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Swallowing occurs under three distinct conditions: volitional, subconscious, and reflexive pharyngeal. Volitional is mostly for nutritional purposes and is so effortless that it is often taken for granted. Subconscious occurs at a rate of one per minute on average and is considered airway housekeeping and reflux clearing. Reflexive pharyngeal is an airway protective mechanism against aspiration of food inadvertently falling into the pharynx during chewing or materials refluxed into the pharynx from the stomach.

Table 3.2 Causes of oropharyngeal dysphagia.

Source: Modified from Cook [21]; and Feldman, Friedman, Brandt [22].

Structural	Posterior pharyngeal diverticulum (Zenker’s) Lateral pharyngeal diverticulum Cricopharyngeal bar Cervical or esophageal web Oropharyngeal tumors – benign or malignant Thyroid enlargement or tumor Esophageal tumors – benign or malignant Post‐surgical Cervical stenosis Vertebral spur or osteophytes Esophageal stricture Inflammatory disease (pharyngitis, tonsillar abscess) Foreign body Cervical lymphadenopathy Vascular anomalies Diffuse idiopathic skeletal hyperostosis
Central nervous system	Stroke Parkinson’s disease Huntington’s chorea Wilson’s disease Brainstem tumor Amyotrophic lateral sclerosis Multiple sclerosis Cerebral palsy Spinal cord injury Tabes dorsalis Spinocerebellar degeneration Syringobulbia Progressive bulbar paralysis Alzheimer’s disease Other congenital or degenerative disorders or motor neuron diseases
Peripheral nervous system	Spinal muscular atrophy Guillain‐Barré syndrome Poliomyelitis, post‐polio syndrome Diabetes Recurrent laryngeal nerve palsy (mediastinal tumor, post‐surgery) Transection or injury Diphtheria Rabies Lead poisoning Other neurotoxins
Myogenic	Myasthenia gravis, botulism Inflammatory myopathies Polymyositis/dermatomyositis Scleroderma
	Mixed connective tissue disease Inclusion body myositis Muscular dystrophies (oculopharyngeal muscular dystrophy, myotonia dystrophica) Sarcoidosis Hyperthyroidism Myxedema Stiff‐man syndrome Cricopharyngeal dysfunction Paraneoplastic syndromes Mitochondriopathies
Miscellaneous	Chronic obstructive pulmonary disease Xerostomia Botulinum toxin Procainamide Cytotoxins, phenothiazines Benzodiazepines Amiodarone Alcohol Cholesterol‐lowering drugs

From a functional perspective, there are four phases of swallowing: preparatory, oral, pharyngeal, and esophageal. Preparatory and oral are volitional and require cognition. However, the pharyngeal phase is essentially reflexive. Preparatory, oral, and pharyngeal are all responsible for bolus transit while protecting the airway. The esophageal phase is involved in bolus transit and is discussed in Chapter 5.

During the preparatory phase, the bolus essentially remains in the oral cavity, is altered physically by being subjected to mastication, and altered chemically by mixing with saliva, all resulting in a bolus with suitable characteristics for safe transit through the aerodigestive tract [23]. It is during this phase that the bolus is sized, shaped, and positioned on the dorsum of the tongue for initiation of the upcoming oral phase of swallowing [24].

During the oral phase, a sequential contraction of the tongue against the hard and soft palates, a peristaltic pressure wave, is generated that propels the bolus from the oral cavity into the pharynx [25–27]. It is in the pharyngeal phase that the pharynx, UES, and larynx [26] are all elevated, and three of the four routes for exit from the pharynx (namely the nasal cavity, oral cavity, and larynx) become sealed off, while the fourth route, the UES, opens. Contraction of the superior pharyngeal constrictor and elevation of the soft palate and its contact with the posterior pharyngeal wall (velopharyngeal closure) close off the nasopharynx. The oral cavity is closed by elevation of the tongue base and its contact with the hard and soft palates [25]. The bolus is then transported into the esophagus by rapid, forceful posterior tongue movements that persist from the oral phase, as well as the peristaltic contraction of the pharyngeal constrictors against the soft palate, base of the tongue, and larynx.

It is during the oropharyngeal swallow that important biomechanical events involving the intrinsic glottis, as well as supra‐ and infra‐hyoid muscles, take place that result in the closure of the airway [28–30], as well as in the opening of the UES [31–33]. These events include (i) adduction of the true vocal cords and arytenoids (first tier of airway closure), followed by vertical approximation of the adducted arytenoids to the base of the epiglottis (second tier of closure) (Figure 3.1); (ii) descent of the epiglottis covering the closed glottis, thereby closing the laryngeal vestibule (third tier of closure); and (iii) the entire larynx being pulled upward and forward by the contraction of the suprahyoid muscle group at the time of vocal cord closure or shortly thereafter. This displacement results in the positioning of the closed larynx under the tongue base, away from the path of the bolus, thereby providing additional protection against aspiration [35, 36] (Figure 3.2).

During oropharyngeal swallowing, the UES transiently relaxes and is subsequently pulled upward/forward by the contraction of the same suprahyoid muscles that displace the larynx. This traction results in active opening of the UES, which is also modified by the bolus size [38–40]. The temporal relationship of the events that take place during the oropharyngeal phase of swallowing is shown in Figure 3.3. Under normal conditions, oropharyngeal swallowing begins with the closure of the vocal cords, marking the initiation of airway protection [30], and ends when the cords return to their resting positions. During this time, respiration is reflexively inhibited, and the protective mechanisms of swallowing are fully activated. Finally, the esophageal phase of swallowing transports the bolus further into the esophagus and stomach.

As stated earlier, the elaborate mechanism of oropharyngeal swallowing ensures two important functions: (i) transit of the bolus and (ii) protection of the airway. Normal oropharyngeal swallowing is therefore defined as complete transit of the ingested material from the mouth into the esophagus, without compromising the airway. OPD may result when the efficacy and/or coordination of either the transport or protective aspect of oropharyngeal swallowing are compromised.

Figure 3.1 Still frames of deglutitive vocal cord closure seen by (A) transnasal videoendoscopy in a normal volunteer and (B) transtracheal videoendoscopy in a patient with tracheostomy. (A) A, Glottis immediately before initiation of swallow. Vocal cords are open at their resting position. B, Complete deglutitive vocal cord and arytenoid adduction. C, Adducted arytenoids have approximated the base of the epiglottis. D–F, Obscured view because of pharyngeal contraction and laryngeal elevation. G, Vocal cords can be seen still adducted following the descent of the larynx and opening of the pharynx after passage of the bolus. H, Vocal cords are beginning to open at the completion of swallow. (B) A, Inferior view of the glottis at rest. The introitus to the trachea is wide open immediately before the initiation of swallow. B and C, Vocal cords are in the process of adduction narrowing the introitus. D, Cords are in contact with each other in the anterior part. However, the posterior gap is still open. E, Posterior gap is now closed, resulting in complete closure of the introitus to the trachea. F, Posterior gap is partially reopened while the anterior parts of the cords are still in contact. G and H, Cords are further opened, returning to resting position. Note that contrary to the transnasal view, in the transtracheal view, the introitus to the trachea remained visible during the entire period of swallowing.

Source: Reproduced from Shaker et al. [34], with permission.