larynx, or at most a catarrh, but a sudden spasm occurs with great distress and urgent dyspnoea. The obstruction may be even fatal. The dyspnoea is accompanied by the crowing respiratory sound and hoarse cough which have given rise to the name Croup.