Swallowing is a complex, precise sequence of movements that is fundamentally mediated by brainstem mechanisms, but volitionally modulated by cortical processes. Although swallowing biomechanics are fairly well defined, little is known about the underlying pathophysiological mechanisms that lead to biomechanical impairments due to peripheral or central neurological damage. Behavioural rehabilitation approaches for dysphagia focus primarily on weakness as a presenting aetiology. This assumption is largely due to limitations in differential diagnosis of underlying pathophysiology. Research in the motor speech and limb literature suggests that peripheral muscle damage results primarily in weakness (decreased force generation), while central neurological lesions may be associated with impaired skill (decreased spatiotemporal precision of movement), with or without weakness. There is a need for accurate and specific diagnostic techniques in swallowing, so that improved and targeted rehabilitation strategies can be provided to patients with dysphagia. The aim of this research programme was to clinically classify patients with neurogenic dysphagia due to stroke or myopathy into subtypes based on objective measures of swallowing-related strength and movement precision.

Study 1 is a methodological study investigating the range of submental muscle activity utilised during minimum-, regular- and maximum-effort swallowing. Skill-based training and assessment protocols targeting movement precision in swallowing have used targets placed at submaximal levels of muscle contraction, to avoid any possible strengthening effects that might occur with maximum effort swallowing. Regular effort swallowing has decreased amplitude of submental surface electromyography (sEMG) activity compared to maximum effort swallowing. However, it is unknown whether, and to what extent, individuals can volitionally reduce magnitude of muscle contraction below that of regular effort swallowing. This information is important so that targets are not set below these minimal effort requirements. Forty-three healthy adults (22 female) representing four age groups (20-39, 40-59, 60-79, and 80+ years) participated in the study. They were verbally cued to swallow saliva and 5 mL water boluses using maximum, regular, and minimum levels of effort, in randomised order. Maximum peak amplitude and duration of each swallow were measured using sEMG. Results demonstrate that, on average, the minimum sEMG amplitude needed to generate functional swallowing was 31% of the participant’s maximum swallowing muscle activity, suggesting that submaximal biofeedback targets used in skill-based protocols should not be set lower than this threshold. In addition, magnitude and duration of muscle activity during regular swallowing were more similar to minimum effort swallowing than maximum effort swallowing, highlighting the possible functional relevance of submaximal swallowing over maximal effort swallowing in training paradigms.

In Study 2, healthy controls (n=40) and patients with dysphagia due to stroke (n=55) and myopathy (n=19) participated in a novel clinical assessment developed to differentiate between strength and movement precision impairments in swallowing. These groups were chosen because they were likely to have specific patterns of swallowing pathophysiology based on lesion location. They were assessed on the following four measures: swallowing strength, jawopening strength, swallowing movement precision, and jaw-opening movement precision. Submental muscle peak sEMG amplitude during effortful swallowing was utilised as a proxy for swallowing strength, while jaw-opening isometric strength was measured using dynamometry to provide comparative information from a volitional, non-swallowing task. Movement precision was measured as 1) hit rate, and 2) error in timing and force of submental muscle activation to place the peak of the sEMG signal in an on-screen target, during swallowing and non-swallowing jaw-opening tasks. Results indicated that compared to healthy controls, stroke patients demonstrated impaired performance on all strength and movement precision tests (p < .01) except for swallowing amplitude error, while patients with myopathy were impaired on strength tests only (p < .01). Hierarchical cluster analysis assigned participants to one of four clusters based on test performance. Cluster 1 contained primarily healthy controls and presented with better performance on strength and precision tests compared to other clusters. A second cluster comprised both myopathic and stroke patients, and was characterised by decreased performance on strength tasks but relatively intact precision. Stroke patients were further assigned to a third cluster (reduced performance in strength and jaw precision tests) or fourth cluster (deficits in strength and swallowing precision tests). Measures of movement precision were better able to classify participants into clusters, with prediction accuracy probabilities of 73 – 89%, compared to strength measures. Results from this study reveal that several subtypes of swallowing pathophysiology may be identified after stroke. It is possible that these different clusters of patients with dysphagia would benefit from different rehabilitation approaches. The novel clinical assessment was able to differentiate between groups that were expected to have distinct patterns of strength and movement precision impairments, suggesting potential as an adjunct clinical test for differential diagnosis of underlying pathophysiology. Assessment of movement precision in swallowing may be an important but overlooked aspect of rehabilitation that should be further explored in controlled studies.

In clinical practice, measures of hyoid movement on videofluoroscopic swallowing studies (VFSS) are sometimes used to make judgements about swallowing strength, although the relationship between biomechanical measures of hyoid movement and underlying pathophysiology are unclear. Study 3 was an initial attempt to explore this relationship. Eight stroke patients who participated in the novel strength and movement precision assessment in Study 2 also participated in a swallowing assessment of 5 mL liquid and 5 mL puree using VFSS. Biomechanical measures of hyoid displacement, hyoid burst duration, hyoid burst velocity, and stage transition duration obtained from VFSS were correlated with physiological measures of strength and movement precision from dynamometry and sEMG. Overall, hyoid trajectory was not specifically associated with measures of strength nor movement precision, suggesting that there are many complex factors contributing towards hyoid movement, and it may be difficult to infer physiology from visualisation of hyoid movement on VFSS.

Results of these research studies challenge the prevailing assumption that reduced force generation is the predominant cause of dysphagia, particularly after central neurological damage such as stroke. This thesis identifies the presence of several subgroups in patients with post-stroke dysphagia, and provides characterisation of each subgroup’s impairment patterns. We propose that decreased movement precision is one of the possible underlying mechanisms of swallowing impairment that may co-occur with weakness. Initial feasibility of a clinical assessment of strength and movement precision has been established, setting the framework for further research in the physiological, rather than the purely biomechanical, assessment of dysphagia. Improving the accuracy and specificity of diagnosis of swallowing pathophysiology is fundamental to the effective management of dysphagia.