- Email: [email protected]
General Principles of Swallowing
SECTION I
ANATOMY AND PHYSIOLOGY OF SWALLOWING
CHAPTER 1
General Principles of Swallowing DINESH K. CHHETRI, MD
INTRODUCTION Normal swallowing plays a critical human function: safe ingestion of food and liquid is vital to our survival and comfort. Human physiology requires regular and frequent eating and drinking for nutrition, and it is imperative that swallowing is not a nuisance or a burden. The goal of swallowing is to safely and efficiently propel food and liquid from the mouth to the stomach. In addition to serving the nutritional function, normal swallowing is necessary to clear saliva and secretions from the oral cavity and pharynx. Adult humans produce over 2 L of saliva daily and spontaneously swallow on average one and a half times a minute (over 2000 times per day). When an individual has difficulty managing saliva due to dysphagia, drooling (sialorrhea) or aspiration may occur. Normal swallowing is also critical for maintaining a satisfying human quality of life (QOL). We humans are a highly social species who find it pleasurable and desirable to eat, drink, and socialize with others. We design special rooms in our homes for cooking and eating. Outside our homes, we gather to barbecue, picnic, potluck, and party and even hunt for food. Family gatherings, celebrations, and rites of passages in all stages of human life nearly always include a moment of eating together in celebration of shared emotions. Religious gatherings often involve a component where food and liquid are consumed or shared. However, when a person has difficulty swallowing or is unable to swallow safely, the possibility of participating in many of these social interactions diminishes. This is psychologically and functionally devastating, bringing depression and anxiety, and significantly diminishing the human QOL. Although a person can certainly be fed and kept alive using a gastric tube, this is clearly neither considered pleasurable nor psychologically meaningful as
eating by mouth. Nonoral feeding is typically considered artificial rather than “human.” In fact, it is not unusual in otolaryngology to encounter patients who would trade their voice for the opportunity to swallow normally again. Therefore, it is imperative that we understand the etiologies of dysphagia and how to manage them, not only to prevent morbidity and mortality but also to improve and maintain satisfying QOL. Dysphagia practice is a relatively new clinical focus in otolaryngology. Currently, otolaryngologists often refer patients with dysphagia to other specialties such as gastroenterologists or speech and language pathologists for evaluation and management. I suggest that the otolaryngologist is the logical physician to manage patients’ oropharyngeal dysphagia, given the field’s extensive knowledge of head and neck anatomy. It is a reasonable extension for otolaryngologists to learn normal and abnormal swallow physiology, as well as how to assess and treat dysphagia. Successful assessment and management of dysphagia is a multidisciplinary endeavor. The otolaryngologist who is well versed in oropharyngeal dysphagia can make the appropriate referrals to gather the necessary data to understand how to help patients with swallow issues. Managing dysphagia is highly satisfying, once the clinician becomes familiar with contemporary assessment and management techniques and understands which patients may be helped. Thus, the purpose of this book is to make dysphagia evaluation and management accessible to all otolaryngologists.
BASIC CONCEPTS OF SWALLOWING The human developmental anatomy that provides voice and speech also creates one major challenge to swallowing: humans must swallow without food or liquids
1
2
SECTION I
Anatomy and Physiology of Swallowing
accidentally spilling into the airways. At birth, the human larynx is at the level of the oropharynx and the epiglottis tip is positioned at the superior oropharynx at the level of the soft palate. This allows the neonate to breathe through the nose and at the same time swallow liquids via mouth and through the lateral oropharyngeal channels. The descent of the laryngopharynx occurring as early as 6 months after birth with postnatal growth suddenly brings the laryngeal inlet directly into the path of the food bolus. While postnatal descent of the larynx allows for the glottic larynx to modulate the pulmonary airflow to produce voice that is further manipulated by the oral and pharyngeal structures to produce speech sounds, the crossing of the airway and the food way places human species at risk for aspiration and death when food falls below the level of the vocal folds into the tracheobronchial airway. A complex laryngeal protective mechanism therefore must be activated during each swallow. When this protective mechanism fails, aspiration and its complications may ensue. The major adverse consequence of aspiration is pneumonia which may result in death. However, more commonly, chronic aspiration leads to debilitation, malnutrition, chronic lung infection, and morbidity and mortality of chronic lung disease. What is dysphagia? Often it is simply described as subjective difficulty with swallowing. It is also the inability to swallow safely, or reduced intake such that the person has difficulty maintaining adequate nutrition and/or hydration. Dysphagia can also be an objective finding of dysfunction in the phases of swallowing. A complex swallowing mechanism controls sequential passage of food bolus from the mouth to the stomach. Functionally and anatomically, the swallowing apparatus can be separated into three sections: