Tongue control for swallowing in Parkinson’s disease and normal aging: Effects of bolus texture, swallowing rate, and medication
P.H.H.M. van Lieshout and C.M. Steele
Funder: CIHR

Swallowing impairments (dysphagia) occur in approximately half of individuals with Parkinson’s disease (PD). The most commonly reported features of dysphagia in PD are impaired tongue control, increased durations of bolus transit, and reduced strength and amplitude of tongue movement during bolus propulsion to the pharynx. These deficits increase the risk of aspiration (entry of material into the airway) due to prolonged presence of food and liquid in the pharynx, both before the pharyngeal swallow (due to poor tongue control) and after the swallow (weak tongue propulsion leaves residues behind).

Symptoms are more marked with liquids than with semi-solids or solids; for this reason, clinical management frequently involves bolus texture modification and swallowing rate control strategies to minimize risks. However, the response of the tongue to these interventions remains unstudied and we lack an understanding of the mechanisms by which improved safety and efficiency of oral stages in swallowing can be achieved. Dysphagia has significant health and quality-of-life consequences for individuals with PD. Those who aspirate are at increased risk for developing pneumonia, which is reported to have the highest mortality risk of all comorbidities in PD. Even in early stages of PD, swallowing problems are reported to increase the risk of malnutrition; in advanced phases of the disease, weight loss can be striking. Poor nutritional status is associated with decreased immunity, higher morbidity and mortality. In short, swallowing problems have a major impact on quality-of-life in PD, which is particularly prevalent in people above 50 years of age.

Although swallowing problems are not an immediate concern for the elderly population, recent studies of aging report decreased flexibility of the oral motor system for swallowing. It is important that changes in swallowing motor function in individuals with PD be studied in the context of normal age-related changes. The tongue plays a primary role in swallowing and is responsible both for the controlled transfer of material from the mouth to the pharynx, and for the initiation of the bolus transport sequence which carries the bolus through the pharynx to the oesophagus. How aging affects tongue movements during swallowing is largely unknown. The proposed investigation is a series of related studies, exploring the separate influences of bolus consistency, bolus delivery-rate, aging, and Parkinson’s disease on tongue movement patterns during swallowing.

We will use electromagnetic midsagittal articulography to measure tongue movements with high spatial and temporal resolution. In our first study, we will investigate the influence of bolus consistency on tongue movements using 6 different liquids with well-defined rheological and material characteristics (density and viscosity). In our second study, we will measure the effects of swallowing rate on tongue movements. In both these studies, we will compare the data from subjects with PD to data from an age- and gender-matched control group; data from both of these subject groups will be further compared to data from a group of healthy, gender-matched controls under the age of 30. These comparisons will enable us to clearly discriminate between effects of normal aging and disease (PD). Individuals with PD are typically prescribed medication to enhance their motor function.

A variety of drugs is used at different stages of the disease, with Levodopa being the most common. However, the long-term use of Levodopa can itself lead to abnormal involuntary movements (dyskinesias) and motor fluctuations; motor function of the tongue for swallowing may be directly affected by anti-Parkinsonian medications. We therefore propose a third study in the PD subject group, which will compare tongue movement patterns in swallowing, in on- and off-medication states within individual subjects. Finally, in a fourth related study, the PD subjects will be followed longitudinally for 4 years, repeating the previously described data collection procedures every 6 months, to document potential changes in tongue behaviours during swallowing over time. This will allow us to substantiate presumed links between tongue behaviours in PD and swallowing function.

This investigation will be the first to address all of these issues in a controlled manner, and will establish an empirical basis for swallowing intervention strategies in PD with respect to tongue control.