Maneuver-dependent differences in pharyngeal pressures were observed primarily in the UES, not in the velopharynx or tongue base. Thus, our hypotheses were partially supported as we anticipated that altering the relative position of the larynx, pharyngeal walls, and tongue may lead to pressure changes in the pharynx at the level of the tongue base and UES, but little change to the velopharynx. However, the results do not completely support our assumptions that the velopharynx is insulated from the changes in anatomic relationship which occur with postural maneuvers, as durational changes were noted. Somewhat contrary to our predictions for the region of the tongue base, maximal tongue base pressures were the highest for the neutral position, followed by head turn and the lowest for the chin tuck. There was no change in rate of pressure rise and there was no change in absolute pressure gradient, but an interesting shift in the entire gradient values toward lower pressures with both of the maneuvers appeared.
The absence of more notable changes at the tongue base region should not be interpreted as a universal effect as it will be important to evaluate this feature in a disordered subject population. Further, we studied this in a small number of participants and a small sample size can lead to beta error in results interpretation. However, in another study, a similar lack of effect during chin tuck was reported in hypopharyngeal intra-bolus pressure and duration in patients with moderate to severe dysphagia (5
). In contrast, head rotation has also been shown to increase
hypopharyngeal (valleculae and pyriform sinus) pressure when the head is rotated toward the pyriform sinus occupied by the catheter, but decrease when the head is rotated away from the catheter (15
). The bolus was directed away from the side of head rotation which would infer that the intra-bolus pressure was best recorded with head rotation away from the catheter (15
). It should be noted that this study used a unidirectional sensor, and thus their high pressure findings may be more related to catheter position rather than true intra-bolus pressures. In the current study, we recorded circumferential pressures, which eliminates directionality of pressure recording and this may account for the discrepancy in hypopharyngeal pressures among studies.
Perhaps of most interest were our observed differences in the duration of pressure events. The duration of time above baseline pressures for the velopharyngeal region was significantly longer for head turn and discernibly longer for the chin tuck. These durations coincide with longer time of minimum pressure (opening time) in the UES, which is not surprising as the velopharyngeal port would usually remain sealed during UES opening to facilitate bolus flow toward the esophagus. This infers either a passive or active interaction between these two distant pharyngeal sites during swallow. However, even as individual components of the swallow showed durational change, the total time of swallow was unaffected (). Duration of tongue base pressure above baseline was longer for head turn, but actually shorter for chin tuck, although these differences were not significant. It would be interesting to evaluate this in a disordered population, as we speculate that some changes to the duration of tongue base activity may be vulnerable to disease processes that affect tongue strength and timing.
Pressure and duration effects were observed in the UES region for both maneuvers. Pre-swallow maximum pressures were the greatest in the neutral position and a head turn led to a significant pressure decrease, with pre swallow mean maximum pressures averaging 227 mmHg in the neutral position and 118 mmHg with head turn. This is consistent with the study of normal subjects by Logemann et al., where an 18 mmHg mean drop in UES pressure was noted with head rotation (1
). Our post-swallow pressures were greater with the head turn and significantly lower with the chin tuck as compared to neutral. Both maneuvers increased the duration of UES opening, though this measure did not reach statistical significance. These findings are in accord with previous work using a sleeve catheter in the UES (1
). According to radiographic studies, both head turn and chin tuck alter cricopharyngeal position, facilitating UES opening and bolus passage into the esophagus (15
). As such, these maneuvers may have caused passive changes in UES physiology via structural change.
