Physiological principles

Stridor is defined in Dorland’s Illustrated Medical Dictionary (28th edition) as ‘a harsh, high-pitched respiratory sound such as the inspiratory sound often heard in acute laryngeal obstruction’. Although this definition is strictly correct, it is not all that helpful and gives no information about how and why stridor comes about. Stridor is a harsh, high-pitched noise heard predominantly during inspiration. Consideration of the physiological principles underlying this fact gives some clue as to the site of the lesion causing the stridor. The presence of an added respiratory sound implies an obstruction to the free flow of gas through the airway tree. This obstruction is usually known as flow limitation. Flow limitation in a compliant tube, such as the airways, is accompanied by fluttering of the walls, which occurs to conserve energy when driving pressure exceeds the pressure required to produce the maximal flow. The fluttering of the walls produces a respiratory noise. When this phenomenon occurs during inspiration the resultant noise is known as stridor, and when it occurs during expiration the noise is known as wheeze.

During breathing, there are pressure gradients between the airway opening and the alveoli. Inspiration occurs when alveolar pressure is lowered below atmospheric pressure and air flows in to equalize the pressures. At the onset of expiration, alveolar pressure exceeds atmospheric pressure and air flows out. There are also pressure gradients across the airway wall and these tend to alter airway calibre. The pressure around the extrathoracic airways, that is, those above the thoracic inlet, is atmospheric, whereas the pressure around the intrathoracic airways essentially is equal to the pleural pressure. As illustrated in Figure 14.2.1, the pressure gradients across the airway wall during inspiration means that there is a net force tending to narrow the extrathoracic airways and to dilate the intrathoracic airways (Fig. 14.2.1A). During expiration, the direction of the forces is opposite, resulting in a tendency to narrow intrathoracic airways and dilate extrathoracic airways (Fig. 14.2.1B).

Fig. 14.2.1 The distribution of pressures throughout the respiratory system during (A) inspiration and (B) expiration. Atmospheric pressure is shown as zero. During inspiration, the expansion of the thorax results in pleural pressure falling below atmospheric. This relatively negative pressure is transmitted to the alveoli and a pressure gradient is established between the airway opening and the alveoli. Gas flows into the lungs along this pressure gradient. The pressure outside the airways is essentially pleural pressure, and results in net forces that tend to expand intrathoracic airways and to collapse the extrathoracic trachea. As shown in B, the pressure gradients are opposite during expiration.

As stridor is an inspiratory noise, the predominant site of obstruction (the site responsible for the flow limitation) is generally in the extrathoracic airways. Stridor with an expiratory component, that is, where the noise can also be heard at the beginning of expiration, can result either from a severe obstruction producing flow limitation during expiration as well, or from a lesion that extends into the intrathoracic airways.

Differential diagnosis

When considering the differential diagnosis, several factors need to be taken into consideration. These include:

Classification

Characteristics of the more important causes of stridor

Laryngomalacia

Laryngomalacia, which is sometimes known as infantile larynx, is the most common cause of persistent stridor. The supraglottic tissues appear as if they are too large for the size of the glottis and narrow the glottic aperture during inspiration instead of the more normal widening during inspiration. This can occur in a number of ways, the most common being:

• a long, curled (sometimes called omega-shaped) epiglottis collapsing during inspiration so that the lateral walls touch, restricting the free passage of air (Fig. 14.2.2A)

• floppy arytenoid processes prolapsing into the glottic aperture during inspiration (Fig. 14.2.2B)

• a long epiglottis collapsing against the posterior pharyngeal wall during inspiration.

Fig. 14.2.2 Flexible bronchoscopy images showing (A) omega-shaped epiglottis and (B) prolapse of the arytenoids into the glottic aperture.

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