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42 CLINICAL MANAGEMENT OF SWALLOWING DISORDERS
op
ophagu
s
Larynx
The larynx is often thought to be the main organ of
swallowing. It works in conjunction with the oral
and pharyngeal phases of swallowing. Adjunctively,
it aids in lifting and defecation by closing tightly and
building pressure to help those activities. Indeed,
the larynx functions as an organ of respiration, phonation, and deglutition.
The larynx shares the swallowing passageway
for both airway and deglutitive functions; thus, it
requires specific anatomical and physiological adaptation for each function.
10
In the normal individual,
the larynx helps to regulate the passage of air either
to or from the lungs or food and liquid into the
esophagus. Figure 2–11A shows a superior view of
the larynx with the vocal folds open. Note that the
esophagus is barely visible when the vocal folds are
open. Figure 2–11B shows a sagittal view of the larynx with the innervation of the vagus nerve (CNX).
For swallowing, the larynx elevates and moves anteriorly. Normally, closure occurs once the bolus passes
Right and left
Vagus nerves
Superior
laryngeal nerves
Piriform recess
Vocal folds
Epiglottis
vallecula
Epiglottis
Pharyngeal branches
of Vagus nerve
Tongue
A
FIGURE 2–11. A. Superior view of the larynx with the right and left vagus nerve endings. continues
2. ANATOMY AND PHYSIOLOGY OF THE SWALLOWING MECHANISM 43
f
to
g
the anterior faucial pillars. At that point, the pharyngeal transport is under involuntary control, although
increases in pressure may voluntarily take place. This
movement can be impaired as a result of radiation,
surgery, or CNS damage as in a cerebrovascular accident. Placing one finger over the thyroid prominence
allows one to grossly feel this elevation during a dry
Vagus nerve
Pharyngeal
branch of
Vagus nerve
Vallecula
Epiglottis
swallow. Without this movement, foods and especially liquids are likely to enter the trachea, leading to
aspiration. Without a strong cough to clear the foods,
continued entry of liquids or foods into the airway
may lead to aspiration pneumonia. When the cough
is weak and aspiration is suspected, esophageal function is best appreciated with a fluoroscopic exam.
Base of
tongue
laryngeal nerve
Vagus nerve
Esophagus
B
Superior
Vocal folds
To
stomach
To
lungs
FIGURE 2–11. continued B. Sagittal view of the laryngopharynx to the upper
esophagus.
44 CLINICAL MANAGEMENT OF SWALLOWING DISORDERS
Esophagus
The esophagus is a tube that connects the pharynx to
the stomach. It begins superiorly (see Figure2–11A)
at the base of the cricopharyngeus muscle and
enters the stomach through the diaphragm muscle.
The esophagus serves as a conduit to pass food
through the thoracic cavity into the abdominal cavity for digestion and absorption. Note the location
of the esophagus in relation to the airway shown
in Figure 2–11B. Pope reported on the description and function of the esophagus describing a
series of peristaltic waves following the relaxation
of the cricopharyngeus muscle.
11
A wide variety
of disease processes may affect the muscles of the
esophagus, with scleroderma representing the most
common.
THE NORMAL SWALLOW
Traditionally, the normal swallow has been described
as a series of 4 phases that relate to the passage of
the bolus through specific anatomical structures.
These phases are the oral preparatory, oral, pharyn-
www
geal, and esophageal. Refer to Videos 1–1 and 1–2
for examples of normal swallowing.
Martin-Harris and colleagues found that the
onset of laryngeal closure and onset of hyoid
excursion were strongly related and formed the
first functional unit in the swallow.
They also noted that these events always occurred
prior to the opening of the pharyngoesophageal segment. For most of their subjects, however, the hyoid
returned to its rest position after breathing resumed.
Thus, the interrelationships between breathing and
swallowing demonstrated an overlap at the onset and
offset of the swallow task.
14
The video examples from
Chapter 1 (Videos 1–1 and 1–2) show the interrelated
actions of swallowing in normal subjects.