Our data roughly correspond with those presented in previous studies using conventional manometry (), with a few key differences. Maximum pharyngeal pressure, defined in this study as tongue base pressure, was higher than in previous studies (3
). This could be attributed to HRM measuring pressure across a greater length of the pharynx than traditional manometry, as more sensors allow for increased measurement precision. Additionally, HRM may detect higher pressures in asymmetric regions which are undetectable using unidirectional manometers. UES opening time, defined in this study as the time between the pre-swallow pressure peak and post-swallow pressure peak, was longer than UES relaxation time reported elsewhere (3
). We chose to use a peak to peak definition as it reflects the entire period of sphincter opening including the times of transition from closed to open and open to closed. Others have used the points of pressure decrease and increase equal to half the UES baseline pressure (14
), pressure curve analysis to identify transition points between bolus and contractile pressures (19
), or the time of low pressure between the high pressure peaks produced by a single recording catheter placed at the level of the UES baseline high pressure zone, the ‘M’ wave (8
). As we were focusing on intrasubject changes, we believe our method was simple, reliable, and appropriate for the research questions. Further, our ability to select a group of sensors covering the entire UES region, as opposed to a single sensor somewhere within the region, most likely accounts for these differences.
Data presented by other manometric studies.
Intersubject variability probably partially accounts for some of the non-significant findings as we identified a wide range of pressure events even in this small set of normal healthy swallowers. It is likely that this variability is a characteristic of human swallowing and will need to be considered when working with dysphagia patients. Subtle disruptions in a unique swallow pattern will lead to a dysphagia which may be difficult to characterize with isolated pressure recordings. It is possible that these maneuvers would indeed facilitate functional pressure changes in disordered individuals with abnormal pressures.
Although topical anesthesia was used in this study to diminish participant discomfort, we do not believe it significantly altered swallow physiology with regard to our measurements. Omitting this in pilot experiments led to increased gagging and resting UES pressure, confounding data collection. As swallowing is a sensorimotor phenomenon, impairing afferent nerves in the pharynx could potentially alter swallow physiology. However, mechanoreceptors in the pharynx (deep to the mucosa) are largely responsible for modulating swallow physiology (20
) and these fibers were probably not affected by our topical anesthetic. Additionally, the oral mucosa was minimally affected, and afferent information from this area is also important to swallow modulation. We believe that the trade-off for increased comfort at the expense of short-term pain/temperature afferent alteration improved the reliability of our data.
HRM provides the opportunity to evaluate pressure gradients during swallowing. However, even with high resolution capabilities, our definition of “pressure gradient” is simplified to a measure of difference in pressure along the course of known fluid movement. In a stricter sense, pressure gradient is defined as a vector quantity of a three dimensional scalar field where every point in the field has an assignable magnitude value, in this case pressure, and a determinable vector influenced by pressure of adjacent points within the field. To a certain extent, we are able to assume information about the pressure in two of the three directions due to boundary conditions at the walls of the pharynx, leaving only one direction for change to occur (the pharyngo-esophageal conduit where the HRM catheter is located). This assumption breaks down in the dysfunctional swallow where sphincteric failure at the oral cavity, nasopharynx, and larynx can modify these simplified boundary conditions.
It is also important to recognize that HRM fails to measure any of the pressure events that occur in the oral cavity during the swallow as the moving bolus and its surrounding pressure events reach the catheter with established momentum. Thus, we must keep in mind that we are measuring many of the important swallow related pressures, but not all of them.
This study examined pressure and timing measures using a small water bolus (5 ml). It would be useful to examine these same parameters in different bolus volumes and consistencies in a larger cohort of both normal and disordered subjects. With larger volumes, pressure changes may be found at the tongue base region; however, the identified changes at the UES level appear to support the use of these maneuvers regardless of the absence of an observed change in the “tongue driving force”. Though significant differences could be observed for some parameters across swallowing tasks, increasing sample size in the future may reveal more evident trends. Trends may also be more apparent in patients with dysphagia, for whom maneuvers are often necessary to restore functional swallowing. Exploring the effects of other maneuvers such as effortful swallow, Mendelsohn’s maneuver, or supraglottic swallow may also be valuable, as these maneuvers will have different effects on pharyngeal pressure patterns. Performing an objective, quantitative evaluation of various maneuvers in a variety of bolus volumes and consistencies may allow for selection of an optimal maneuver for use by individual dysphasic patients.