Page 151 - Functional impairment and cues for rehabilitation of head and neck cancer patients -

INTRODUCTION
Swallowing is a complex mechanism facilitated by over thirty muscles of the head and neck area (1, 2). Coordination of these muscles facilitate the four phases of swallowing (i.e., oral preparatory phase, oral phase, oropharyngeal phase, and esophageal phase) (2). Interruption of this mechanism, dysphagia, is a common problem after treatment for head and neck cancer (HNC) (3-6). This is frequently caused by impaired laryngeal elevation, mainly accomplished by the suprahyoid muscles, and/or pharyngeal constriction, accomplished by the pharyngeal muscles (7-10). Impairment of the pharyngeal constrictors results in impaired bolus transportation, and reduced laryngeal elevation results in both impaired laryngeal closure and reduced opening of the upper esophageal sphincter (7-9). This results in an increased risk for aspiration and pharyngeal residue. Also, tongue strength, which plays an important role in moving the food bolus from the oral cavity into the pharynx, can be reduced (11). Trismus is another highly prevalent effect of head and neck cancer treatment, which can negatively impact chew function (12).
Swallowing muscle strength exercises are known to be effective in restoring swallowing function (13-16). These exercises are designed to target the abovementioned causes of functional impairment (2). A weakness of these standard swallowing exercises is that exercise intensity can only be modified by changing the number of repetitions or the duration of the contraction whereas the optimal strengthening of muscles requires progressive overload (17). Therefore, we recently developed the Swallow Exercise Aid (SEA), a tool enabling three different muscle strength swallowing exercises with progressive load to activate the suprahyoid, tongue, pharyngeal and jaw opening musculature, with the objective to improve effectiveness of the strength training (18). The feasibility and effectiveness of the SEA have been studied with healthy individuals as well as with dysphagia patients, with positive results for exercise compliance and effectiveness (18, 19). However, precise knowledge on which muscles are activated while performing swallowing exercises, especially with the SEA, is still lacking.
Muscle activation causes an altered water distribution within the muscle, which can be detected as increased transverse relaxation time constant by means of Magnetic Resonance Imaging (MRI) (20, 21). Transverse relaxation time constants, also known as T2-values, are a measure of the life-time of the transverse magnetization of water protons within a voxel. T2 mapping of muscles, derived from muscle functional-MRI (mfMRI) scans, is a non-invasive quantitative technique which can be used to visualize muscle activation patterns during/at the end of an exercise session (22-24). The technique has been validated to demonstrate muscle activation in a variety of muscle exercises including those of the lower limb and core as well as those of the head and neck area (10, 20, 21, 24-34).
The objective of this explorative biomechanical study is to investigate which muscles are activated during swallowing muscle strength exercises with the SEA (chin tuck against resistance, jaw opening against resistance and effortful swallow) and without an exercise tool providing external load (conventional effortful swallow, Masako and Shaker) by means of mfMRI.
6
Muscle activation during swallowing exercises
149