In order to understand bolus preparation and
bolus transit, the phases of swallowing are described
sequentially. Nonetheless, the reader should remember that these phases overlap, and normal swallowing involves the integration of phases. Moreover, it is
also important to understand the interaction between
the organs of swallowing and respiration function
prior to, during, and after swallowing. These temporal relationships may affect bolus transit, retention
and/or penetration, and aspiration. Moreover, these
relationships impact therapy targets.
Oral Phase
www
The phases of swallowing were generally
thought to occur sequentially; however, recent
studies have shown that the oral and pharyngeal
phases are interdependent.
12
Martin-Harris and colleagues originally presented evidence obtained from measuring the
onset of the swallow and specific respiratory timing patterns associated with swallowing.
13
Using
confirmatory factor analysis, they concluded that
there is an overlap between the start of the oral
and pharyngeal phases of swallowing. They identified 4 distinct patterns of breathing and swallowing.
They also identified 4 functional units of swallowing
that overlap.
The oral phase of swallowing involves bolus retention, mastication, and bolus transfer. These 2 functions usually operate sequentially; as mastication
is completed, the bolus is then transferred. For the
swallow to be normal, the anatomical structures of
the upper aerodigestive system must be intact, and
their function in sequence with each other must be
appropriately timed. This requires the integrity of
both the motor and sensory nervous systems. The
2 parts of the oral phase, mastication and bolus
transfer, are reviewed separately in order to better
understand each. Both aspects rely on lip seal, an
important aspect of the oral phase that should not
be overlooked during the oral assessment. Closure
of lips and their ability to remain closed under tension are both important.
2. ANATOMY AND PHYSIOLOGY OF THE SWALLOWING MECHANISM 45
Mastication
The oral preparatory function involves mastication
and bolus formation. The lips retain the food in the
oral cavity and direct it back to the tongue and teeth
for bolus mastication and transfer. The movements
of the tongue, mandible, dentition, soft palate, and
muscles of the buccal cavity are temporally integrated to grind and position the food.
During the mastication and bolus preparation
segment, the tongue arranges the bolus in a way
that allows it to be moved to a location and position
where it can be chewed. In the normal swallow, this
usually results in the food being placed in the region
of the molar teeth. At this point, the food is reduced
through mastication, mixed with saliva, and forms
the bolus that eventually will be swallowed.
Exercise devices such as the TheraBite aid in
mastication.
15
At this time, factors such as taste, temperature,
and the viscosity and size of bolus are sensed, and
appropriate lip, tongue, buccal, and dental manipulations are carried out to finally prepare the bolus.
The trigeminal nerve (cranial nerve CN V),
through its second and third divisions, provides
sensory and motor innervations, respectively, to the
muscles of mastication. Sensory information related
to taste is mediated by CN VII (anterior two-thirds
of the tongue) and CN IX (posterior one-third of
the tongue).
Bolus Transfer
is depressed, and the anterior and middle portions
of the tongue differentially elevate and begin the
propulsion of the bolus to the oropharynx. Recent
findings suggest that tongue strengthening exercises
aid in bolus transfer. Yeates et al found improvements in tongue strength and pressure-generating
precision in older adults with dysphagia.
16
It should be emphasized that the tongue is the
primary manipulator of food during the oral phase.
Any injury, neurological damage, or surgical treatment to the tongue will affect the oral phase of
swallowing. Injury to the lips may complicate the
problems in the oral phase. If lip closure and the
maintenance of lip pressure are inadequate, the oral
phase of swallowing is affected due to lack of bolus
containment. If normal bolus transit does not occur
due to either a lack of lip closure or a lack of pressure buildup, there is a high probability that materials will be found in the cricopharyngeus area after
the swallow is completed. When the food is properly
masticated and formed into a bolus, the entire bolus
is propelled into the oropharynx by the action of
the tongue and the pressure buildup supported by
adequate lip seal. This leads to the next stage of
swallowing.
When there is damage to the cranial nerves
involved in swallowing, the oral phase of swallowing becomes abnormal. This usually leads to further abnormalities as the bolus reaches or fails to
reach the next phase in a coordinated manner. In
the oral phase of swallowing, results from functional
magnetic resonance imaging (fMRI) studies have
revealed that the primary motor and sensory areas
of the brain, as well as the anterior cingulate cortex
and insular cortex, are active during swallowing in
healthy adults.
