Swallowing Function and Implications in Head and Neck Cancer

Introduction

The annual incidence of head and neck cancer (HNC) is estimated at 686,328 new cases and 376,000 cancer-related deaths worldwide. Developing in the mucosal lining of the oral cavity, oropharynx, nasopharynx, hypopharynx, and larynx, the histology of HNC is predominanly squamous cell carcinoma (90%) with human papilloma virus–related SCC being one of the main causes for the growing incidence of oropharyngeal HNC cancer. Patients with HNC are at a high risk for developing dysphagia due to the presence of the tumor itself or the cancer treatment. Up to two-thirds of patients present with dysphagia at the time of HNC diagnosis. Short-term or long-term swallowing complications following treatment can significantly impact nutritional intake and quality of life. With increasing cases of HPV-related SCC in younger individuals, coupled with improved survival of HPV-SCC over non-HPV SCC, the demands on treatment and long-term rehabilitation, including dysphagia services, are only expected to increase in the future. The purpose of this chapter is to describe swallow physiology and pathophysiology associated with HNC, the procedures for the assessment of dysphagia, and the effects of surgical and nonsurgical HNC treatment on swallowing function.

Oral Preparatory Phase

The oral preparatory or bolus preparation phase of swallow involves oral manipulation and/or preparation of saliva, fluids, and food. There are multiple muscle groups that work synergistically to facilitate the oral preparatory phase. The orbicularis oris is a circular muscle of the lips that serves to maintain oral competency during bolus preparation and swallowing. Tension activated in the buccinators prevents pooling of the bolus in the lateral and anterior sulci. To maintain oral control of fluids, the posterior tongue elevates to the soft palate to prevent the fluid bolus from entering the pharynx. Mastication by the teeth enables the solid bolus to soften and reduce in size, which is then mixed with saliva and formed into a cohesive bolus through cyclical movements of the jaw and tongue. The four extrinsic muscles of the tongue (genioglossus, palatoglossus, styloglossus, and hyoglossus) work with the intrinsic tongue muscles (superior longitudinal, inferior longitudinal, transverse, and vertical) to move and position the bolus in preparation for swallowing. The hyoid bone is interestingly active during the oral preparatory phase, because it has muscle connections to the suprahyoid and infrahyoid muscles, thereby influencing the movements of the jaw and tongue to some degree. Under voluntary control, the oral preparatory phase of swallowing may be halted or modified at any time.

Oral Phase

The oral phase involves timely and efficient transport of the bolus into the pharynx. The tongue elevates to the superior alveolar ridge then moves in an anterior to posterior direction. The soft palate elevates through contraction of the levator veli palatine and musculus uvulae, as the hyoglossus muscle of the tongue contracts and depresses, providing an opening for the bolus to be propelled into the pharynx ( Fig. 99.1 ). Intrabolus propulsion and pharynx driving forces propel the bolus into the pharynx within approximately 1 to 1.5 seconds. Elevation of the soft palate closes the velopharyngeal port, preventing nasal regurgitation during the oropharyngeal phase of swallow.

Pharyngeal Phase

The pharyngeal stage of the swallow is physiologically important because airway protection occurs during this phase in normal swallowing. The oral preparatory and oral phases of the swallow may be bypassed by reducing the consistency of food to liquid, by syringing food into the back of the mouth, or by positioning the head back so that gravity carries the food into the pharynx. The pharyngeal stage of the swallow cannot be bypassed. The pharyngeal phase involves bolus transport through the hypopharynx into the proximal esophagus. The pharyngeal phase begins as the pharyngeal swallow is triggered. This occurs once the leading edge of the bolus has passed any point between the anterior faucial arches and where the tongue base crosses the lower rim of the mandible. It is theorized that a sensory recognition center in the nucleus tractus solitarius of the medulla is responsible for decoding sensory information detected from the oropharynx and tongue, identifying the swallow stimulus, and relaying this information to the nucleus ambiguous, which initiates the pharyngeal swallow motor pattern. Research has demonstrated variability in the “normal” onset of the pharyngeal phase of swallowing associated with age and bolus-related variables, including consistency, size, and use of verbal cueing. The normal “aging” pharyngeal swallow has been found to be triggered later (i.e., below the lower rim of the mandible) without impacting the frequency of airway intrusion.

A number of physiological responses occur as a result of pharyngeal swallow triggering: (1) elevation and retraction of the velum and complete velopharyngeal closure to prevent nasal regurgitation and sustain interbolus pressures; (2) base-of-tongue retraction to contact the posterior pharyngeal wall (PPW); (3) progressive superior-inferior contraction of the pharyngeal constrictor muscles creating a driving force or pressure on the bolus; (4) elevation and anterior movement of the hyoid and larynx facilitating approximation of the hyoid with anteriorly tilting arytenoid cartilages, and deflection of epiglottis, combined with (5) closure of the larynx at the true vocal folds and false vocal folds prevents airway penetration and aspiration during the swallow; and finally, (6) relaxation of the upper esophageal sphincter (UES) to allow bolus passage from the pharynx into the esophagus. In normal swallowing function, the pharyngeal phase occurs within 1 to 1.5 seconds depending on bolus volume and viscosity.

The following sections discuss the features that facilitate a safe and efficient pharyngeal swallow.

Cricopharyngeal (Upper Esophageal Sphincter) Opening

The cricopharyngeus muscle acts in opposition to the function of the constrictor mechanism of the pharynx. At rest, the constrictors are relaxed, and the cricopharyngeus muscle or UES is in tonic contraction to prevent air intake into the esophagus concurrent with inhalation into the lungs. The contracted cricopharyngeus muscle also prevents backflow from the esophagus into the pharynx. During the swallow, as the constrictor mechanism is contracting, the cricopharyngeus muscle relaxes at the appropriate moment. The anterosuperior movement of the larynx opens the UES, and the bolus passes into the esophagus. The duration of UES opening and airway closure increases as bolus volume increases.

In summary, the pharyngeal stage of the swallow is responsible for transit of material into the esophagus and for airway protection. Fig. 99.2 illustrates the progression of the bolus through the pharynx. If an anatomic or neuromuscular disorder affects the pharyngeal stage of the swallow, poor bolus clearance and airway invasion incorporating laryngeal penetration or aspiration may result.

Signs and Symptoms of Abnormal Swallow Associated With Head and Neck Cancer