The development of feeding skills is an extremely complex process influenced by multiple anatomic, neurophysiologic, environmental, social, and cultural factors. Oral feeding in infants should be efficient to preserve energy for growing. Moreover, it should be safe so as to avoid aspiration, and it should not compromise respiratory status. This can only be achieved if sucking, swallowing, and breathing are properly coordinated [1]. This entire process is dynamic because of ongoing growth and development. Functional feeding skills, which depend on the integrity of anatomic structures, undergo changes based on neurologic maturation and experimental learning. Eating/feeding requires active effort by infants who must have exquisite timing and coordination of sucking, swallowing, and breathing to be efficient [2].

A variety of neurological , neuromuscular conditions in children and infants can impair the physiological phases of sucking/swallowing and cause disorders of feeding and dysphagia . The causes of feeding and swallowing problems include combinations of structural deficits, neurologic conditions, respiratory compromise, feeder–child interaction dysfunction, and numerous medical conditions such as genetic, metabolic, and degenerative disease [3].

In the recent years, there has been an increase in infant swallowing disorders as a result of improved survival rates for infants born prematurely or with life-threatening medical disorders. Negative experiences related to feeding, such as intubation, tube feeding, or airway suctioning may further disturb sucking and swallowing development [4].Disorders of feeding and swallowing in children are serious and potentially fatal problems. Aspiration due to dysphagia may lead to severe pulmonary disease, and impaired oral and pharyngeal function may rapidly result in failure to thrive. Prompt evaluation of swallowing disorders is therefore critical.

The differential diagnosis of dysphagia in children is wide. The diagnostic work-up can be extremely difficult and exhaustive in many cases. Because of this complexity, multidisciplinary team evaluations should be conducted.

Successful rehabilitation of children with swallowing disorders requires knowledge of the parameters of normal and abnormal swallowing plus skill in the integration of a variety of essential therapeutics techniques.

Data on the incidence of swallowing disorders are lacking, because in clinical practice, disorders of swallowing are often considered in the general context of a feeding disorder . Feeding (or eating) is different from swallowing. Eating is primarily an oral phase function that includes oral preparation and oral transit of a bolus [5]. Feeding is a complex process that involves a number of phases in addition to the act of swallowing, including the recognition of hunger (appetite), the acquisition of the food, and the ability to bring the food to the mouth [6]. The estimated prevalence of feeding problems in the pediatric population ranges from 25–35 % in normally developing children to 40–93 % in children with developmental delay [7, 8] . Early sucking and swallowing problems were reported to be present in 35–48 % of infants with different types of neonatal brain injury [9]. However, knowledge of the true epidemiology of pediatric dysphagia remains largely unavailable because of the lack of a standardized reporting system assessing dysphagia in all of the possible contexts that may occur in infants and children [10] .

Disorders of sucking/swallowing may be caused by multiple etiologic factors that may interfere with the child’s ability to coordinate swallowing and breathing maneuvers and may be manifested as a unique set of symptoms . Potential causes responsible for three broad categories include: immaturity, delay, or a defect in neuromuscular control; an anatomic abnormality of the aerodigestive tract; and/or systemic illness. The magnitude of the dysfunction depends on the balance between the extent of the structural or functional abnormality and the child’s compensatory adaptations [11]. Disorders associated with sucking/swallowing difficulties are reported in Table 20.1 [12, 13] .

The fetus is capable of swallowing amniotic fluid in utero, indicating that the motor program for swallowing is functioning before gestation is complete .

However, oral feeding is not initiated in preterm infants before 32 weeks of postconceptional age, partly because the coordination of sucking, swallowing, and respiration is not established [14]. Even at 34 weeks, minute ventilation during sucking decreases more than that of infants at 36–38 weeks. Therefore, the coordination between swallowing and breathing is not yet fully organized at 34 weeks of postconceptional age [15, 16].

