Influence of altered tongue contour and position on
deglutitive pharyngeal and UES function
G. N.
Ali,
I. J.
Cook,
T. M.
Laundl,
K. L.
Wallace, and
D. J.
De
Carle
Department of Gastroenterology, St. George Hospital, University
of New South Wales, Sydney 2217, Australia
 |
ABSTRACT |
The potential
influence of altered lingual position and contour during the bolus
loading phase of the swallow in mediating the swallowed bolus
volume-dependent regulation of upper esophageal sphincter (UES)
relaxation and opening was studied in 15 healthy volunteers using
simultaneous videoradiography and manometry. A maxillary dental splint
modulated tongue deformity during the early oral phase of deglutition.
We examined the effect of the splint and swallowed bolus density on
bolus volume-dependent changes in the timing of events in the swallow
sequence and on hypopharyngeal intrabolus and midpharyngeal pressures.
Peak midpharyngeal pressure (P = 0.001) and hypopharyngeal intrabolus pressure
(P = 0.04) were significantly reduced
by the splint. The normal volume-dependent earlier onset of sphincter
relaxation and opening was preserved with the splint in situ. The
splint significantly delayed the onset of hyoid motion and UES
relaxation and opening without influencing transit times or total
swallow duration. Alterations in tongue contour and position reduce
intrabolus pressure and pharyngeal contraction without influencing
normal bolus volume-dependent regulation of timing of UES relaxation
and opening.
receptor; afferent; oral; pharynx; upper esophageal sphincter; radiology; manometry; physiology
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INTRODUCTION |
AFFERENT INPUT FROM ORAL and pharyngeal receptors may
be important in regulating the temporal relationships among motor
events during the oral-pharyngeal swallow. The sequence of motor events is not stereotyped, and the timing of hyoid motion and upper esophageal sphincter (UES) relaxation and opening can be modified by alterations in the characteristics of the swallowed bolus such as volume or viscosity (7, 8, 14). Blockade of all oral-pharyngeal mucosal receptors
by topical anesthesia does not influence the temporal relationships
among the deglutitive oral-pharyngeal motor events, suggesting that
these volume- and viscosity-dependent temporal shifts are not mediated
by tactile mucosal sensory receptors (2). It is possible that other
receptors deep to the mucosa, such as mechanoreceptors, might mediate
the observed temporal shifts. Alterations in tongue contour and
position have been shown to serve important functions of bolus
containment and loading before bolus propulsion (15). These processes
involve deformation of the tongue at swallow initiation and are to an
extent dependent on bolus volume (16). It is possible that a sensory
cue that is modified by bolus variables might emanate from lingual
mechanoreceptors at swallow initiation. We hypothesized that
modification of bolus forces on the tongue, at or shortly after swallow
initiation, influences bolus volume-dependent temporal events and that
lingual mechanoreceptor activity is likely to be involved in this
process. The aim of this study was to determine whether experimental
modulation of tongue contour and position and thus lingual
mechanoreceptor activity mediates volume-dependent changes in swallow
coordination. We used this experimental procedure on the premise that
it is likely to alter mechanoreceptor activity. Hence a positive
finding would support the hypothesis that mechanoreceptor activity is involved in volume-dependent changes in swallow
coordination.
 |
METHODS |
Subjects.
We studied two groups of healthy volunteers recruited by advertisement
from the community. Group 1 consisted
of 6 healthy subjects (mean age 27.2 ± 6.3 yr) in whom we confirmed
that the maxillary splint caused lingual deformity and
group 2 consisted of 15 healthy
volunteers (mean age 30 ± 11 yr) in whom the physiological effects
of dental splints were examined. All were carefully screened, and none
had swallowing difficulties, medical illnesses, or were taking any
medications. Ethical approval was granted by Southern Sydney Area
Health Service Ethics Committee, and all subjects gave written informed
consent.
Videoradiography.
Subjects were studied by simultaneous videoradiography and manometry as
previously described (7). Briefly, subjects were studied seated, and
images of bolus swallows were recorded in the lateral projections using
a 9-in. Toshiba (Kawasaki, Japan) image intensifier. Fluoroscopic
images were recorded on videotape at 25 frames/s by a VHS video
recorder (Panasonic, AG6500, Osaka, Japan) for later analysis. The
correction factor for magnification was determined before each study by
placing two metallic markers set 3 cm apart in the field of the image
intensifier, above the subject's head but in the plane of the UES.
