Tongue function and swallowing in individuals with temporomandibular disorders

Abstract The tongue participates in the oral phase of swallowing by pushing the food bolus toward the oropharynx. This relationship between tongue function and swallowing is little addressed addressed in individuals with temporomandibular dysfunction (TMD). Objective: To analyze the association of functional tongue conditions on swallowing in individuals with TMD. Methodology: After approval by the Institutional Review Board, the study was conducted on 30 individuals of both sexes, aged 18 to 28 years, with TMD, and not treated for the disorder. Tongue function was assessed as to the mobility, pressure, and oral motor control. Swallowing was analyzed by clinical assessment during ingestion of solid (wafer biscuit) and liquid (water). Data regarding mobility and swallowing were collected using the orofacial myofunctional evaluation protocol. Tongue pressure was measured by the Iowa Oral Performance Instrument, during elevation, protrusion, swallowing, and resistance test. The oral motor control was assessed by the oral diadochokinesis (DDK) test by rapid and repeated emissions of syllables “ta” and “ka”. Data were statistically analyzed by the Spearman correlation coefficient, at a significance level of 5%. Results: Relationships were found between tongue function and swallowing for the following aspects: mobility (r=0.741), pressure in protrusion (r=-0.366), swallowing of saliva (r=-0.499), mean DDK rate in emissions “ta” (r=-0.424) and “ka” (r=-0.446), and mean DDK period in emissions “ta” (r=0.424) and “ka” (r=0.446). Thus, the greater the change in tongue mobility, the lower the tongue pressure in protrusion and swallowing of saliva, the lower the emissions per second, the longer the mean time between vocalizations, and the worse the swallowing of individuals with TMD. Conclusion: The functional conditions of the tongue regarding mobility, pressure, and oral DDK were associated with swallowing in individuals with TMD.


Introduction
The tongue and the temporomandibular joint (TMJ) are components of the stomatognathic system, and the musculature regarding the action of these structures should work in coordination for the adequate performance of orofacial functions, such as swallowing. 1 In this function, the tongue participates in the oral phase by pushing the food bolus toward the oropharynx, applying force against the palate with sufficient magnitude and timing, initially at the anterior and then at the posterior region. 2-5 This pressure against the palate developed by the tongue is influenced by the food bolus consistency 6,7 and is temporally related with the movement of hyoid and mandible during the process. 8,9 This description of the tongue function in orofacial functions refers to the normal physiological conditions. However, one of the clinical conditions that impair the stomatognathic system is the temporomandibular dysfunction (TMD), which is related with a combination of alterations affecting the TMJ, masticatory muscles, and associated structures. 10 The TMD may cause compensations and adaptations to the stomatognathic functions when present, 11,12 indicating the need to understand the myofunctional alterations by detailed analysis, to define the adequate treatment planning. 13 Table 1.
Despite the few alterations observed, a relationship was found between tongue function and swallowing (Table 2). It showed a strong positive correlation with mobility, indicating that, with the alteration in mobility, the worse the swallowing performance. There was also weak negative correlation between tongue pressure in protrusion and the score in swallowing, as well as moderate negative correlation between tongue pressure in swallowing of saliva and the score in swallowing, demonstrating that the lower the tongue pressure, the worse the performance of swallowing for individuals with TMD.   structures without the need for treatment, unless the patient presents some complaint. 25 However, no longterm studies assessed and followed the performance of these individuals' tongue and swallowing function to confirm the orofacial myofunctional adjustments in this population. Also, the degree of TMD severity was low, indicating mild symptomatology.
The aforementioned characteristics may justify the low score observed in the assessment of tongue mobility, as well as swallowing, since in studies that found myofunctional alteration, the included individuals were patients searching for treatment 11,12,17 and presenting dysfunction for a long time. 18,26,27 Regarding the tongue pressure, the values found   The lower speed of repetition of syllables "ta" and "ka", consequently with higher mean of the period of these repetitions, may be associated with worse control of tongue movements. 30 The negative correlation between the score in the assessment of swallowing and the mean DDK rate, and the positive correlation with the mean DDK period indicated that, with the alteration in swallowing, fewer emissions were produced per second and the longer the mean time was between vocalizations, for both the anterior and posterior tongue regions, by the emission of "ta" and "ka", respectively. Since the tongue contact with the palate is synchronically performed on the anterior, middle, and posterior regions during swallowing, 2-4 the alteration in the oral motor control regarding the tongue may impair the performance of swallowing in individuals with TMD.
As observed, there was association between tongue function regarding mobility, pressure and oral DDK with swallowing. Considering that the literature indicates the occurrence of myofunctional disorder as a compensatory mechanism of the altered stomatognathic system in the presence of TMD, 11,12 even though individuals in this study did not present significant damage to the analyzed functions, maybe, because of the associations observed, they might be performing myofunctional adaptations of the stomatognathic system to achieve better functional performance, despite the mild symptoms.
Further studies should be conducted to continue this investigation, considering groups with specific dysfunction classifications, separately analyzed, as well as compared with a control group, and should follow these individuals in the long-term to investigate the functional adjustments performed and confirm the impact on orofacial functions.
Even though no important damage was observed on the tongue and swallowing functions compared with the literature, when these aspects were related, an association was observed for tongue mobility and pressure and oral DDK. Thus, when therapeutic intervention is indicated for a patient with characteristics that are similar to those of this study, these aspects may be considered when assessing the treatment plan, especially for orofacial myofunctional therapy.

Conclusion
The functional conditions of the tongue regarding mobility, pressure, and oral DDK was associated with swallowing in individuals with TMD.