Normal Upper Airway Anatomy And Physiology

The upper airway extends from the posterior margin of the nasal septum to the larynx. It is divided into four anatomical regions (Fig. 1):

■ Nasopharynx: between the nares and hard palate.

■ Retropalatal: between the hard palate and caudal margin of the soft palate.

■ Retroglossal: between the distal margin of the soft palate and the base of the epiglottis.

■ Hypopharynx: from the base of the tongue to the larynx.

The physiologic functions of the upper airway include ventilation, phonation, and deglutition. The evolutionary adaptations necessary to allow for such functional diversity have resulted in a structure that is heavily dependent on muscle activity and intrinsic airway collapsibility to maintain airway patency. The collapsibility of the upper airway represents a balance between opposing forces. The negative intraluminal pressure generated by the diaphragm during inspiration and the positive extraluminal pressure from surrounding tissue constitute the collapsing factors. The action of the upper airway pharyngeal dilator muscles counteracts the collapsing forces.

FIGURE 1 Mid-sagittal magnetic resonance imaging (MRI) in a normal subject highlighting the four upper airway regions: (A) nasopharynx; (B) retropalatal; (C) retroglossal; and (D) hypopharynx. Fat is white on an MRI scan.

Several surrounding tissue and craniofacial structures contribute to the morphology of the airway. A majority of patients with OSA experience narrowing or closure in the retropalatal and retroglossal regions. For this reason, the boundaries discussed below pertain specifically to these regions of the upper airway. The anterior oropharyngeal wall is formed mainly by the soft palate and tongue, while the posterior wall is bounded by the superior, middle, and inferior pharyngeal constrictor muscles. These lie anterior to the cervical spine. The lateral walls are complex structures that constitute muscles (hyoglossus, styloglossus, stylohyoid, stylopharyngeus, palatoglossus, palatopharyngeus, and the pharyngeal constrictors), lymphoid tissue, and adipose tissue (parapharyngeal fat pads). The lateral wall tissue is bounded by the parapharyngeal fat pads in the retropalatal region and by the mandibular rami in the retroglossal region (Fig. 2). The biomechanical relationships between the upper airway soft tissue and craniofacial structures are complicated and not fully understood. Nonetheless, upper airway imaging studies have begun to unravel these relationships.

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