Anatomic structures, which are essential to competent feeding skills, undergo growth that changes their physical relationship to one other and consequently affects their function. The swallowing mechanism, by which food is transmitted to the stomach and digestive organs, is a complex action involving 26 muscles and 5 cranial nerves. The neurophysiologic control involves sensory afferent nerve fibers, motor efferent fibers, paired brainstem swallowing centers, and suprabulbar neural input. Structural integrity is essential to the development of normal feeding and swallowing skills [17]. The central patterning of aeroingestive behavior is based on volitional and reflexive control mechanisms and benefit from sensory feedback to modify the spatiotemporal organization of the feed sequence to allow safe swallow [18]. Central pattern generators (CPGs) are primarily composed of adaptive networks of interneurons that activate groups of motor neurons to generate task-specific motor patterns [19]. The essential components of the masticatory CPG are found between the rostral poles of the fifth and seventh motor nuclei, although they are normally synchronized by commissural axons each hemisection side can generate a rhythm [20]. Mastication patterns differ greatly between foods and change systematically during a chewing sequence based on sensory feedback. Functional imaging has revealed that swallowing is controlled through a network of cortical areas which shares loci with other ororhythmic movements including speech [21] .

Deglutition is generally divided into phases of swallowing based on anatomic and functional characteristics: pre-oral, pharyngeal, and esophageal [22, 23].

An understanding of the anatomy of the pharynx is essential to a thorough understanding of the swallowing process . The anatomy changes during development. The tongue, the soft palate, and the arytenoids mass (arytenoids cartilage, false vocal cords, and true vocal cords) are larger relative to their surrounding chambers when compared with the adult [24]. In the infant, the tongue lies entirely within the oral cavity, resulting in a small oropharynx [25, 26]. In addition, a sucking pad, composed of densely compacted fatty tissue that further reduces the size of the oral cavity, stabilizes the lateral walls of the oral cavity. The larynx lies high in the infant, and the tip of epiglottis extends to and may overlap the soft palate. These anatomic relationships are ideal for the normal infant feeding pattern of suck or suckle feeding from a breast or a bottle in a recumbent position [26]. In the infant, the larynx sits high in the neck at the level of vertebrae C1 to C3, allowing for the velum, tongue, and epiglottis to approximate, thereby functionally separating the respiratory and digestive tracts. This separation allows the infant to breathe and feed safely. By age 2–3 years, the larynx descends, decreasing the separation of the swallowing and digestive tracts [7] .

Swallowing skills develop progressively during fetal and neonatal maturation [27] . At approximately 26 days’ fetal age, the developmental trajectories of the respiratory and swallowing systems diverge and start to develop independently. Swallowing in fetuses has been described as early as 12–14 weeks’ gestational age. A sucking response can be provoked at 13 weeks’ postconceptional age by touching the lips [28]. Real sucking, defined by a posterior–anterior movement of the tongue, in which the posterior movement is dominant, begins at 18–24 weeks’ postconceptional age [29]. Between 26 and 29 weeks’ there is probably no significant further maturation of sucking [30]. By week 34, most healthy fetuses can suck and swallow well enough to sustain nutritional needs via the oral route, if born at this early age. Sucking movements increase in frequency during the final weeks of fetal life due to an increase in amniotic fluid swallowed by a fetus during pregnancy from initially 2–7 ml a day to 450 ml a day. This is approximately half of the total volume of amniotic fluid at term [31]. The normal maturation of sucking and swallowing during the first months of life after full-term birth can be summarized by increased sucking and swallowing rates, longer sucking bursts and larger volume per suck [16]. The skill of safe and efficient oral feeding is based on oral-motor competence, neurobehavioral organization, and gastrointestinal maturity [32]. Two forms of sucking are distinguished: nutritive sucking (NS) and nonnutritive sucking (NNS). NS is an infant’s primary means of receiving nutrition while NNS is regarded as an initial method for exploring the environment . The rate of NNS is approximately twice as fast as that of NS [33]. In NS, however, the ability to integrate breathing with sucking and swallowing is essential for coordinated feeding [1] and it becomes consistent by 37 weeks’ gestation [34]. By increasing the intraoral space, the infant begins to suppress reflexive suckle patterns and starts to use voluntary suck patterns. In contrast to suckling, true sucking involves a raising and lowering of the body of the tongue with increased use of intrinsic musculature. Most of the infants complete the gradual transition from suckling to true suck by 9 months of age. This is considered a critical step in the development of oral skills that will permit handling of thicker textures and spoon-feeding [35] .

