The upper aerodigestive tract is a single conduit that must integrate and separate two very different vital functional systems: breathing and swallowing. In the healthy state, the anatomic structures and their neural substrates exquisitely coordinate each function through sensory-motor integration. The ability to swallow safely is critical for nutrition, hydration, and quality of life; however, its significance is often taken for granted when the physiology and pathophysiology are not fully appreciated. Understanding the anatomy and physiology of deglutition is not only required to properly evaluate and treat patients with dysphagia but also necessary to maximize functional outcomes after surgical interventions for head and neck cancer.
Dysphagia is not a disease, but rather a symptom or a collection of symptoms that broadly describe difficulty swallowing. Under the umbrella term dysphagia, finer distinctions are made that refer to the phase of swallowing where impairments exist. For example, patients may have oral, pharyngeal, oropharyngeal, pharyngoesophageal, or esophageal dysphagia. Dysphagia can also vary in terms of severity from mild to severe. However, it is unknown how much aspiration in a particular patient will result in pneumonia. Some patients can only tolerate a minimal amount of aspiration, while others tolerate larger amounts. The end point of dysphagia is pneumonia or airway obstruction. Unfortunately, to date, there is no universally accepted, standardized system that can be used to objectively define severity levels. In addition, the evaluation and treatment of dysphagia are often complicated by the fact that some patients are acutely aware of difficulty swallowing, while others may not perceive its presence or recognize the symptoms.
The swallowing difficulty can develop gradually from neurologic or respiratory disease, or be acquired suddenly as a result of injury or surgery. The multitude of different etiologies for dysphagia can often be delineated with a thorough history and physical examination. Instrumental assessments of swallowing function provide additional information that will further characterize swallowing function so that treatment can be planned. Effective therapeutic interventions are based upon each patient’s specific physiology. Swallowing exercises can increase muscle strength and range of motion of swallowing structures. Skill training can instruct patients to use swallowing maneuvers such as breath-holding before drinking. Compensatory strategies such as changing head position can also be used to alter swallowing biomechanics. Diet alterations can be instituted so that unsafe food consistencies are eliminated. Surgical interventions, such as vocal fold augmentation or esophageal dilatation, may be required as the primary treatment, or in addition to swallowing therapy.
The ability to eat and drink safely and efficiently is fundamental to our quality of life. The wide variety of food and liquid that we enjoy throughout each day requires precise management because of the shared function of the upper aerodigestive tract. Indeed, modern research has shown that the pharyngeal swallow is not a stereotypic reflex as it was first described; rather, it is a patterned motor response to sensory input. As such, pharyngeal muscle force and contraction duration must rapidly and consistently adjust depending upon what is to be swallowed (1,2). To accomplish this feat, separate and overlapping phases of swallowing use sensory-motor integration. Swallowing dysfunction can occur in each phase individually or collectively across them.
The anticipatory phase is often considered the first true step in swallowing because visual information, olfactory stimulation, and experience interact to form the initial motor plan (3). A common statement in the culinary profession is, “we eat with our eyes first.” The adult has a well-developed motor plan for swallowing different types of food and drink that is based upon appearance (visual processing), consistency (tactile processing), taste (chemical processing), and bolus size (proprioceptive processing). Most have experienced a sudden awareness that an item placed in the mouth did not match what was expected. This observation can be viewed as practical evidence that a plan had been violated. Another example that illustrates this concept can be seen when we perceive a liquid to be very hot or a food is suspected of being very spicy. When experience indicates these possibilities, a slower presentation to the mouth will follow and care will be taken to ensure a smaller bolus size because of the anticipated negative effects.
It is also at the anticipatory level where distinctions between food and nonfood items are made and where palatability is determined. Oral intake may be avoided as a result of the information perceived and processed during this phase. If an individual is forced to eat or drink an item that he or she has predetermined (anticipated) is unsafe or undesirable, then gagging, coughing, and dysphagic symptoms may be observed. It is important to note that these are not behaviors that can be controlled; rather, they are in response to sensory stimulation. The anticipatory phase provides the initial sensory input to cortical swallowing structures. In patients with dementia or other cognitive impairments, the sensory processing of visual, olfactory, and experiential information can be altered. As such, the motor plan implemented to swallow may not match that of the bolus itself, putting the patient at risk for dysphagia.
TABLE 56.1 SUMMARY CHART OF PERIPHERAL SENSORY AND MOTOR COMPONENTS OF DEGLUTITION
Once solid and semisolid food is placed in the mouth, it must first be prepared for the pharyngeal phase. The combination of mastication and saliva production is considered to be the first step in digestion. Saliva from the submental, sublingual, and parotid glands flows into the oral cavity to mix with the bolus and begin the chemical process of food breakdown. The mandible closes to crush solid food and the rotatory motion of mastication enables the molars to shear and shred the food. The muscles of mastication are the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles (Table 56.1). The masseter, temporalis, and medial pterygoid muscles elevate the mandible, while the lateral pterygoid depresses the mandible, thereby opening the mouth. Contraction of the masseter and medial pterygoid muscles also results in lateral mandible movements required for the rotary motions. Sensory and motor innervation to the muscles of mastication and salivary glands is provided by several branches of the trigeminal nerve. The cyclic motion of the mandible and motions of the tongue are mediated, in part, by a cortical central pattern generator (CPG). A CPG is essentially neural circuitry that generates rhythmic movement. It is important to remember that during this phase, the airway is open and active breathing continues. During mastication, respiratory rate becomes more rapid and irregular when compared to tidal breathing (4). If the bolus is not well controlled, solid food may fall into the open airway resulting in aspiration or asphyxiation. Also during mastication, the circular fibers of the orbicularis oris muscle work to actively close the lips and maintain the food or liquid within the oral cavity. An incompetent oral sphincter will result in drooling of saliva or loss of food and liquid out of the mouth. This circumstance is very embarrassing for patients and severely impacts the social aspects of meals. Neurologic diseases and surgical resections that prevent labial closure or impair sensation can cause anterior oral loss of food or liquid.
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