17
The food bolus is transported via the action of the
tongue and its interaction with the palate, tongue,
teeth, and cheeks. The oral segment is primarily a
delivery system. Contact of the back of the tongue
with the soft palate retains the bolus in the oral cavity, preventing early spillage into the pharynx. Once
the bolus is prepared, it is positioned posteriorly on
the tongue. The velum then elevates as the lips and
buccal muscles contract to build pressure and reduce
the volume of the oral cavity. The posterior tongue
Pharyngeal Phase
The pharyngeal phase of swallowing begins when
the bolus reaches the level of the anterior tonsillar
pillars. Normal function of the pharyngeal phase is
dependent on the consistency of the bolus, the size
of the bolus, and whether swallowing is a single or
continuous event. A small well-organized bolus may
pass the anterior faucial arches rapidly, whereas a
46 CLINICAL MANAGEMENT OF SWALLOWING DISORDERS
poorly organized bolus will extend from the oral
cavity into the oropharynx, requiring continuous
interaction of the pharyngeal mechanisms.
The pharyngeal phase of swallowing is independent of both the oral and esophageal phases of
swallowing. It can be stimulated to function.
The pharyngeal phase of swallowing involves
the complex interaction of the tongue, velopharynx,
and larynx. As the tongue elevates, velopharyngeal
closure begins. This activity triggers the forward
motion of the laryngeal structures to increase the
opening of the upper esophageal sphincter. The larynx also elevates, all of which leads to relaxation of
the cricopharyngeus musculature. When these interactions occur with appropriate temporal integration,
the bolus moves through the pharyngeal segment
without penetration or aspiration of the bolus into
the airway. This activity is considered to be involuntary. An endoscopic view of the hypopharynx with
residual bolus remaining is shown in Figure 2–12.
The material in the cricopharyngeus area indicates
that a portion of the bolus did not pass into the
esophagus.
Table 2–7, adapted from Simonian and Gold-
18
berg,
summarizes the signs and possible causes of
dysphagia and its treatment according to the phases
of swallowing. More detailed descriptions of the
treatment options are offered in Chapter 7.
Esophageal Phase
The esophageal body is a muscular tube extending 20 to 25 cm in length from its origin just caudal
to the cricopharyngeus muscle to its termination at
the entrance to the stomach. Figure 2–13 shows the
esophagus with its attachments at the upper esophageal sphincter to the lower esophageal sphincter at
the entrance to the stomach. The esophagus shortens
by about 10% through longitudinal muscle contractions during swallowing. Although the primary function of the esophageal body is passage of injected
materials from the pharynx to the stomach, recent
studies indicate that the esophagus is not merely a
FIGURE 2–12. Flexible endoscopic view showing the epiglottis and vocal folds shortly
after the bolus has passed into the upper esophagus. Note: there is no food in the
airway, although some remains in the valleculae.
2. ANATOMY AND PHYSIOLOGY OF THE SWALLOWING MECHANISM 47
TABLE 2–7. Diagnosis of Dysphagia Showing Signs of Cranial Nerve Deficits and Treatment Options
Type Signs Possible Causes Treatment Options
Oral Preparatory
Oral
Pharyngeal
Lip flaccidity
Labial leakage
Buccal pocketing Facial weakness
Labored mastication Lack of dentition
Premature spill Lingual weakness Chin-tuck position
Delayed swallow
initiation
Decreased laryngeal
elevation
Multiple swallow pattern Decreased pharyngeal
Cough/throat clear
immediately after the
swallow
CN V Place food
Oral motor exercises, present food
Surgical revision
Poor cognition
Poor oral phase
Vagus nerve dysfunction
Prolonged intubation
Tracheotomy
Nasogastric tube
Suprahyoid muscle
peristalsis/contraction
Aspiration secondary to
decreased epiglottic deflection
Poor oral phase
Tracheoesophageal fistula (rare)
to stronger side
Modify food texture
Modify food texture
Thermal stimulation
Posterior tongue strengthening
Tracheotomy cuff deflation, d/c NGT
Edema
Alternate liquid and solid swallows
Supraglottic swallow
Modify food texture
Delayed cough, throat
clear
Change in vocal quality Penetration to the level of the
Esophageal Significantly delayed
aspiration
a
Adapted and revised with permission from Simonian and Goldberg.