As with sucking, chewing patterns emerge gradually during infancy. Between birth and 5 months of age, a phasic bite-release pattern develops. At this series of jaw openings and closing begins as a reflex and evolves into volitionally controlled bite. True chewing develops as the activity of the tongue, cheeks, and jaws coordinates to participate in the breakdown of solid food. The eruption of the deciduous teeth between ages of 6 and 24 months provides a chewing surface and increased sensory input to facilitate the development of chewing [35].

The concept of a “critical period” is relevant to feeding development. A critical period refers to a fairly well-delineated period of the time during which a specific stimulus must be applied to produce a particular action. After such a critical period, a particular behavior pattern can no longer be learned. Critical periods have been described for chewing and for taste. The critical period for chewing is that time following the disappearance of the tongue protrusion reflex that should occur around 6 months of age [36]. Critical periods have also been reported for introduction of tastes. Newborn infants detect sweet solutions, reject sour flavors, and are indifferent to the taste of salt [37]. Mc Farland and Tremblay emphasized that sensory experience is crucial to optimize pattern formation and brain development during the critical period for attainment of swallowing proficiency [38] .

Current knowledge of the swallowing mechanism is derived mainly from radiographic studies, which have been in use since the early 1900s. Plain films of the pharynx were replaced in the 1930s by cineradiography, which was subsequently in the 1970s replaced by videofluoroscopy . Videofluoroscopy permits instant analysis of bolus transport, aspiration, and pharyngeal function [39]. Using this descriptive method, deglutition can be divided into four phases: oral preparatory phase, oral voluntary phase, pharyngeal phase, and esophageal phase (Table 20.2) [13, 40].

The oral preparatory phase occurs after food is placed into the mouth. The food is prepared for pharyngeal delivery by mastication and mixing with saliva. This is a highly coordinated activity that is rhythmic and controlled to prevent injury to the tongue. The tongue is elevated toward the palate by the combined actions of the digastric, genioglossus, geniohyoid, and mylohyoid muscles. Intrinsic tongue muscles produce both the initial depression in the dorsum that receives the food and the spreading action that distributes the food throughout the oral cavity. The buccinator muscles hold food between the teeth in dentulous infants and help to generate suction in neonates. In this phase, the soft palate is against the tongue base, secondary to contraction of the palatoglossus muscles, which allows nasal breathing to continue [7, 41] .

During the oral propulsive phase, the bolus is propelled into the oropharynx. The oral phase is characterized by elevation of the tongue and a posterior sweeping or stripping action produced mainly by the action of styloglossus muscles. This propels the bolus into the pharynx and triggers the “reflex swallow.” The receptors for this reflex are thought to be at the base of the anterior pillars, but there is evidence that others exist in the tongue base, epiglottis, and pyriform fossae. Sensory impulses for the reflex are conducted through the afferent limbs of cranial nerves V, IX, and X to the swallowing center. Oral transit time is less than 1 s [7, 42] .

The pharyngeal phase of deglutition is the most complex and critical. The major component of the pharyngeal phase is the reflex swallow. This results from motor activity stimulated by cranial nerves IX and X. Swallowing is elicited involuntary by afferent feedback from the oral cavity and has a duration of approximately 530 ms [1]. The reflex swallow may be triggered by a voluntary oral phase component or any stimulation of the afferent receptor in and around the anterior pillar [7]. Bolus passage through the pharynx is accompanied by soft palate elevation, lingual thrust, laryngeal elevation, and descent upper esophageal sphincter (UES) relaxation and pharyngeal constrictor peristalsis. The pharyngeal phase commences as the bolus head is propelled past the tongue pillars and finishes as the bolus tail passes into the esophagus [42]. Once it begins, the pharyngeal phase is very quick, 1 s or less [7]. It is characterized biomechanically by the operation of three valves and several propulsive mechanisms. During pharyngeal swallowing, respiration is inhibited centrally [43]. The larynx closes and the palate elevates to disconnect the respiratory tract. The UES opens to expose the esophagus. At the completion of the pharyngeal phase, the airway valves (larynx, palate) open and the UES closes so that respiration can resume [42] .

Pharyngeal bolus transit occurs in two phases: an initial thrust phase and a mucosal clearance phase [44]. Bolus thrust, which propels most of the bolus into the esophagus, is provided by lingual propulsion, laryngeal elevation, and gravity. The tongue has been linked to a piston, pumping the bolus though the pharynx [45]. Patients with tongue impairment cannot generate large bolus driving forces despite an intact pharyngeal constrictor mechanism [46].Laryngeal elevation creates a negative postcricoid pressure to suck the oncoming bolus toward the esophagus, and the elevated larynx holds the pharyngeal lumen open to minimize pharyngeal resistance [45] .

