Comparison of electroglottographic variability index in euphonic and pathological voice
In a recent study we introduced a new approach for analysis of the electroglottographic (ECG) signal. This method is based on the evalua- tion of variation of the EGG signal and its first derivative, through new software developed by the Pisan phoniatric school. This software is designed to extract quantitative indices related to the contacting and decontacting phases of the vocal folds during phonation. The software allows us to study the combined variability of vibration amplitude and velocity (i.e. the first derivative of the EGG signal). Pathological voices show a much more variable EGG signal compared to normal voices, since cordal vibration is made irregular due to the presence of glottis plane pathologies. With the aim of demonstrating the differences between normal and pathological voices relevant to combined vibration amplitude and velocity variability, we have introduced a new quantitative parameter named “variability index, VI”. We studied 95 subjects (35 normal and 60 with pathological voice); among pathologic subjects, 15 showed functional dysphonia and 45 showed organic dysphonia. Subjects affected by organic dysphonia presented: 15 bilateral vocal nodules, 15 unilateral polyps and 15 unilateral cysts. All subjects were studied with videolaryngostroboscopy; electro-acoustic parameters of the voice were analysed with the KayPENTAX CSL (Model 4500) system. The EGG signal was recorded using KAY Model 6103 connected to the CSL system. The new software for the analysis of the EGG signal allows us to obtain not only a VI total value relevant to variability during all the recording, but also partial VI values relevant to the different glottis cycle phases. In fact, plotting the amplitude variation and its first derivative on a Lissajous graph, it is possible to divide the whole glottis cycle into four phases (each represented by four quadrants on the graph): the initial vocal folds contacting activity (VI-Q1), the last phase of vocal folds contacting (VI-Q2), the first phase of vocal folds decontacting (VI-Q3) and the last phase, up to the complete decontacting of vocal folds (VI-Q4). For each quadrant, it is also possible to work out the percent variability index. By comparing the variability indices in the normal and pathological groups, we obtained the following results: the total VI was sig- nificantly higher in the pathological subjects (0.25 vs 0.18; p = 0.01); the absolute value of VI was higher in pathological subjects, although the difference was not significant (VI-Q2, 0.041 vs 0.029; VI-Q3, 0.065 vs 0.058; VI-Q4, 0.054 vs 0.052). The percent variability in the Q2 quadrant (VI-Q2%) was significantly higher in pathological subjects compared to normal subjects (0.22 vs 0.16) (p = 0.01). The results of this study confirm that our new software for analysis of EGG signal can distinguish normal voice from pathological voice based on the new quantitative parameter VI, “variability index”. Moreover, this study emphasises that the final contact phase of vocal folds is the most representative of the difference between the normal and pathological voice and shows a wider variability in terms of amplitude and vibra- tion velocity. Further studies on larger groups of subjects will be required to confirm these results and assess differences in the EGG signal among the various vocal fold pathologies.