Eating and swallowing are complex neuromuscular activities, involving over 30 muscles to perform. They have two crucial biological features: food passage from the oral cavity into stomach and protecting material from entering the airway. Normal swallowing can be divided into four stages, the oral preparatory stage, the oral stage, pharyngeal stage and esophageal stage. The four stages are dynamic and overlapping.
Oral Preparatory Phase
The oral preparatory stage begins when food is introduced into the oral cavity. Food is chewed into smaller pieces and mixed with saliva to form a bolus of material. Mastication and mixing the food with saliva require the muscles of mastication including the masseter, temporalis, medial and lateral ptergoids and facial muscles including the obicularis oris and buccinator muscles.
After liquid is taken into the mouth from a cup or by a straw, the liquid bolus is held in the anterior part of the floor of the mouth or on the tongue surface against the hard palate surrounded by the upper dental arch (upper teeth). The oral cavity is sealed posteriorly by the soft palate and tongue contact to prevent the liquid bolus leaking into the oropharynx before the swallow. There can be leakage of liquid into the pharynx if the seal is imperfect, and this leakage increases with aging.
Oral Transit Phase
The oral transit phases is a voluntary phase that begins with the posterior propulsion of the bolus by the tongue and ends with initiation of the pharyngeal swallow.
Pharyngeal swallow is a rapid sequential activity, occurring within a second. The pharyngeal phase begins with the initiation of a voluntary pharyngeal swallow which in turn propels the bolus through the pharynx via involuntary peristaltic contraction of the pharyngeal constrictors. The pharyngeal phase has two crucial biological features:
- food passage, propelling the food bolus through the pharynx and UES to the esophagus; and
- airway protection, insulating the larynx and trachea from the pharynx during food passage to prevent the food from entering the airway. Breathing momentarily stops and the vocal folds come together.
The esophagus is a tubular structure from the lower part of the UES to the lower esophageal sphincter (LES). The lower esophageal sphincter is also tensioned at rest to prevent regurgitation from the stomach. It relaxes during a swallow and allows the bolus passage to the stomach.
Breathing and Swallowing Coordination
Eating, swallowing and breathing are tightly coordinated. Swallowing is dominant to respiration in normal individuals. Breathing ceases briefly during swallowing, not only because of the physical closure of the airway and neural suppression of respiration in the brainstem. Swallowing usually starts during the expiratory phase of breathing. There is a respiratory pause during swallowing, and respiration usually resumes with expiration. This resumption is regarded as one of the mechanisms that prevents inhalation of food remaining in the pharynx after swallowing.
Dysphagia (difficulty swallowing) can result from a wide variety of functional or structural deficits at any stage of swallowing. There can be oral, pharyngeal, esophageal dysphagia or a combination. Dysphagia can result in aspiration, which is when material such as food, liquid, or saliva passes below the vocal folds into the trachea. Consequences of dysphagia can include: pneumonia, weight loss, malnutrition, dehydration, electrolyte imbalance, psychosocial affects, alternative nutrition, hospitalization, choking, death.
It is important to understand normal swallowing in order to determine if a patient has dysphagia and how to best treat it. Patients with tracheostomy and mechanical ventilation are at high risk for dysphagia and aspiration due to many factors. See Swallowing Management for Patients with Tracheostomy and Mechanical Ventilation for more information.