Subjects swallowed duplicate 5- and 10-ml boluses of two different
densities: 1) high-density liquid
barium suspension [250% (wt/vol), E-Z-HD, E-Z EM, Westbury,
NY] and 2) a low-density
water-soluble iodinated radiopaque contrast MD-Gastroview (Mallinckrodt
Medical, St. Louis, MO). The rationale for using two densities was to
examine the effect of bolus weight on the tongue. All boluses were
delivered to the mouth by a syringe. Included in the field of view were the incisor teeth anteriorly, hard palate superiorly, cervical spine
posteriorly, and proximal cervical esophagus inferiorly.
Manometry.
Pharyngeal pressures were measured with a catheter combining three
solid-state transducers (Gaeltec, Dunvegan, Isle of Skye, Scotland)
measuring pharyngeal pressures and a 6-cm perfused sleeve assembly
(Dentsleeve, Belair, South Australia) measuring UES pressure. The
solid- state catheter (OD 2.3 mm) was inlaid into a six-lumen silicon
rubber-polyvinyl chloride perfused manometric catheter (ID each lumen
0.51 mm). The overall catheter diameter was 6 mm, and the sleeve
assembly distal to the transducers had a 5 × 3 mm oval
cross section to maintain its anteroposterior orientation within the
UES (13). The manometric assembly was passed transnasally, and the
transducers and the sleeve were orientated posteriorly with the sleeve
straddling the UES. The posterior orientation of the transducers was
readily verified radiographically. The solid-state transducers were
spaced 3 cm apart with the middle transducer lying at the level of the
valleculae in the midpharynx and the distal transducer lying at the
upper margin of the sleeve in the hypopharynx, just proximal to the UES
at the time of maximal sphincter ascent during swallowing. Four
perfused side holes, spaced at 1.5-cm intervals in the pharynx with the
distal side hole located at the midsleeve position, were used to aid in
positioning the sleeve such that its midpoint was in the center of the
UES high-pressure zone at rest. The sleeve assembly and side holes were
perfused by a low-compliance pneumohydraulic perfusion system at 0.6 ml/min, and UES pressures were registered by external transducers (Spectramed Medical Products), and all signals were amplified and
digitized at 200 Hz by preamplifiers (Neomedix Systems, Sydney, Australia) and recorded on a Macintosh computer (Apple, Cupertino, CA)
using Gastromac software (Neomedix Systems). The height-adjustable perfusion pump was positioned so that the external transducers were
level with the midsleeve position. All pressures were referenced to
basal hypopharyngeal (atmospheric) pressure. A purpose-built, video-digital timer unit (Practel Sales, Holden Hill, South Australia) imprinted simultaneously the elapsed time on the video images in
hundredths of seconds and a signal on the pressure tracing each whole
second, to permit precise temporal correlation of video images with
pressure.
Validation experiment.
A validation experiment was conducted in the six group
1 subjects in whom we examined radiologically the
tongue surface contour relative to the palate to confirm that a
maxillary splint did exaggerate tongue deformity for a given bolus
volume. In these six subjects we applied three tantalum
markers (3-mm diam) to the ventral surface of the tongue in the
midsagittal plane with denture adhesive (Poly-grip,
Stafford-Miller, New South Wales, Australia). The anterior tantalum
marker was placed 0.5 cm from the tongue tip, the posterior marker was
positioned adjacent to the last lower molar tooth, and the middle
marker was placed midway between the other two. A reference marker was
placed on the hard palate in the midline (Fig.
1A).
We then recorded lateral videoradiographic images of duplicate 5- and
10-ml water swallows to determine splint-induced alteration in tongue
contour at rest and during oral bolus delivery. Rays from the hard
palate reference marker were traced to each tongue marker, and
distances along these rays were measured at three time points:
preswallow rest position (T1); at swallow onset (T2), defined as the initial movement of the tongue tip
against the posterior surface of the maxillary incisors; and at the mid oral phase (T3), 200 ms after swallow onset.

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Fig. 1.
Effect of maxillary splint on tongue surface contour relative to palate
before and during tongue action for 5-ml water bolus swallow.
A: rays from reference marker on hard
palate were traced to each of 3 tantalum markers applied to ventral
surface of tongue in midsagittal plane. Distances along these rays were
measured at 3 time points: T1 (rest position), T2
(swallow onset, or initial movement of tongue tip against posterior
surface of maxillary incisors), and T3 (mid oral phase, or
T2 + 200 ms). At T1 with maxillary
splint in situ, distance between reference and middle and posterior
tongue markers was greater than would be predicted by thickness of
splint. This increased palatolingual separation was significantly more
pronounced posteriorly at rest
(* P = 0.02) and at
T2 (* P = 0.02).
