Larynx

Outline of voice production

The primary function of the larynx is to act as a sphincter to prevent the entry of foreign material into the tracheobronchial tree. To achieve this, the vocal folds close tightly (as do the false vocal folds (vestibular folds)) during swallowing. In addition, the larynx facilitates expulsion of secretions from the trachea and lungs during a cough: the act of coughing requires the vocal folds to abduct rapidly during exhalation, causing an explosive release of air and expulsion of material. A further function of the larynx is to increase the intrathoracic and/or intra-abdominal pressure (the so-called Valsalva manœuvre); the vocal folds tightly adduct during attempted expiration, as in straining to lift weights or to increase intra-abdominal pressure during defecation or parturition.

The final function of the larynx is phonation. With the vocal folds adducted, increasing subglottic air pressure overcomes the muscular force of adduction and pushes the vocal folds apart; a fraction of a second later, the vocal folds then come together again. This is achieved partly because of the elastic recoil of the vocal folds and partly because of the Bernoulli effect (the flow of air through the rima glottidis causes a negative pressure, drawing the vocal folds together). This cyclical movement of the vocal folds causes movement of air, which is perceived as sound ( Fig. 17.1 and Video 17.1 ). The sound generated by the vocal folds is then modulated by the changing shape of the rest of the vocal tract (pharynx, oral cavity and so on) to produce intelligible voice.

In order for the epithelium of the vocal folds to vibrate in a cyclical way, the vocal folds must be pliable. The layered microstructure of the vocal folds means that the epithelium is separated from the vocal ligament by a loose collagenous layer known as ‘Reinke's space’. The superficial layer of the lamina propria forms Reinke's space, while the intermediate and deep layers of lamina propria constitute the vocal ligament. The smooth, regular cyclical vibration of the vocal epithelium over the ligament, with the two separated by Reinke's space, is known as the ‘mucosal wave’ and can be seen with stroboscopic examination techniques ( Fig. 17.2 ).

Surgical approaches and considerations

Historically, the larynx would have been examined with a mirror held in the oropharynx. This technique was devised by Manuel Garcia in the mid-19th century. In modern practice, examination of the vocal folds is usually achieved with a flexible endoscope (either a fibreoptic endoscope or a distal chip endoscope) introduced through the nose and passed into the pharynx. This gives the view seen in Fig. 17.3 . Examination of epithelial pliability relies on seeing the vocal fold epithelium moving in slow motion. In clinical practice, this is achieved using stroboscopic techniques.

In broad terms, voice disorders fall into the categories listed in Table 17.1 . Normal phonation relies on vocal folds with straight edges, adequate glottic closure and pliability of the vocal folds. Laryngeal surgery may aim to achieve any or all of these goals. There are several different approaches to laryngeal surgery.