Swallowing, formally known as deglutition, is an intricate biological function that transitions a substance from the mouth to the stomach. This complex, often unconscious process requires the precise coordination of over 30 muscle pairs and several cranial nerves. The system performs the dual roles of permitting digestion while simultaneously protecting the airway from foreign material. Failure of this system can lead to serious health complications like aspiration pneumonia.
The Anatomy of Swallowing
The mechanics of moving a food or liquid bolus rely on three primary groups of muscles working in sequence.
Muscles attached to the hyoid bone, a unique floating bone in the neck, are responsible for vertical and forward movement of the entire laryngeal structure. The suprahyoid muscles, including the digastric and mylohyoid, contract to pull the hyoid bone and the attached larynx upward and forward. This elevation is necessary for airway protection and for opening the top of the esophagus.
The throat is governed by the pharyngeal constrictor muscles, which are arranged in three overlapping layers: superior, middle, and inferior. These muscles contract sequentially from top to bottom in a wave-like motion (peristalsis) once the bolus enters the pharynx, squeezing the food downward toward the entrance of the esophagus.
Movement of the bolus within the mouth is handled by the intrinsic and extrinsic muscles of the tongue, such as the genioglossus and hyoglossus. These muscles manipulate food, form it into a cohesive mass (bolus), and then propel it backward toward the pharynx. The epiglottis, a flap of cartilage, folds over the laryngeal opening, diverting the descending bolus into the esophagus rather than the trachea.
The Coordinated Stages of Swallowing
The entire swallowing process is divided into three distinct phases, beginning with the voluntary oral stage. This phase involves the muscles of mastication to chew food, and the tongue muscles to mix it with saliva and form a bolus. The oral phase concludes when the tongue propels the prepared bolus to the back of the mouth, initiating the reflexive part of the swallow.
Once the swallow reflex is triggered, the pharyngeal phase begins, lasting approximately one second and requiring precise coordination. The soft palate elevates to seal off the nasal cavity, preventing food from entering the nose. Simultaneously, the suprahyoid muscles pull the larynx up and forward, causing the epiglottis to cover the trachea. The vocal cords adduct, or close tightly, providing a secondary layer of airway defense.
The pharyngeal constrictor muscles then contract rapidly in a wave, generating pressure to propel the bolus toward the upper esophageal sphincter.
The final part is the esophageal phase, which is entirely involuntary and moves the bolus toward the stomach. This phase is characterized by a slower, rhythmic wave of muscle contraction, or peristalsis, in the esophageal walls. The upper and lower esophageal sphincters must relax in sequence to allow the bolus to pass into the esophagus and then into the stomach, ensuring that the material moves in one direction.
Common Causes of Swallowing Difficulty
Difficulty swallowing, known as dysphagia, often results from a disruption in the precise muscular coordination described above.
One major cause is neurological damage, which interferes with the nerve signals that control the swallowing muscles. Conditions like a stroke can impair swallowing in at least 50% of patients. Progressive diseases such as Parkinson’s disease and Amyotrophic Lateral Sclerosis (ALS) disrupt the signaling pathways; up to 30% of ALS patients may present with swallowing impairment at diagnosis.
A second set of causes involves structural or muscular damage in the head and neck region. Age-related weakening of the swallowing muscles, termed presbyphagia, increases the risk of difficulty, with prevalence in independently living older adults reaching 33% in those over 80. Damage from radiation therapy for head and neck cancers can cause stiffness and scarring, reducing the mobility and strength of the muscles involved in laryngeal elevation and pharyngeal constriction. Other muscular disorders, such as Myasthenia Gravis, affect the connection between nerves and muscles, leading to generalized weakness that impairs the ability to swallow effectively.
Strengthening and Rehabilitation Techniques
Intervention for swallowing difficulties is typically managed by a Speech-Language Pathologist (SLP) who focuses on improving muscle function and coordination. Rehabilitation often involves specific exercises designed to target weakened muscle groups.
One common technique is the Effortful Swallow, where the patient is instructed to swallow “long and strong,” squeezing all throat muscles as hard as possible to increase pharyngeal pressure and bolus clearance.
The Mendelsohn Maneuver is another technique that aims to improve the coordination and duration of laryngeal elevation. Patients learn to intentionally hold the larynx at its peak height during the swallow, which prolongs the opening of the upper esophageal sphincter.
Another exercise, such as the Shaker exercise or its modification, the Chin Tuck Against Resistance (CTAR), is designed to strengthen the suprahyoid muscles. Strengthening these muscles directly improves the height and forward movement of the larynx, which is important for airway protection.
In addition to muscle training, rehabilitation includes compensatory strategies to make eating safer immediately. These strategies involve modifying the consistency of foods and liquids, such as thickening liquids, to make them easier to control and reduce the risk of material entering the airway. The choice of exercise and strategy is individualized based on the specific physiological impairment identified by the SLP.