Effect of splint on deformity of posterior tongue had diminished at
middle oral phase T3 (P = 0.08).
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Experimental protocol.
Swallows in each of the 15 group 2 subjects were
recorded under control conditions (no oral splint) and separately under
two experimental conditions, after insertion of either a mandibular splint or a maxillary splint. Full arch mandibular and maxillary splints were made from Hydroplastic (Tak Systems, Boston, MA), a custom
formulated plastic that softened in hot water to enable molding. A
"U-shaped" mandibular splint of uniform thickness covered the
occlusal, lingual, and labial surfaces of the lower teeth. The
maxillary splint covered the mucosa of the hard palate in addition to
covering the upper teeth. The maxillary splint served to reduce the
supralingual, subpalatal space, thereby exaggerating bolus forces on
the tongue for a given bolus volume and altering tongue contour.
Because the maxillary splint had the potential to influence
extralingual mechanoreceptors (e.g., periodontal and temporomandibular
joint proprioceptors) and increase salivation, the purpose of making
the same measurements with the mandibular splint was to control for
these potential nonlingual confounders, as the mandibular splint has
the same mechanical influences on the mouth as the maxillary splint
without influencing the supralingual, subpalatal space.
Data analysis.
The timing of swallow events was referenced to the swallow onset,
defined as the initial movement of the tongue tip against the posterior
surface of the maxillary incisors. All these temporal measures have
been defined previously (7). UES opening, closure, and duration of UES
flow were defined fluoroscopically (6). UES relaxation onset was
defined as the time point when the basal UES pressure began to fall
abruptly. Maximum UES relaxation was defined as the point in time when
the UES relaxation profile ceased to fall rapidly and leveled off.
Because the proximal sleeve margin projects into the hypopharynx, the
sleeve registers prematurely the apparent termination of UES relaxation
(14). Accordingly, termination of UES relaxation was measured from the
tracing recorded by the side hole 1.5 cm distal to the proximal sleeve
margin, which was seen fluoroscopically to lie within the UES at the
time of sphincter closure. We measured the onset of superior and
anterior motion of the hyoid and larynx as well as timing of peak
anterosuperior motion of these structures. Total swallow duration was
defined as the interval between swallow onset and UES relaxation. Peak pharyngeal pressure was measured at the two distal solid-state transducers positioned in the hypopharynx and the midpharynx. Hypopharyngeal intrabolus pressure measured at the side hole
immediately proximal to the UES represents the pressure within the
bolus as it traverses the sphincter (7).
Duplicate values for each subject were averaged before calculation of
group mean data for each volume swallowed. Statistical inferences were
made regarding the bolus volume effect, splint effect, and
splint-volume interaction using a two-way mixed design analysis of
variance for repeated measures (3). All values, both numeric and
graphic, are represented as means ± SE unless stated otherwise.
 |
RESULTS |
The lingual marker study confirmed that, with maxillary splint in situ,
the distance between the reference and tongue markers was greater than
would be predicted by the thickness of the splint. This increased
palatolingual separation was more pronounced posteriorly and at rest
(P = 0.02). This effect was also
significant at swallow onset T2
(P = 0.02) but diminished as the oral
phase progressed (Fig. 1). The effects were comparable for 5- and 10-ml
water boluses.
With the mandibular splint in situ the timing of swallow events did not
differ from that under control conditions. Hence all subsequent
comparisons were made between control state (i.e., no splint) and
maxillary splint in situ. Under control conditions the timing of UES
relaxation onset and UES opening relative to the swallow onset occurred
earlier with increasing swallowed bolus volume (Table
1), and this effect was preserved with the
maxillary splint in place (Fig.
2). However, for any given
bolus volume the maxillary splint caused a delay in the onset of hyoid
motion, the onset and termination of UES relaxation, and UES opening
and closure (Fig. 3). This effect was
statistically significant with low-density bolus swallows
(P < 0.006) but was less marked and did not reach statistical significance with high-density bolus swallows. Hence the volume-dependent earlier timing of UES relaxation and opening was preserved for both bolus densities, whereas for low-density boluses, the maxillary splint causes a rightward
temporal shift in these variables. The timing of UES
relaxation onset, maximum relaxation, and opening did not differ
significantly between high- and low-density swallowed boluses,
indicating that density per se did not influence coordination (Fig.