oral, pharyngeal, and esophageal. Each section can be conceptualized as a conduit with a pump and valves. Valves at either end of the section open and close to propel food bolus toward the stomach and prevent improper backward or sideway leakage. Pumps squeeze sequentially and generate the propulsive force that facilitates an anterograde movement of the food bolus toward the stomach. In the oral phase, the lips anteriorly and the oropharyngeal faucial arch and base of tongue posteriorly form valves that hold liquid in the oral cavity tube prior to swallow onset (the posterior oral valve may remain open during solid food mastication to allow aroma to reach the nasopharynx). The tongue pumps the food and liquids into the oropharynx at the onset of swallowing while the lips stay sealed and the posterior oral cavity valve opens. In the pharyngeal phase, the tongue base and velum
valves close and the upper esophageal sphincter (UES) valve opens while the pharyngeal muscle pump pushes the food toward the upper esophagus. The laryngeal protective valve is also activated during this phase. The pharyngeal phase is completed in 1 s, a testament to the highly coordinated neuromuscular control required to swallow without aspirating! Once food is in the esophagus, the UES closes and the esophageal pump propels the food bolus toward the lower esophageal sphincter, which opens to allow food into the stomach. Details of the swallowing events are described in detail in the following chapters. The valves, pumps, and pipes paradigm obviously simplifies the swallowing mechanism which is complex anatomically and functionally in the three dimensions. In addition, the pump and the pipe are often the same and changes shape with swallowing. The central and peripheral neural control is complex. However, this concept illustrates normal swallow function and identifies areas of dysfunctions in evaluating and managing dysphagia. The next chapters of this book will present the developmental anatomy, functional anatomy, and physiology of the swallowing apparatus in great detail. Then the swallow assessment tools will be covered in detail. Lastly, the most common swallow dysfunctions encountered by the otolaryngologist and management of these disorders will be covered in detail.
ETIOLOGIES OF DYSPHAGIA Conceptualization of the normal swallow anatomy and physiology as a biomechanical event lends itself to determining the etiologies and pathophysiology of dysphagia. There should be no defects in the walls of the pipe, the valves should close adequately and open sequentially, and the pumps should be intact in their muscular components and innervation. In regard to neural control, while motor nerves are critical to maintaining the compliance of the swallow compartments and pump function, afferent sensory system should be intact as well to act as a feedback system for controlled flow of food and liquids. One can witness the devastating consequence of neuromuscular dysfunction in patients with multiple cranial neuropathies. The glossopharyngeal (CN IX), vagus (CN X), and hypoglossal (CN XII) nerves are the critical cranial nerves for safe oropharyngeal swallowing. The trigeminal (CN V) and facial (CN VII) nerves play a crucial function as well, especially for oral phase swallowing. The importance of the afferent system is seen in the more severe swallow dysfunction seen in high vagal injury compared to recurrent laryngeal nerve injury, and challenges in
CHAPTER 1 General Principles of Swallowing swallow therapy training in patients with insensate laryngopharynx. Silent aspiration is often present in patients with injuries to the laryngopharyngeal sensory nerves. Surgical and nonsurgical treatment of head and neck diseases can cause dysfunctions of the many valves in the swallowing system. Lip valve dysfunction leads to oral incompetence and external leakage of saliva and food, velopharynx valve dysfunction may lead to nasopharyngeal regurgitation of food, laryngeal valve dysfunction may lead to penetration and aspiration, upper esophageal valve dysfunction may lead to inefficient handover of food bolus from the oropharynx to esophagus, and lower esophageal valve dysfunction may lead to esophageal stasis. Valve dysfunction also may lead to decreased or absent bolus pressure and release of swallow pressure strength in the improper direction. Proper pump function is arguably the most important requirement of the normal swallow taskd and the most difficult to treat when dysfunctionaldand pump failure leads to food stasis and residue in the respective compartments. The tongue pump, pharyngeal muscle pump, and esophageal muscle pumps are
3
all under voluntary and involuntary cortical neuromuscular control, and sequential activation of the pumps is another major requirement for normal swallowing function.
CONCLUSIONS Eating and drinking are essential for human survival and should be pleasurable and effortless. Normal swallowing is a highly coordinated physiologic activity involving over 30 paired muscles and multiple cranial nerves under voluntary and involuntary control. Food and liquid are passed sequentially and seamlessly from the mouth to the stomach. For the purposes of assessment and treatment, the swallow process can be anatomically and functionally separated into three compartments, where each compartment consists of valves, pipe, and pump. This allows for manageable conceptualization of swallow function and assessment and treatment of dysphagia. Treatment of dysphagia can be is a highly satisfying clinical activity as you will help patients achieve their goal to restore a basic and essential function of normal human life.
