Advances in the Treatment of Dysphagia in Neurological Disorders: A Review of Current Evidence and Future Considerations

Figures & data

Table 1 Different Stages of Swallowing, Muscle Involvement and Innervation at Each Stage and Potential Treatment Options for Disorders at Each Stage

Stages of Swallowing	Action	Muscles Involved	Innervation	Potential Treatment Options*
Oral preparatory	Voluntary Fluid: containment of fluid within oral cavity Solid: mastication to break down larger bolus and mix it with saliva to form a cohesive bolus	Orbicularis oris Buccinator Risorius Lip elevators Lip depressors Superior and inferior longitudinal muscles of tongue Transverse and vertical muscles of tongue Genioglossus Hyoglossus Styloglossus Palatoglossus Temporal Masseter Lateral pterygoid Medial pterygoid	Facial Nerve (CN VII) Glossopharyngeal (CN IX) Hypoglossal (CN XII)	Bolus texture modification Rehabilitative exercises that involve lip and tongue muscles^+ Rehabilitative exercises with biofeedback NMES TRP channel agonists
Oral propulsive	Voluntary Positioning the bolus of fluid on the superior surface of the tongue The tip of the tongue then forces the bolus toward the pharynx.
Pharyngeal	Involuntary Soft palate elevates to seal the opening of nasopharynx Vocal cords and arytenoids abduct to seal the opening of trachea Epiglottis inverts to cover the laryngeal vestibule Hyoid bone and larynx move superiorly and anteriorly which widens and elevates the pharynx and the upper esophageal sphincter (UES)	Tensor veli palatine Palatoglossus Palatopharyngeus Levator veli palatine Musculus uvulae Anterior digastric Geniohyoid Stylohyoid Styloglossus Superior, middle and inferior pharyngeal constrictors Palatopharyngeus Palatoglossus	Mandibular branch of trigeminal nerve (CN V3) Pharyngeal branch of vagus nerve (CN X) Inferior alveolar nerve Hypoglossal nerve (CN XII) Facial nerve (CN VII) Vagus nerve (CN X)	Rehabilitative exercises with biofeedback EMST CTAR NMES TRP channel agonists PES rTMS tDCS
Esophageal	Bolus travel downward along the esophagus by esophageal peristalsis and gravity Lower esophageal sphincter relaxes, allowing bolus to enter the stomach	Cricopharyngeus upper esophageal sphincter lower esophageal sphincter	Vagus nerve (CN X)	Esophageal endoprosthetic stents to expand the esophagus in patients with esophageal strictures

Notes: *Treatment options that have been discussed in this review. ⁺Clinical efficacy to improve swallowing has not been established.

Abbreviations: CN, cranial nerve; NMES, neuromuscular electrical stimulation; TRP, transient receptor potential, EMST, expiratory muscle strengthening training; CTAR, chin-tuck against resistance; PES, pharyngeal electrical stimulation; rTMS, repetitive transcranial magnetic stimulation; tDCS, transcranial direct current stimulation.

Table 2 Comparison Between the Characteristics of Repetitive Transcranial Magnetic Stimulation (rTMS) and Transcranial Direct Current Stimulation (tDCS)

		rTMS and tDCS
Similarities		No surgical procedures involved
		Capable of inducing neuroplasticity in the human swallowing system
		Induced neuroplasticity is associated with changes in swallowing behaviour
		Capable of reversing “virtual lesion” in the swallowing motor cortex
		Showed preliminary treatment benefits for patients with post-stroke dysphagia
		rTMS	tDCS
Differences	Proposed mechanism of action	Electromagnetic induction High intensity of induced current causes depolarization of nerve cells and triggers action potential	Direct electric current Low intensity current alters the threshold of membrane polarization
	Safety	May induce seizure, but the risk is extremely low (< 0.03%) with no permanent damage	Transient adverse effects such as itchiness on scalp, burning sensation and headache
	Size of equipment	Bulky	Small and portable
	Cost	Relatively expensive (Approximately USD 35,000)	Relatively cheap (Approximately USD 1500)