4).

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Fig. 2.
Bolus volume-dependent swallow coordination referenced to swallow
onset. Shown here schematically are group mean timings in response to
2- and 10-ml high-density barium bolus swallows. Arrows under stylized
manometric profile of upper esophageal sphincter (UES) pressure curves
represent UES relaxation onset, maximum relaxation, and termination of
relaxation. Note that under control conditions there is
volume-dependent earlier onset of timing of UES relaxation onset and
UES opening (P < 0.03). Slope of
dashed lines connecting bottom 2 curves confirms that this effect was
preserved in presence of maxillary splint
(P < 0.03).
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Fig. 3.
Effect of maxillary splint on swallow coordination referenced to
swallow onset. In this example, swallows of 2- and 10-ml low-density
boluses are measured. When bolus volume is held constant, maxillary
splint alone caused significant delay in onset of anterior hyoid
motion, UES relaxation onset, and UES opening
(P < 0.006 for 2- and 10-ml
boluses).
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Fig. 4.
Effect of modification of bolus density on UES coordination when
referenced to swallow onset. Group mean data in response to 10-ml
high-density barium boluses and low-density boluses are shown. There
was no significant difference in swallow coordination between high- and
low-density boluses.
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Oral and pharyngeal transit times, total swallow duration, and
pharyngeal clearance times were not significantly influenced by the
maxillary splint (Fig. 5). Because the
splints occupied space within the oral cavity, we examined whether this
effect might cause earlier entry of the bolus head into the pharynx. The timing of the arrival of the bolus head into the pharynx was not
significantly influenced by either splint.

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Fig. 5.
Effect of mandibular and maxillary splints on regional transit times
and pharyngeal clearance time. Data for 10-ml barium bolus swallows
shown. Neither total swallow duration, oral and pharyngeal transit
times, nor pharyngeal clearance time were influenced by splints.
Presence of oral splints did not result in premature entry of bolus
head into pharynx.
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Bolus volume did not influence peak pharyngeal pressure. Midpharyngeal,
but not hypopharyngeal, peak pressure was significantly reduced by both
splints during high-density barium bolus swallows (P < 0.01; Table
2). Hypopharyngeal intrabolus
pressure was also significantly reduced by both maxillary and
mandibular splints during high-density barium boluses
(P < 0.05; Table 2) but not during
low-density boluses.
 |
DISCUSSION |
In this study we hypothesized that modification of bolus forces on the
tongue, at or shortly after swallow initiation, influences bolus
volume-dependent temporal events and that lingual mechanoreceptor activity is likely to be involved in this process. We demonstrated that
the presence of a maxillary splint during the swallow alters tongue
contour during the early stages of the swallow. The maxillary splint
delays the onset of anterior hyoid motion, UES relaxation, and UES
opening without influencing bolus transit times or total swallow
duration. However, contrary to our initial hypothesis, the maxillary
splint did not influence the normal bolus volume-dependent earlier
onset of anterior hyoid motion, UES relaxation, or UES opening.
Hence the splint is capable of modifying neuromuscular coordination, and this effect is not a result of altered bolus transit.
We have also shown that both maxillary and mandibular splints reduce
pharyngeal pressures.
