An Analysis of Lingual Contribution to Submental Surface Electromyographic Measures and Pharyngeal Pressure During Effortful Swallow

Participants 

We recruited 20 healthy female research participants between the ages of 20 and 35 through advertisement at a university to provide data for this project. Participants reported no history of dysphagia or neurologic disease. Informed consent was obtained from all participants prior to initiating data collection; ethics approval was obtained by the appropriate regional health ethics review board.

Procedure 

To avoid confusion between tasks and to minimize the potential for fatigue due to a lengthy recording session, we collected data in 2 sessions of approximately 90 minutes in duration completed within a 1-week period. Research methods for both days were identical with the exception of instruction in how swallowing maneuvers were to be executed. For both sessions, data were collected in a Swallowing Rehabilitation Research Laboratory located in a medical facility. After information was provided and questions answered, electrodes were secured to measure the activity of the anterior suprahyoid and floor of mouth muscles during swallowing. Active triode surface electrodes were positioned lengthwise to the undersurface of the chin between the spine of the mandible and the superior palpable surface of the thyroid cartilage, that is, activity of the collective floor of mouth and anterior suprahyoid muscles during swallowing. The ground electrode was positioned laterally. The resulting rectified and averaged signal was displayed on a computer monitor within view of the research participant. Before proceeding with further sensor placement or data collection, the subjects were given demonstration and directions concerning the performance of a noneffortful saliva swallow and a contrasting effortful saliva swallow. For the effortful swallow task, instructions were provided for 2 strategies of execution, which would be counterbalanced across the 2 data collection sessions. In 1 strategy condition, participants were instructed that during execution of the effortful swallow they should restrict tongue-to-palate contact and should utilize the floor of mouth and pharyngeal muscles to complete the tasks (“As you swallow, I want you to squeeze hard with the muscles of your throat, but not use your tongue to generate extra force”). In the alternative strategy condition, they were instructed that during execution of maneuvers they should exaggerate tongue-to-palate contact (“As you swallow, push really hard with your tongue”). Participants were allowed to practice these tasks using submental surface electromyographic output to guide performance and mastery. No participant reported difficulty with task completion at the end of the training session.

Subsequent to this instructional period for both sessions, we placed a solid state manometric catheter with 3 pressure transducers and a pair of bipolar surface electromyography electrodesa (diameter, 2.1mm) into an unanesthetized nares, using a lubricant gel to facilitate passage. As the catheter reached the upper pharynx, identified by resistance at the posterior pharyngeal wall, the participants were asked to rapidly ingest a glass of water through a straw. In doing so, the catheter was swallowed into the proximal esophagus. Each participant was asked to swallow until the catheter had been pulled down approximately 40cm as measured from the tip of the nose. The catheter was then slowly pulled out again until all sensors were positioned appropriately. During this procedure the subjects were asked not to swallow, not to speak and not to cough. A pull-through technique was utilized to guide catheter placement with the third manometric sensor at the upper borders of the high pressure zone of the UES; correct placement was confirmed by clear visualization of the M wave on swallowing (fig 1).14, 15 In this resting location, the uppermost manometric sensor was positioned approximately even with the tip of the epiglottis, the second manometric sensor was placed 13mm below in mid-pharynx and the third sensor was positioned at the superior border of the UES. Manometric sensors measuring 2×5mm were oriented toward the posterior pharyngeal wall. Refer to figure 2 for a radiographic representation of catheter placement at rest. The catheter was taped securely to the external nose to limit movement of the catheter in a superior to inferior orientation. Additionally, a measurement in millimeters was taken from the tip of the nose to the end of the catheter to facilitate consistency in placement during the second recording session. Visualization of indelible orientation markers on the superior, posterior aspect of the catheter was utilized to assure continued orientation of the sensors to the posterior pharyngeal wall. Although the lowermost UES sensor was utilized to facilitate placement of the catheter, behavior of the UES was not the focus of this research and thus data from this sensor were not included in the final analysis. Finally, a strip of soft plastic incorporating 3 orolingual pressure sensors was secured to the palate using a small amount of polymer tissue adhesive.b The most anterior sensor was placed at the junction of the central incisors and the alveolar ridge. The middle sensor was approximately mid-palate and the most posterior sensor was approximately at the junction of the hard and soft palates. Data from the most anterior sensor were not analyzed for this study.

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Fig 1. Sample waveform obtained from pharyngeal manometry. Location of the lowermost sensor in the UES producing high resting amplitude and a characteristic M wave with slightly increased pressure immediately preceding* and following§ the drop in pressure with UES opening.

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Fig 2. Lateral pharyngeal radiograph with manometric catheter in situ. Three manometric sensors are identified: (A) proximal manometric sensor, (B) mid-pharyngeal sensor, and (C) UES sensor.

Data Collection 

Each subject then completed 15 repetitions of 3 research tasks: noneffortful (saliva) swallows, effortful (saliva) swallows, and the Mendelsohn maneuver, as instructed during the training period. These 15 trials were completed in 3 sets of 5 repetitions to avoid within-task fatigue. The order of task completion was randomized within each research participant. Data from the Mendelsohn maneuver will not be described in this article. All data were collected and analyzed using the integrated Kay Elemetrics Digital Swallowing Workstationc (suprahyoid surface electromyography, orolingual manometry, pharyngeal manometry).