Aspiration after the swallow
secondary to pooling in the
pharynx
vocal cords
Vocal cord weakness
Reflux, stricture Medication
hollow, passive conduit for food transport. Rather,
it has several active functions for acid control and
mucosal protection.
Peristalsis, or sequential contraction of the
esophagus and relaxation of the lower esophageal
sphincter, characterizes the esophageal phase of
swallowing. The bolus is propelled through the
Utilize dry swallow, alternating liquid
and more solid swallows
Modify food texture
Modify foods
GI referral
18(p367)
esophagus by contraction above and relaxation
below the bolus. This relaxation is referred to as
descending inhibition.
Primary peristalsis occurs when a swallow
induces peristaltic activity, whereas secondary peri-
stalsis refers to the initiation of a propagated contraction wave in the absence of a swallow. Initiation
48 CLINICAL MANAGEMENT OF SWALLOWING DISORDERS
Upper
esophageal
sphincter
Diaphragm
FIGURE 2–13. The esophagus with its attachments at the upper esophageal sphincter
to the lower esophageal sphincter at the entrance to the stomach.
of secondary peristaltic contractions is involuntary
and normally is not sensed. Ultimately, in a normal
swallow, the bolus passes from the proximal to the
distal esophagus and into the stomach.
Phase Relationships
It is clear from previous research
of swallowing involve a type of parallel processing
from the cortex to the PNS. Problems during any
one phase of swallowing may lead to problems during other phases. As pointed out in the remainder
13,19
that the phases
Lower
esophageal
sphincter
of this chapter, CNS structures control the motor
sequencing of the transfer of the bolus from the lips
to the stomach. However, these central processes
require the coordination of the PNS to carry out the
functional passage of the bolus to the stomach. The
phase relationships in swallowing are somewhat
variable, affected by the type, size, and consistency
of the bolus. Video 2–1 presents a patient with difficulty initiating the proper sequence, thus resulting
in significant pooling of the bolus. Similarly, this is
shown in a videofluoroscopic swallow study exam
of a patient following cerebrovascular accident in
Video 2–2.
www
www
2. ANATOMY AND PHYSIOLOGY OF THE SWALLOWING MECHANISM 49
Even in the normal individual, the order of
events and especially the timing of those events may
not be entirely consistent. Moreover, the timing of
events may vary for an individual depending on the
conditions under which they are swallowing (distraction, multiple swallows, rapid swallows, etc.).
Nonetheless, Martin-Harris and colleagues have suggested an approximate order for the events of the
normal swallow to be initiated.
14,19
The course of
these events and the variation in timing are variable,
but they follow the pattern described by Rosenbek
and Jones
20
and outlined in Table 2–8.
CRANIAL NERVES INVOLVED IN SWALLOWING
Vagus Nerve (CN X)
The vagus nerve provides motor and sensory innervation to the palate, larynx, pharynx, esophagus,
stomach, and respiratory tract and is intimately
involved in the regulation of blood pressure. Central contributions include motor innervation from
TABLE 2–8. Events in a Normal Swallow Are Initiated in
Approximately This Order
Apnea onset
Oral bolus transport
Hyoid excursion
Laryngeal closure
Maximum laryngeal closure
PES opening
Maximum hyoid excursion
Laryngeal opening
Swallowing inspiration
Apnea onset
Last PES opening
Hyoid return
Abbreviation: PES, pharyngoesophageal segment.
a
Reprinted with permission from Rosenbek and Jones.
a
20
the nucleus ambiguus and sensory innervation from
the nucleus solitarius.
Recurrent Laryngeal Nerve
The recurrent laryngeal nerve innervates all of the
muscles of the larynx, except the cricothyroid. It is
responsible for glottic closure during swallowing.
Superior Laryngeal Nerve
The superior laryngeal nerve (SLN) bifurcates into
2major divisions: an internal and an external division. The external division innervates the cricothyroid muscle, which retracts the posterior cricoid
facet from the thyroid lamina, tensing the vocal
fold and thus lengthening the anterior-to-posterior
dimension of the glottis and changing the vocal
pitch.