The normal control of swallowing involves afferent and
efferent pathways. The medullary swallow center is responsible for the
coordination and sequencing of the entire swallow (21, 22). The timing
of onset of hyoid motion, an event occurring early in the swallow
sequence when the bolus is still in the mouth, is bolus volume
dependent (7, 8, 14). This observation suggests that lingual afferents
are likely to be mediating this effect. The mouth is not merely a motor
organ but is densely innervated by afferent receptors sensitive to
tactile and pressure stimuli (9). We have recently shown that neither
the ability of the individual to initiate a normal pharyngeal swallow
response nor temporal relationship among swallow events are dependent
on the integrity of oral-pharyngeal mucosal tactile receptors (2). These observations suggest that mechanoreceptors are likely to mediate
volume-dependent regulation of UES timing. Oral and pharyngeal mechanoreceptors have been identified histologically (11, 17, 18, 23,
26), as have tongue and periodontal proprioceptors (1, 24). In this
study we manipulated bolus density and supralingual capacity to
modulate the extent of tongue deformity independent of bolus
volume. We hypothesized that either increased bolus density or the
maxillary splint, or both, would result in earlier onset of temporal
swallow events because of increased lingual deformity or bolus forces
on the tongue for a given volume. Contrary to our prediction, bolus
density had no effect. Furthermore, the maxillary splint did delay UES
relaxation and opening, despite the total swallow duration remaining
unchanged, but had no effect on the normal volume-dependent earlier
onset of anterior hyoid motion, UES relaxation, or UES opening. The
former finding may reflect inadequate differences in density between
bolus type. We were able to confirm that the maxillary splint did
exaggerate anterior and middle tongue deformity without influencing
bolus transit and that the splint did delay the onset of UES relaxation and UES opening. However, our results do not permit us to conclude whether this response is simply due to mechanical effects or to reflex
modulation by lingual or other mechanoreceptors because we had no way
of quantifying mechanoreceptor stimulation. Hence our findings do not
support the hypothesis that modulation of tongue contour and position,
and thereby lingual mechanoreceptor activity, mediates volume-dependent
changes in UES relaxation and opening. The possibility that receptors
distal to the faucial pillars might be implicated cannot be ruled out,
but this is an unlikely explanation because the response seems to be
mediated very early in the swallow sequence (7, 14). If it were
dependent on stimulation of pharyngeal receptors by the bolus, it is
doubtful whether there would be sufficient time for this to take place because the bolus head enters the oropharynx before onset of anterior hyoid motion for bolus volumes >5 ml.
An alternative explanation is that the observed effects, such
as delayed hyoid motion and delayed UES relaxation, might not be
reflexive but rather mechanically mediated. The possibility exists that
the new tongue position at swallow onset might alter the tension on the
hyoid exerted by the hyoglossus thereby delaying anterior hyoid motion
effected by genioglossus and anterior belly of digastric muscle.
Hyolaryngeal traction force does contribute to UES opening (7, 12) and
delay in the onset of this force or reduction of its magnitude could
possibly delay the timing of manometric UES relaxation and sphincter
opening.
With either maxillary or mandibular splint in situ, we showed a
significant reduction in peak midpharyngeal but not hypopharyngeal contraction amplitude during swallowing. Unlike UES relaxation and
opening pharyngeal peristaltic velocity, contraction duration, and
maximal contraction amplitude are stereotyped among swallow volumes
(16). As distinct from the hypopharynx the midpharyngeal chamber is
bound anteriorly by the tongue base. Augmentation of supralingual,
retrolingual, and hypopharyngeal peak pressures by bolus viscosity
suggests that bolus propulsion is a function of both tongue and
pharyngeal constrictor activity (10, 25). Alterations in mandibular
position by a maxillary splint can influence tongue function (5), and
electromyographic recordings confirm that changes in the mandibular
position have a definite effect on tongue activity (19). These
observations indicate that alterations in mandibular position, with or
without altered tongue activity, may influence pharyngeal propulsive
forces.
The finding of reduced hypopharyngeal intrabolus pressure is also
likely to be due to modification of lingual forces by the splint.
During pharyngeal bolus propulsion the initial phase of the intrabolus
pressure domain is generated by tongue forces, whereas the later
segment is due to a combination of tongue and pharyngeal constrictor
forces (4, 20). The intrabolus pressure domain is similar in the
retrolingual and hypopharyngeal regions (10). The reduction of the
intrabolus pressure domain in our study was uniform and therefore not
delineating the exact contribution of the tongue in the generation of
this pressure domain.
Although both splints reduced hypopharyngeal intrabolus and
midpharyngeal peak pressures, the pressures observed under experimental conditions still lay within the normal range. It is likely that the
magnitude of change was insufficient to influence bolus transit time.
Alternatively, the absence of any effect on transit time might reflect
activation of nonlingual mechanoreceptors. The changes in propulsive
forces demonstrated in our study may have implications for bolus
transport in patients with oral prostheses and dentures, because,
although unknown, it is highly likely that maxillary dentures may have
similar effects. Therefore any attenuation of propulsive forces during
deglutition with such devices might further impair bolus transport in
the context of preexisting compromise of pharyngeal function.
 |
ACKNOWLEDGEMENTS |
This study was supported by a project grant from the National
Health and Medical Research Council (NHMRC) of Australia. G. N. Ali is
a recipient of an NHMRC Australia Postgraduate Research Scholarship.
 |
FOOTNOTES |
Address reprint requests to I. J. Cook.
Received 18 January 1995; accepted in final form 1 August 1997.
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