Changes in both pressure and timing events were observed as bolus volume was varied. Discernible effects were seen at all three areas of interest, demonstrating the widespread effects that bolus conditions have on pharyngeal pressure generation. Generally, as bolus volume increased, the maximum pressure and pressure rise time increased at the velopharynx and in the UES, there was a rise in the minimum pressure during bolus transport. Conversely, pressures at the tongue base decreased with increased bolus volume. With regard to timing, pressure duration above baseline in the velopharynx, UES opening duration, and the total swallow duration increased with larger bolus volumes (). No durational changes were observed at the tongue base. However, despite the overall trend in those directions, not all findings were statistically significant. Statistically significant differences were observed for total swallow duration, UES opening time, and minimum UES pressure. Additional variables such as maximum velopharyngeal pressure, duration of velopharyngeal pressure above baseline, and maximum tongue base pressure showed a correlation with bolus volume and approached, but did not reach significance at the 0.05 level.
Figure 3 Spatiotemporal plots displaying swallows at each volume from one subject. As bolus volume increased, velopharyngeal pressure (box with solid lines) and upper esophageal sphincter activity time (arrow) increased while tongue base pressure decreased (box (more ...)
Increased maximum pressure and duration of pressure above baseline in the velopharynx likely achieves two main goals. First, this reflects the need to ensure a tight seal at the velopharynx when swallowing a large bolus to prevent nasal regurgitation. Second, these changes were predictably accompanied by an increase in UES opening duration as bolus volume increased. Elevated velopharyngeal pressure, combined with a relaxed cricopharyngeal muscle and consequently open UES creates a large pressure gradient favoring bolus propulsion toward the esophagus. No change in velopharyngeal pressure rise rate was detected, attributable to offsetting increases in both maximum pressure and rise time.
Contrary to previous studies that did not observe a change in tongue base pressure,12,25
we found tongue base pressure had an inverse relationship with bolus volume. Both previous investigations used a pressure catheter with a single sensor. It is possible that this sensor did not capture relevant pressure data which could have revealed the negative trend found in this study using HRM. A discernible decrease in tongue base pressure rise rate was also observed, likely due to a decrease in maximum pressure while pressure rise time stayed relatively constant. Lower pressures may be due to larger boluses capitalizing on their gravitational force,13
decreasing the necessary muscular force required for successful swallowing. Probably the most influential physiologic change involves movement of the hyoid bone. Degree of hyoid bone movement has been strongly associated with bolus volume.16,17,21,26
Increases in the anterosuperior excursion of the hyoid certainly affects the shape and position of the tongue base (and at the same time, UES opening). This creates a larger cavity during bolus flow, which may be measurable in the oropharynx as lower pressures with larger volumes.
Both UES opening duration and total swallow duration had a direct relationship with bolus volume. Longer and wider opening of the UES accommodates a larger bolus, as reported in previous videofluoroscopic studies.14,15,17,19
While the effect of bolus volume on UES opening duration is consistently reported, the effect on total swallow duration has been disputed. Previous manometric studies using only three sensors reported a decrease in swallow duration, although the small number of sensors may not have captured all relevant pressure data. The increased number of sensors employed with HRM (up to 36) provides a more comprehensive assessment of both timing and pressure events, eliminating potential “blind spots” which may occur if using a manometric catheter with one to three sensors. Not all 36 sensors were used in our analysis, as maximum pharynx length was 12.6 cm which would require only 13 sensors to span the pharynx. The high number of sensors ensures that when structures such as the UES move during swallowing, they do not move beyond the catheter. As total swallow duration can reliably be determined using videofluoroscopy, it would be interesting to evaluate the effect of increasing bolus volume using simultaneous HRM and videofluoroscopy. Previous videofluoroscopic findings are not consistent, as some studies report an increase in total swallow duration,18
while others did not find a significant change.14,20
Coupling HRM with videofluoroscopy may help clarify this issue.
Sex and pharynx length were incorporated into our analysis as covaraiates, as changes in pharyngeal pressures may be dependent on pharynx size. No differences in pressure or timing events were observed between males and females, similar to the findings from Takasaki et al.7
Pharynx length was significantly greater in males, and of the three regions of interest, differences were most evident at the tongue base. Increased duration of tongue base pressure above baseline approached significance for the male participants and a similar trend occurred for rise rate.
A notable distinction between our study and previous investigations of bolus volume is the measurement of UES opening time, defined manometrically as the time lapse between pre-opening and post-closure maximum UES pressures. Previous studies have recorded UES opening duration11,13,15,24
or cricopharyngeal relaxation time,22
for which similar values have been reported. Our UES opening duration, however, is significantly longer. Others used points along the declining and then rising pressure slopes which results in shorter measured durations,13,15,24
yet revealed similar increasing duration with increasing bolus volume. When recording cricopharyngeal EMG signals, though, no bolus effect was seen between one, five, and ten ml water volumes.22
Cricopharyngeal EMG identifies a robust post-closure electromyographic signal burst, but there is no pre-opening parallel muscle activity signal burst. Only loss of the continuous, low baseline muscle activity can be observed. While the post-closure EMG signal likely coincides with UES post-closure peak pressure pattern, the pre-opening pressure and pressure events are possibly due in part to non-cricopharyngeal muscle events such as laryngeal positioning adjacent to cervical spine soft tissues and laryngopharyngeal posturing prior to bolus delivery. The variation before cricopharyngeal relaxation measurable with EMG and consequent UES opening measured with manometry may account for the differences.
Interestingly, minimum UES pressure increased as bolus volume increased. It is important to note that this increase only approached zero, with all average minimum UES pressures recorded below atmospheric pressure and thus as a negative number. Dantas et al. found a similar positive correlation between minimum UES pressure and bolus volume, though all pressures were greater than zero.14
The negative UES pressure is thought to be generated in part by laryngeal elevation and serves to move a bolus into the esophagus. It is possible that with larger bolus volumes, less negative pressure at the bolus head is needed as the positive pressure at the bolus tail and increased opening duration are sufficient for the bolus to traverse the pharynx. However, we are recording a complex event in which bolus volume changes appear to be accommodated for primarily with prolonged duration of low pressure at the UES, yet complete accommodation is lacking, leading to the measured increase in minimum pressure during bolus transport through the UES.
Two aspects of this study will be the subject of future investigations seeking to improve HRM analysis. First, while the measurement of regional maximum pressure can provide valuable information on swallowing physiology and can readily be compared to previous studies, it does not utilize the full potential of the HRM multi-sensory array. Analyzing pressure patterns, rather than simply pressure values, may provide a more comprehensive evaluation of the pharyngeal swallow. Second, the pressures measured in this study were likely a result of luminal closure and may not necessarily represent bolus driving forces. Measuring bolus driving forces would require implementation of a bulb sensor as in Pouderoux et al.,25
simultaneous HRM and videofluoroscopy, and further evaluation of pressure gradients, which are the force underlying bolus propulsion.
Knowledge of how swallowing physiology changes with varying bolus conditions can be used during swallowing therapy and rehabilitation. At larger volumes, the pattern of greater velopharyngeal pressure, less tongue base pressure, and increased duration of UES opening is characteristic of typical swallowing. Therefore, changing bolus size may alter physiology in ways that can compensate for or exacerbate deficits. Although this strategy is already employed in routine therapy, these data provide further evidence for its use. This preliminary HRM study provides some new insights and confirms results of prior studies yet leaves many questions unanswered. Additional studies using simultaneous videofluoroscopy or electromyography could address many of these basic and clinically important questions.