The overall goal of direct laryngoscopy is to visualize the glottic opening between the vocal cords for placement of an endotracheal tube. The ease with which this goal is accomplished is dependent on anatomic variations of the relevant anatomy, including the mouth, oropharynx, posterior pharynx, and larynx.
The size of the mouth opening, size and distribution of teeth, and size of the tongue play important roles in ease of laryngoscopy and intubation.
The oropharynx includes the structures of the upper airway from the soft palate to the level of the hyoid bone. The oropharynx opens anteriorly into the mouth, is bounded laterally by the two palantine arches and tonsils, and includes the posterior tongue. The laryngopharynx includes the epiglottis and larynx, which in turn comprise the vocal folds and vocal cords; the thyroid, cricoid, and arytenoid cartilages; and the intrinsic muscle of the larynx. The larynx begins where the upper airway divides to form the laryngeal inlet and the upper esophagus. The epiglottis is a cartilaginous flap attached to the posterior tongue, which forms a protective flap between the trachea and the upper esophagus. During swallowing, the hyoid bone is elevated, pulling the larynx upward and folding the epiglottis downward over the laryngeal inlet, diverting material into the upper esophagus.
Many major nerves contribute to pharyngeal and laryngeal function. Pharyngeal function is governed by cranial nerves IX, X (pharyngeal plexus), and XI (pharyngeal branch), which control elevation and shortening of the pharynx, alterations in palate position, the size of the pharyngeal lumen, and bolus transport of food in the esophagus. Motor innervation of the larynx is via the external and recurrent laryngeal branches of cranial nerve X. In addition, the facial nerve (7th cranial nerve) and cervical spinal nerves (C1-3) contribute to motor function of the upper airway via actions of the mylohyoid muscle on the hyoid bone to pull the larynx up and the infrahyoid muscles, which pull the larynx down. Mandibular action during phonation is also controlled by the mandibular branch of the trigeminal nerve. Sensory innervation of the larynx is via the internal laryngeal branch of the vagus nerve (cranial nerve X) above the vocal folds and the recurrent laryngeal branch of the vagus nerve below the vocal folds.
Important internal landmarks involved in laryngoscopy and intubation include the epiglottis, the vallecula, the intrinsic cartilages of the larynx (arytenoids, thyroid, cricoid), the true and false vocal cords, and the hyoid bone.
External Anatomic Features
The external anatomy of the larynx is also important in airway management. Operators should be able to identify the external landmarks of the mental protuberance of the mandible, thyroid cartilage, hyoid bone, and cricoid cartilage. Anatomic features of direct relevance to laryngoscopy include extent of mouth opening, extent of jaw subluxation, ability to flex and extend the neck, neck circumference, tongue size and protrusion, ability to visualize the uvula or soft palate with mouth opened and tongue protruded in the sitting posting without phonation (Mallampati score), mandibular size, mento-hyoid distance, thyromental distance, and protuberant dentition. The sensitivity and specificity of any of these measures to predict limited view on direct laryngoscopy is limited. Taken together, limitations of movement, low Mallampati score, and short anatomic distances may warrant alternative strategies to endotracheal intubation. Each of these aspects has been studied extensively in conjunction with radiologic investigations of airway anatomy in awake and unconscious patients. The reader is encouraged to consult the considerable literature on the subject for a more thorough discussion.