The internal branch of the SLN provides mucosal touch and proprioceptive sensory input from the
supraglottic larynx, cricoarytenoid joints, posterior
aspect of the larynx, and pharyngeal mucosa in the
piriform sinuses. A loss of sensation due to damage
to the SLN results in anesthesia of the supraglottis
and piriform sinuses, which may lead to aspiration.
Despite secretions into the trachea, patients do
not cough when the SLN is not functional. Jafari
21
etal
found evidence that damage to the SLN alone
without additional lesions in the brain or airway
obstructions or diseases can result in aspiration.
They suggested that following various conservative
laryngeal surgeries, SLN injury was a main factor in
dysphagia and aspiration.
Trigeminal Nerve (CN V)
The third division of the trigeminal nerve supplies
sensory innervation to the tongue (lingual nerve)
and to the inferior alveolus, buccal mucosa, and
lower lip (inferior alveolar nerve). Although sensation of the base of the tongue is supplied through
CN IX (glossopharyngeal nerve), innervation of the
oral tongue is conferred via the lingual nerve. The
trigeminal nerve also supplies motor innervation to
the mastication muscles, including those with mandibular and maxillary insertions.
50 CLINICAL MANAGEMENT OF SWALLOWING DISORDERS
Glossopharyngeal Nerve (CN IX)
This nerve provides sensory innervation to the oropharynx and the base of the tongue and supports
taste fibers at the base of tongue. Its motor innervation is to the stylopharyngeus muscle.
Hypoglossal Nerve (CN XII)
The hypoglossal nerve (CN XII) controls critical
movements of the tongue. Since the tongue is the
primary mover of the bolus while in the mouth,
these movements contribute significantly to the
overall swallowing process.
Problems with the hypoglossal nerve often result
in the inability of the bolus to be moved in place
for chewing and for transmitting the bolus to the
oropharynx. If there is damage to CN XII, the
tongue may be weak or paralyzed. Speech also
becomes disrupted.
chamber to facilitate the propulsion of the bolus
into the next chamber. Damage to any of the sphincters may affect disruption of the normal swallow.
The specific sphincter actions of the upper
aerodigestive tract involved in swallowing are discussed later. Sphincter relaxation in a normal swallow generally precedes the onset of pharyngeal
transit (bolus entry into the pharynx) and the initiation of swallow. Manometric studies have proven
useful in interpreting the actions of the upper esophageal sphincter in the act of swallowing.
22,23
During
a swallow of more than 5 mL of fluid, failure to
coordinate the onset of pharyngeal transit (entry of
the bolus into the pharynx) with the onset of swallow gestures can result in nasal reflux, aspiration,
or regurgitation. In other swallows, however, particularly those associated with mastication, the linguopalatal sphincter may open repeatedly to allow
small amounts of the bolus into the oropharynx and
valleculae long before the swallow sequence is initiated. Opening of the linguopalatal sphincter usually coincides with the onset of pharyngeal transit
(bolus entry into the pharynx) and the initiation of
the swallow.
If paralysis is present, the tongue deviates to the
paralyzed side. When CN XII is damaged because of
strokes, infections, or tumors, it can become weak
or paralyzed, which can in turn lead to problems
with communication, chewing, or swallowing. Eventually, the tongue will begin to atrophy or shrink if
the hypoglossal nerve is paralyzed, causing further
problems with the bolus while in the oral cavity and
making speech almost unintelligible.
SPHINCTERS
Swallowing can be visualized as the passage of the
bolus through a series of dynamic chambers. These
chambers are separated by sphincters (gates) that
help to prevent spillage of the material before it
enters the next chamber.
Sphincters maintain a watertight closure that
aids in building up the pressure in the particular
Velopharyngeal Sphincter
Failure to close the velopharyngeal sphincter results
in leakage of the bolus or air into the nasopharynx and a diminished ability to generate appropriate oropharyngeal pressures to propel the bolus
through the oropharynx. Data by Pauloski and colleagues
23
in a study of manometry and fluoroscopy
showed that increased tongue base activity resulted
in increased pressure on the bolus, resulting in a
more efficient swallow that was characterized by
shorter transit times and better bolus clearance.
