Factors governing the timing of pharyngeal-swallow onset in healthy aging and acquired neurogenic dysphagia.  
C.M. Steele and P.H.H.M. van Lieshout
Funder: CIHR Institute of Aging

Traditional descriptions of the physiology of liquid swallowing have divided events into four stages: the oral preparatory, oral propulsive, pharyngeal and esophageal stages. In clinical settings, the timing of the onset of pharyngeal stage activity is widely accepted as a marker of swallowing integrity. The latency between arrival of the bolus in the upper oropharynx, and the onset of hyoid elevation has been proposed as a durational measure of the integrity of pharyngeal swallow timing, known as stage transition duration (STD). With discrete (i.e. single bolus) liquid swallows, STDs greater than 0.35 seconds represent a delay between arrival of bolus in the pharynx and the initiation of pharyngeal stage swallowing activity; this is considered abnormal, and is associated with a greater risk of aspiration and incomplete bolus clearance.

Understanding the temporal integrity of pharyngeal swallow onset is an essential requirement for designing effective interventions for individuals with dysphagia (swallowing difficulties). Two explanations for prolonged STDs have been proposed (either or both of which may occur in selected patients). The first explanation presumes a motor deficit, whereby the tongue fails to contain the bolus in the oral cavity. When this occurs, the bolus spills prematurely into the pharynx without intentional transfer by the tongue. The second explanation presumes a more complex sensory-motor control deficit, resulting in a problem with generation of the swallowing motor command in the swallowing central pattern generator (CPG) in the brainstem. This results in delayed initiation of pharyngeal stage swallowing events relative to lingual transfer of the bolus into the pharynx.

Delayed pharyngeal swallow is thought to be one of the most common deficits of swallowing in individuals with neurogenic disorders. Various sensory stimulation therapy techniques have been proposed for delayed pharyngeal swallow initiation, predicated on the assumption that priming or heightening sensory input to the swallowing CPG may elicit more-timely onset of the motor components of the pharyngeal swallow. The current proposal seeks to clarify the influence of two different types of sensory (bolus) stimulus manipulation on the sensory-motor process of pharyngeal swallow initiation.

Specifically, we first propose to investigate the influence of bolus flow characteristics by studying the relative contributions of bolus viscosity and density to stage transition durations. Secondly, we propose to investigate the influence of bolus taste on STDs. These influences will be studied both in healthy control subjects across the age span, and in individuals with radiographic evidence of abnormally prolonged STDs secondary to acquired neurogenic dysphagia.

Electro-magnetic midsagittal articulography (EMMA) is a non-radiographic method, used to trace structural movement inside the upper aerodigestive tract during speech or swallowing. In our laboratory, we have pioneered the use of this method for tracing and measuring the kinematic characteristics of tongue and jaw movement during swallowing {Steele & Van Lieshout, in press; Steele, 2003}. The proposed research will build upon previous studies by using EMMA in combination with time-locked submental surface electromyography (sEMG) to detect a biomechanical correlate of STD, i.e. the timing of hyoid movement, relative to events in the time histories of tongue and jaw movement.

A series of three experiments will be undertaken. Experiment one will explore variations in swallow event-timing related to bolus viscosity and density in healthy subjects, in four 15-year age-cohorts between ages 20 and 80. Experiment two will explore timing variations related to bolus taste in healthy subjects, again across the age span. Experiment three will investigate the benefits of manipulating bolus flow characteristics and bolus taste to facilitate improved swallow event-timing in individuals with radiographic confirmation of prolonged stage transition durations secondary to acquired neurogenic dysphagia.