ANATOMY AND PHYSIOLOGY OF SWALLOWING
CHAPTER 1
General Principles of Swallowing DINESH K. CHHETRI, MD
INTRODUCTION Normal swallowing plays a critical human function: safe ingestion of food and liquid is vital to our survival and comfort. Human physiology requires regular and frequent eating and drinking for nutrition, and it is imperative that swallowing is not a nuisance or a burden. The goal of swallowing is to safely and efficiently propel food and liquid from the mouth to the stomach. In addition to serving the nutritional function, normal swallowing is necessary to clear saliva and secretions from the oral cavity and pharynx. Adult humans produce over 2 L of saliva daily and spontaneously swallow on average one and a half times a minute (over 2000 times per day). When an individual has difficulty managing saliva due to dysphagia, drooling (sialorrhea) or aspiration may occur. Normal swallowing is also critical for maintaining a satisfying human quality of life (QOL). We humans are a highly social species who find it pleasurable and desirable to eat, drink, and socialize with others. We design special rooms in our homes for cooking and eating. Outside our homes, we gather to barbecue, picnic, potluck, and party and even hunt for food. Family gatherings, celebrations, and rites of passages in all stages of human life nearly always include a moment of eating together in celebration of shared emotions. Religious gatherings often involve a component where food and liquid are consumed or shared. However, when a person has difficulty swallowing or is unable to swallow safely, the possibility of participating in many of these social interactions diminishes. This is psychologically and functionally devastating, bringing depression and anxiety, and significantly diminishing the human QOL. Although a person can certainly be fed and kept alive using a gastric tube, this is clearly neither considered pleasurable nor psychologically meaningful as
eating by mouth. Nonoral feeding is typically considered artificial rather than “human.” In fact, it is not unusual in otolaryngology to encounter patients who would trade their voice for the opportunity to swallow normally again. Therefore, it is imperative that we understand the etiologies of dysphagia and how to manage them, not only to prevent morbidity and mortality but also to improve and maintain satisfying QOL. Dysphagia practice is a relatively new clinical focus in otolaryngology. Currently, otolaryngologists often refer patients with dysphagia to other specialties such as gastroenterologists or speech and language pathologists for evaluation and management. I suggest that the otolaryngologist is the logical physician to manage patients’ oropharyngeal dysphagia, given the field’s extensive knowledge of head and neck anatomy. It is a reasonable extension for otolaryngologists to learn normal and abnormal swallow physiology, as well as how to assess and treat dysphagia. Successful assessment and management of dysphagia is a multidisciplinary endeavor. The otolaryngologist who is well versed in oropharyngeal dysphagia can make the appropriate referrals to gather the necessary data to understand how to help patients with swallow issues. Managing dysphagia is highly satisfying, once the clinician becomes familiar with contemporary assessment and management techniques and understands which patients may be helped. Thus, the purpose of this book is to make dysphagia evaluation and management accessible to all otolaryngologists.
BASIC CONCEPTS OF SWALLOWING The human developmental anatomy that provides voice and speech also creates one major challenge to swallowing: humans must swallow without food or liquids
1
2
SECTION I
Anatomy and Physiology of Swallowing
accidentally spilling into the airways. At birth, the human larynx is at the level of the oropharynx and the epiglottis tip is positioned at the superior oropharynx at the level of the soft palate. This allows the neonate to breathe through the nose and at the same time swallow liquids via mouth and through the lateral oropharyngeal channels. The descent of the laryngopharynx occurring as early as 6 months after birth with postnatal growth suddenly brings the laryngeal inlet directly into the path of the food bolus. While postnatal descent of the larynx allows for the glottic larynx to modulate the pulmonary airflow to produce voice that is further manipulated by the oral and pharyngeal structures to produce speech sounds, the crossing of the airway and the food way places human species at risk for aspiration and death when food falls below the level of the vocal folds into the tracheobronchial airway. A complex laryngeal protective mechanism therefore must be activated during each swallow. When this protective mechanism fails, aspiration and its complications may ensue. The major adverse consequence of aspiration is pneumonia which may result in death. However, more commonly, chronic aspiration leads to debilitation, malnutrition, chronic lung infection, and morbidity and mortality of chronic lung disease. What is dysphagia? Often it is simply described as subjective difficulty with swallowing. It is also the inability to swallow safely, or reduced intake such that the person has difficulty maintaining adequate nutrition and/or hydration. Dysphagia can also be an objective finding of dysfunction in the phases of swallowing. A complex swallowing mechanism controls sequential passage of food bolus from the mouth to the stomach. Functionally and anatomically, the swallowing apparatus can be separated into three sections: oral, pharyngeal, and esophageal. Each