The manometric data (both orolingual and pharyngeal) and the submental surface electromyographic data were displayed to the researcher but not the participant during data collection and were stored on the swallowing workstation for subsequent analysis. Confidentiality was assured by assigning each participant a coded identification number. Peak amplitudes and durations of manometric and electromyographic recordings were extracted from the data files and subjected to statistical analysis.

Data Analysis 

We collected data from this study over 2 sessions per participant, counterbalanced for the strategy used to perform the effortful swallow task (with or without tongue-to-palate emphasis). To account for potential intersession variability, normalization of the data within subjects and across the 2 sessions was required. Each participant’s raw data for surface electromyography and orolingual and pharyngeal pressures were transformed to a value relative to their own greatest amplitude obtained during noneffortful swallows. For example, if the raw amplitude of a participant’s maximum noneffortful swallow was measured at 120μV, this value would be re-expressed as 100μV; subsequently a raw effortful swallow amplitude of 180μV would be re-expressed as 150μV. Mean transformed values for each variable were then calculated across each set of 5 repeated swallows. To appreciate the impact of manner of execution on the effortful swallow task, each effortful swallow datapoint was converted to a difference score from the mean noneffortful swallow value prior to statistical analysis. Repeated-measures analyses of variance (ANOVAs) with factors of strategy (no tongue emphasis vs tongue emphasis) and set (3 task repetitions) were performed on the effortful swallow difference score data using SPSS.d The α criterion for statistical significance was set at α equal to .05. Box’s M and Mauchly tests were conducted for all ANOVA results, to identify any violations of the assumptions of homogeneity of variance, covariance matrix circularity, and compound symmetry. No violations of these assumptions were detected.

Noneffortful Saliva Swallows 

Noneffortful saliva swallows were the reference task in this investigation, and were performed in the same manner on both data collection days. Descriptive statistics for noneffortful saliva swallows are shown in table 1, together with descriptive statistics for the effortful swallow difference scores under each condition (no tongue emphasis, tongue emphasis). Within the noneffortful swallow data, surface electromyographic amplitudes did not differ significantly across sessions (F1,19=.138, P=.714). No statistically significant differences were found in the amplitudes of noneffortful swallow orolingual pressures across sessions at either sensor location: mid (F1,19=0.7, P=.413) or posterior (F1,19=.133, P=.72). Similarly, pharyngeal pressures for the noneffortful swallow task did not differ significantly across sessions at either sensor location: upper (F1,19=.133, P=.719) or lower (F1,19=1.003, P=.329). These analyses also confirmed the absence of any statistically significant differences across set repetitions for 4 of the 5 variables: surface electromyography (F2,18=1.467, P=.257); mid tongue pressure (F2,18=3.81, P=.04); posterior tongue pressure (F2,18=.530, P=.598); upper pharyngeal pressure (F2,18=1.173, P=.332); and lower pharyngeal pressure (F2,18=.02, P=.980). No statistically significant set by session interactions were found. On this basis, set was removed from the model for the subsequent statistical analyses.

Table 1. Descriptive Statistics

	Variable	Mean ± SE	95% Confidence Interval
	Surface electromyography (μv)
	Noneffortful swallow	64.14±3.17	54.25to62.14
	Effortful swallow difference scores
	No tongue emphasis	21.73±7.93	5.14to38.32
	Tongue emphasis	120.81±26.30	65.76to175.86
	Mid-tongue pressure
	Noneffortful swallow	51.33±2.06	47.20to55.46
	Effortful swallow difference scores
	No tongue emphasis	−15.85±5.83	−28.06to−3.64
	Tongue emphasis	116.06±16.09	82.37to149.74
	Posterior tongue pressure
	Noneffortful swallow	60.00±74	56.52to63.47
	Effortful swallow difference scores
	No tongue emphasis	3.44±7.00	−11.21to18.10
	Tongue emphasis	85.97±9.80	65.45to106.48
	Upper pharyngeal pressure (mmHg)
	Noneffortful swallow	62.14±1.95	58.24to66.03
	Effortful swallow difference scores
	No tongue emphasis	6.93±4.77	−3.05to16.92
	Tongue emphasis	20.09±4.08	11.54to28.63
	Lower pharyngeal pressure (mmHg)
	Noneffortful swallow	65.61±1.50	62.61to68.62
	Effortful swallow difference scores
	No tongue emphasis	10.21±3.53	2.82to17.61
	Tongue emphasis	19.62±6.02	7.03to32.22

Abbreviation: SE, standard error.

Effortful Swallow by Strategy 

There were no significant effects of task repetition (P range, .48−.79) and no significant interactions between task repetition and type of swallow at any sensor, suggesting that the data were stable across multiple measures (P range, .11−.83). Consequently, task repetition was not included in further analysis. Statistical analyses were then performed using a general linear model ANOVA to evaluate the effect of strategy of effortful swallow execution on pressure measurements (tongue emphasis; no tongue emphasis). Results are tabulated in table 2. Statistically significant effects of strategy were observed for all five variables. In all cases, the strategy of tongue-to-palate emphasis produced a greater change from normal swallowing pressure than the strategy of tongue-to-palate inhibition.

Table 2. General Linear Modeling ANOVA Results Comparing Strategy of Effortful Swallow Execution (tongue emphasis vs no tongue emphasis)

	Source	F	df	P
	Surface electromyography	38.45	1,118	.000
	Mid tongue pressure	166.51	1,118	.000
	Posterior tongue pressure	124.63	1,118	.000
	Upper pharyngeal pressure	11.37	1,118	.001
	Lower pharyngeal pressure	5.15	1,118	.025