Effect of electrical stimulation of tongue in rehabilitation - A systematic review

Abstract

Tongue is unique and each half of the tongue is represented bilaterally in brain and is being used as an effective interface to send signals to central nervous system. Tongue movements and tongue stimulation have been used in various types of rehabilitation. The purpose of this study is to review all articles related to tongue stimulation. All studies including reviews, case, cohort and experimental studies, which dealt with tongue stimulation, periglossal stimulation, or hypoglossal stimulation during the period between 1981 to 2020 were screened and a total of 41 studies comprising of 9 review articles, 30 experimental studies and 2 case reports were included for review. The methodology quality of the experimental studies has been analyzed using PEDro score and level of evidence measure. Electrical stimulation of the tongue was performed on normal subjects to find out the taste sensation and its similarity with other liquids. Hypoglossal stimulation has been found to stimulate the muscles of the tongue and it stiffens the posterior aspect of tongue and walls of the pharynx, thereby reducing the symptoms in obstructive sleep apnea. Stimulation of the muscles of the tongue also improves swallowing functions in dysphagia patients. Tongue stimulation, using a variety of devices and in combination with balance exercises, has also been used to improve balance in stroke, multiple sclerosis and spinal cord injury patients by inducing activity in cerebellum and brainstem, which process the balance networks. Tongue movements and tongue stimulation along with upper limb rehabilitation, have also been used in improving upper limb functions in stroke patients based on the principles of Hebbian theory. Thus, tongue stimulation along with targeted exercise program becomes a novel mode of therapy in inducing neuroplasticity and can be used in wide variety of patients in rehabilitation. This review will input to the clinicians and researcher for conducting further research on tongue stimulation. Further long-term follow-up studies can also be done to find out the neuroplastic changes in brain following tongue stimulation.