section can be conceptualized as a conduit with a pump and valves. Valves at either end of the section open and close to propel food bolus toward the stomach and prevent improper backward or sideway leakage. Pumps squeeze sequentially and generate the propulsive force that facilitates an anterograde movement of the food bolus toward the stomach. In the oral phase, the lips anteriorly and the oropharyngeal faucial arch and base of tongue posteriorly form valves that hold liquid in the oral cavity tube prior to swallow onset (the posterior oral valve may remain open during solid food mastication to allow aroma to reach the nasopharynx). The tongue pumps the food and liquids into the oropharynx at the onset of swallowing while the lips stay sealed and the posterior oral cavity valve opens. In the pharyngeal phase, the tongue base and velum
valves close and the upper esophageal sphincter (UES) valve opens while the pharyngeal muscle pump pushes the food toward the upper esophagus. The laryngeal protective valve is also activated during this phase. The pharyngeal phase is completed in 1 s, a testament to the highly coordinated neuromuscular control required to swallow without aspirating! Once food is in the esophagus, the UES closes and the esophageal pump propels the food bolus toward the lower esophageal sphincter, which opens to allow food into the stomach. Details of the swallowing events are described in detail in the following chapters. The valves, pumps, and pipes paradigm obviously simplifies the swallowing mechanism which is complex anatomically and functionally in the three dimensions. In addition, the pump and the pipe are often the same and changes shape with swallowing. The central and peripheral neural control is complex. However, this concept illustrates normal swallow function and identifies areas of dysfunctions in evaluating and managing dysphagia. The next chapters of this book will present the developmental anatomy, functional anatomy, and physiology of the swallowing apparatus in great detail. Then the swallow assessment tools will be covered in detail. Lastly, the most common swallow dysfunctions encountered by the otolaryngologist and management of these disorders will be covered in detail.
ETIOLOGIES OF DYSPHAGIA Conceptualization of the normal swallow anatomy and physiology as a biomechanical event lends itself to determining the etiologies and pathophysiology of dysphagia. There should be no defects in the walls of the pipe, the valves should close adequately and open sequentially, and the pumps should be intact in their muscular components and innervation. In regard to neural control, while motor nerves are critical to maintaining the compliance of the swallow compartments and pump function, afferent sensory system should be intact as well to act as a feedback system for controlled flow of food and liquids. One can witness the devastating consequence of neuromuscular dysfunction in patients with multiple cranial neuropathies. The glossopharyngeal (CN IX), vagus (CN X), and hypoglossal (CN XII) nerves are the critical cranial nerves for safe oropharyngeal swallowing. The trigeminal (CN V) and facial (CN VII) nerves play a crucial function as well, especially for oral phase swallowing. The importance of the afferent system is seen in the more severe swallow dysfunction seen in high vagal injury compared to recurrent laryngeal nerve injury, and challenges in
CHAPTER 1 General Principles of Swallowing swallow therapy training in patients with insensate laryngopharynx. Silent aspiration is often present in patients with injuries to the laryngopharyngeal sensory nerves. Surgical and nonsurgical treatment of head and neck diseases can cause dysfunctions of the many valves in the swallowing system. Lip valve dysfunction leads to oral incompetence and external leakage of saliva and food, velopharynx valve dysfunction may lead to nasopharyngeal regurgitation of food, laryngeal valve dysfunction may lead to penetration and aspiration, upper esophageal valve dysfunction may lead to inefficient handover of food bolus from the oropharynx to esophagus, and lower esophageal valve dysfunction may lead to esophageal stasis. Valve dysfunction also may lead to decreased or absent bolus pressure and release of swallow pressure strength in the improper direction. Proper pump function is arguably the most important requirement of the normal swallow taskd and the most difficult to treat when dysfunctionaldand pump failure leads to food stasis and residue in the respective compartments. The tongue pump, pharyngeal muscle pump, and esophageal muscle pumps are
3
all under voluntary and involuntary cortical neuromuscular control, and sequential activation of the pumps is another major requirement for normal swallowing function.
CONCLUSIONS Eating and drinking are essential for human survival and should be pleasurable and effortless. Normal swallowing is a highly coordinated physiologic activity involving over 30 paired muscles and multiple cranial nerves under voluntary and involuntary control. Food and liquid are passed sequentially and seamlessly from the mouth to the stomach. For the purposes of assessment and treatment, the swallow process can be anatomically and functionally separated into three compartments, where each compartment consists of valves, pipe, and pump. This allows for manageable conceptualization of swallow function and assessment and treatment of dysphagia. Treatment of dysphagia can be is a highly satisfying clinical activity as you will help patients achieve their goal to restore a basic and essential function of normal human life.