Laryngeal Sphincter
Laryngeal closure occurs in a sequential fashion,
with approximation of the true vocal folds (CNX)
preceding false vocal folds approximation, and
finally approximation of the arytenoids to the petiole of the epiglottis. Failure to close the supraglottic
2. ANATOMY AND PHYSIOLOGY OF THE SWALLOWING MECHANISM 51
and glottic sphincters during the swallow results in
penetration and/or aspiration and in a decreased
ability to generate adequate hypopharyngeal pressures to propel the bolus through the pharyngoesophageal segment and into the esophagus.
Upper Esophageal Sphincter
The upper esophageal sphincter (UES) is a tonically
contracted group of skeletal muscles separating the
pharynx from the esophagus (see Figure 2–6) The
major component of the sphincter is the cricopharyngeus muscle. At rest, the sphincter is in a state
of tonic contraction that minimizes the entrance of
air into the gastrointestinal tract during respiration.
Equally important is its function to prevent the entry
of refluxed material from the esophagus into the
pharynx.
The tonically contracted UES relaxes during
the pharyngeal peristaltic sequence. The relaxation
begins after the onset of swallowing and lasts 0.5
to 1 second. The onset of the pharyngeal peristaltic
wave is marked by apposition of the soft palate to the
pharyngeal wall, generating a contraction that lasts
over 0.1 second and generates a pressure greater
than 180 mm Hg. Pharyngeal peristalsis transverses
the oropharynx and hypopharynx at about 15 cm/s
and reaches the UES in about 0.7 second. After the
relaxation phase, the sphincter contracts with an
increase in force, in which the pressure may exceed
twice the pressure of the resting tone for approximately a second prior to returning to baseline.
Poor coordination of the pharyngoesophageal
segment may occur due to neurological deficits such
as recurrent laryngeal nerve paralysis or brainstem
stroke. Inadequate elevation of the hyoid-laryngeal
complex and/or weakness of the pharyngeal constrictors also affect the function of the pharyngoesophageal segment as a sphincter.
Lower Esophagus
The lower esophagus is a specialized segment of
smooth tubular muscle extending 20 to 25 cm below
the cricopharyngeus muscle. It relaxes to permit the
bolus to enter the gastric cavity and contracts to
prevent gastroesophageal reflux in its resting state.
Gross and histological examinations of the lower
esophageal sphincter have failed to identify a specific sphincteric structure. Compared to adjacent
structures, the lower esophageal muscle also possesses an increased sensitivity to many excitatory
agents, suggesting a greater influence on sphincter
tone by nerves and hormones.
CENTRAL NEURAL CONTROL OF SWALLOWING
Normal deglutition, the act of swallowing, is initiated voluntarily. Central neural control of swallowing can be divided into cortical and subcortical
components. Neural control is composed of a very
complex interaction of afferent sensory neurons,
motor neurons, and interneurons that control voluntary and involuntary/reflexive actions of swallowing.
Cortical regulation includes centers in both
hemispheres of the brain with representation for
the pharynx and the esophagus. These cortical
areas have interhemispheric connections and projections to the motor nuclei of the brainstem. Bilateral hemispheric stimulation produces a greater
response than unilateral impulses, and this response
is intensity and frequency dependent. Both motor
and premotor cortical areas are involved in the initiation of swallowing or at least have the potential
to modulate the contraction of the pharyngeal and
esophageal musculature. Input from these cortical
areas to the pharynx, however, seems greater than
input to the esophagus. Similarly, afferent impulses
from the pharynx, largely from the SLN and glossopharyngeal nerve, have greater effects on cortical
areas than those from the upper esophagus via the
recurrent laryngeal nerve.
The “swallowing center,” identified as an area
within the reticular system of the brainstem that
comprises the nucleus ambiguus (cranial nerves IX,
X, and XI) and the nucleus of the tractus solitarius
(see Figure 2–4) (cranial nerves VII, IX, and X) interact with other nuclei of the cranial nerves (V, IX, X,
and XII).