Aspiration occurs sporadically in the general population, but when it becomes persistent, it is classified as a pathologic condition requiring evaluation and treatment of the cause [1]. One of the causes of aspiration is cerebrovascular disease and more than half of it is silent aspiration [2,3]. Diagnosing dysphagia by conducting videofluoroscopic evaluation and providing appropriate thickness of food as management is crucial [4]. There have been continuous studies objectively and quantitatively analyzing effects of the videofluoroscopic swallowing study (VFSS) [5,6,7]. One of the studies attempted to analyze the movement of the hyolaryngeal complex three-dimensionally to quantify the difficulty of swallowing [6]. Yet, evaluating the functional status only by the movement of the hyolaryngeal complex was difficult, and the result showed that pharyngeal movement was more important than the movement of the hyoid bone. In another study, scoring was conducted on the subsections, such as lip seal, penetration, and pharyngeal residue, to quantitatively analyze the process of swallowing [7]. This study suggested that the amount of pharyngeal residue is related to aspiration, and the study method was used where the amounts of pharyngeal residue were classified into four groups as follows: no residue and residue of below 10%, below 50%, and over 50%. However, this study method had no reference. Thus, this study attempted to confirm the cutoff value of the amount of pharyngeal residue when aspiration occurs.

Facilitative therapeutic approaches to stop aspiration, such as the Shaker exercise, have been studied [8]. Subsequent studies continually presented the neuromuscular electrical stimulation (NMES) as having as much effectiveness as traditional treatments. If combined with traditional treatment methods, the swallowing ability was shown to have more improvement than traditional treatment alone [9,10,11]. However, in conducting NMES, the predictive index on the improvement of dysphagia has not yet been distinctive. Thus, there is a need for an objective index which can help to predict this. Also, pharyngeal residues of valleculae and pyriformis sinuses, which are the index of dysphagia, are predictive factors of improvement when quantitatively measured through VFSS after the VitalStim treatment [12]. The authors confirmed this through a retrospective study.

A previous study attempted to assess the amount of pharyngeal residue required for defining dysphagia. Although this proved the correlation between the two, it assessed the self-defined sections stage by stage and failed to quantitatively assess the amount of pharyngeal residue as a continuous variable [7]. However, with vague evidence of defining the section, our study attempted to quantitatively analyze the amount of residue which is clinically important. Since measuring the absolute amounts of pharyngeal residue is limited due to the variations in anatomical shape and size, we used the ratio of each patient's residue to anatomical structure as an objective parameter. As a result, although some differences existed among food types, the pharyngeal residues of the valleculae in the non-improved group were 36.5%-39.6% for solid and soft foods and 28.1% for liquid food. Also, the pharyngeal residues in the pyriformis sinuses were 31.5%-33.8% in solid and soft foods and 24.4% in liquid food. Residue of more than 1/3 of the total area for solid and soft foods and more than 1/4 for liquid food were expected to be a sign of not responding well to the treatment. Furthermore, the cutoff value of the R/T ratio of different food types was similar in solid and soft foods, but it was reduced significantly in liquid foods. This is considered to reflect the fact that people are more vulnerable to aspiration with liquid foods even with less pharyngeal residue.

There was no statistically significant difference in the decrease of R/T ratio after valleculae treatment between the improved group and the non-improved group. They were 0.062, 0.091, and 0.047 in the improved group and 0.061, 0.104, and 0.066 in the non-improved group. Degree of decrease of R/T ratios of the pyriformis sinuses also showed no statistically significant difference between the improved group and the non-improved group. They were 0.065, 0.078, and 0.058 in the improved group and 0.061, 0.104, and 0.066 in the non-improved group for solid, soft, and liquid foods, respectively.

Positive or negative correlation between valleculae and pyriformis sinuses R/T ratios was not observed in any same type of food.

According to review of literature, patients with intracranial hemorrhage showed more improvement than patients with cerebral infarction in Functional Independence Measure, Motor Assessment Scale, and gait velocity, whether or not their initial neurologic deficits were in similar levels [14,15]. However, it was impossible to confirm the difference in the level of improvement due to the etiology. A study on the predictive factors of improvement after cerebral infarction indicated that predictive improvement or degenerative factors had no statistical significance in the case of hemorrhagic transformation after cerebral infarction [16]. In this study, patients with intracranial hemorrhage were distributed more significantly in the non-improved group. Yet, in order to draw a conclusion that dysphagia shows unfavorable prognosis in intracranial hemorrhage than cerebral infarction, the design of the study should be more sophisticated. The above mentioned studies measured the degree of neurologic deficits, so the severity of dysphagia should be accurately measured and more cases should be added to show statistical significance.

Similar to other studies, the difference in sex did not affect the recovery from dysphagia, since the improvement of R/T ratio did not differ by sex as far as this study is concerned [14].

The term between the onset of stroke and the start of treatment was different from previous studies; it ranged from 20 days to 2 years. However, patients in the relatively early phase of stroke participated in this study, and interpreting and applying the results should be limited to subacute patient with dysphagia occurring due to stroke [7,8,9,10,11,16]. The treatment effectiveness of VitalStim on dysphagia due to subacute phase of stroke is considered to have clinical significance.

In the ROC curve, the pyriformis sinuses R/T ratio was similar in sensitivity, but the area under curve (AUR) and specificity were lower compared to the valleculae residue. This might be the result of error being measured with exaggeration in the smaller pyriformis sinuses area compared to valleculae area. The errors of pyriformis sinuses residual R/T ratio from analyzing only the lateral view of VFSS could be the limitations of this study, which could be more accurately measured in the AP view. Future studies are advised to measure the valleculae R/T ratio, to consider the fact that specificity of the pyriformis sinuses R/T ratio would decline when analyzing only the R/T ratio of the lateral image.

In conclusion, improvement in dysphagia after a stroke with NMES treatment (VitalStim) could be predicted by quantitatively measuring the residue per total area of valleculae and pyriformis sinuses through video-recording by VFSS.