The mechanism of swallowing laterality is unclear. Studies of swallowing laterality are mostly aimed at people who do not have swallowing difficulty, so a study which analyses swallowing laterality as a disease faces an immaterial condition. In the research by Logemann et al.2
on 6 individuals without health problems, 2 had left laterality and 4 did not have it. Moreover, Seta et al.,4
studying 167 individuals without health problems, found that 7% of them had right laterality and 35% of them had left laterality, but 58% of them did not have laterality. Mosier et al.6
reported that the cerebral cortex's activity of the left hemisphere during swallowing was more dominant than the right hemisphere, using the functional MRI. Then, Seta et al.4
suggested, citing the results of the research performed by Mosier et al.6
that the existence of the dominant and non-dominant cerebral hemisphere could be applied in the swallowing function. Mikushi et al.3
proposed the mechanism of the swallowing laterality based on the research about swallowing laterality in Wallenberg syndrome. First, because of the discordance of cricopharyngeal muscles, disability in the opening of the pharynx occurred on the lesion side and, as a consequence, food material passed through the pharynx of the healthy side, resulting in laterality. Second, one side of the pharynx became narrower because of neurogenic atrophy of the cricopharyngeal muscles, causing food material to pass through the pharynx on the healthy side. As a result, laterality developed. Third, unlike the previous two mechanisms, it may be that food material passed through the affected side because it had less resistance than the healthy side due to flaccid palsy on one side of the cricopharyngeal muscles.
This is the first study to our knowledge which has researched the swallowing laterality of hemiplegic patient with swallowing difficulty caused by stroke. We found the direction of hemiplegia was not related to the direction of swallowing laterality and an aspect of swallowing laterality does not significantly affect the severity of swallowing difficulty. The existing literature on swallowing difficulty recommended using the compensatory technique of rotating the head towards the laterality side or tilting it in the opposite direction. In the light of this study, the existing compensatory techniques such as head tilt and head rotation can improve swallowing laterality, but the possibility of no improvement can be inferred, so further study is needed.
The most marked difference between prior studies aimed at disease free people and the results of our research targeted at hemiplegic patients with stroke was that swallowing laterality had occurred more on the right side, even divided and examined with right hemiplegia and left hemiplegia. Supposing that patients before contraction of a stroke mostly had left laterality based on the results of previous research from disease free people, our result suggested the possibility, that a stroke may change the aspect of swallowing laterality. But as the relationship between the direction of hemiplegia and swallowing laterality was not observed, and as evaluation of swallowing laterality before the stroke was realistically impossible, it was difficult to infer what mechanism made the change of swallowing laterality.
The research also studied the influence of swallowing laterality on recovery from swallowing difficulty. The existence and the direction of swallowing laterality did not noticeably influence the severity and recovery of swallowing difficulty.
The limitations of this study are as follows: First, the individuals who participated in the research in order to confirm statistically significant results were too few. Second, there was no evaluation of the swallowing state of patients before the occurrence of stroke. In future research, studying what differences exist between disease free people and patients with stroke, should be done to verify how strokes are involved in swallowing laterality.