As the bolus enters the pharynx and is stripped inferiorly by the combined effects of gravity, the negative pressure mentioned above, and the sequential contractions of the pharyngeal constrictors, the soft palate moves against the posterior pharyngeal wall to close off the nasopharyngeal port. The bolus divides around the epiglottis, combines and passes though the cricopharyngeal muscle, or UES [7] .

UES refers to the high-pressure zone located in between the pharynx and the cervical esophagus. The physiological role of this sphincter is to protect against reflux of food into the airways as well as prevent entry of air into the digestive tract [47]. Posteriorly and laterally the cricopharyngeus muscle is a definitive component of the UES. Cricopharyngeus has many unique characteristics: it is tonically active, has a high degree of elasticity, does not develop maximal tension at basal length, and is composed of a mixture of slow- and fast-twitch fibers, with the former predominating. These features enable the cricopharyngeus to maintain a resting tone and yet be able to stretch open by distracting forces, such as a swallowed bolus and hyoid and laryngeal excursion. Cricopharyngeal, however, constitutes only the lower one-third of the entire high-pressure zone. The thyropharyngeus muscle accounts for the remaining upper two-thirds of the UES. The UES function is controlled by a variety of reflexes that involve afferent inputs to the motorneurons innervating the sphincter [47]. Based on functional studies, it is believed that the major motor nerve of the cricopharyngeal muscle is the pharyngoesophageal nerve. Vagal efferents probably reach the muscle by the pharyngeal plexus, using the pharyngeal branch of the vagi [48]. The superior laryngeal nerve may also contribute to motor control of the cricopharyngeus muscle [6]. Sensory information from the UES is probably provided by the glossopharyngeal nerve and the sympathetic nervous system. There is probably little or no contribution by the sympathetic nervous system to cricopharyngeal control [48] .

The relaxation phase begins as the genioglossus and suspensory muscle pulls the larynx anteriorly and superiorly. The bolus is carried into the esophagus by a series of contraction waves that are a continuation of the pharyngeal stripping action [7]. Proposed functions of the UES include prevention of esophageal distention during normal breathing and protection of the airway against aspiration following an episode of acid reflux [6, 48].Qualitative abnormalities of UES have been documented in infants with reflux disease [49].

The esophageal phase occurs as the bolus is pushed through the esophagus to the stomach by esophageal peristalsis. Esophageal transit time varies from 8 to 20 s [26] .

Dysphagia is an impairment of swallowing involving any structures of the upper gastrointestinal tract from the lips to the lower esophageal sphincter [50] . Dysphagia in children is generally classified as either oral dysphagia (abnormal preparatory or oral phase) or pharyngeal dysphagia (abnormal pharyngeal phase).

Oral dysphagia is seen most commonly in children with neurodevelopmental disorders. Infants with oral dysphagia often have an impaired oral preparatory phase. These children typically demonstrate poor lingual and labial coordination, resulting in anterior substance loss and poor labial seal for sucking or removing food from a spoon. Other abnormal patterns include jaws thrust and tongue thrust on presentation of food. Oral dysphagia also may involve the oral phase of swallowing. Children with impaired oral phase function often have difficulty in coordinating the “suck, swallow, breathe” pattern of early oral intake, resulting in diminished endurance during oral feeds. Apraxia of oral swallow as well as reduction of oral sensation also is common. Other deficits include reduced bolus formation and transport, abnormal hold patterns, incomplete tongue to palate contact, and repetitive lingual pumping [26] .

Oropharyngeal dysphagia results from either oropharyngeal swallowing dysfunction or perceived difficulty in the process of swallowing. Major categories of dysfunction are: (1) an inability or excessive delay in initiation of pharyngeal swallowing, (2) aspiration of ingestate, (3) nasopharyngeal regurgitation, and (4) residue of ingestate within the pharyngeal cavity after swallowing. Each of these categories of dysfunction can be subcategorized using fluoroscopic and/or manometric data [29] .

Clinical signs and symptoms of sucking/swallowing disorders in infants and children are listed in Table 20.3 [13] .