Introduction: The conventional rigid-90° and rigid-70° laryngostroboscopy has been so far considered the gold standard in assessing the vibratory behavior of the vocal folds and the glottal closure configuration during phonation. Meanwhile, this rigid laryngostroboscopy is more and more replaced by flexible chip-on-tip systems. The aim of this study was to evaluate the influence of these different endoscopic techniques on glottal closure configuration and on visibility of the complete focal fold length including anterior commissure during phonation. Methods: Twenty-one euphonic subjects were enrolled (mean age 34.6 ± 9.5; m = 10, f = 11). They were examined with the three laryngoscopic techniques (conventional rigid-90°, rigid-70°, and flexible chip-on-tip laryngoscopy during low and high voice pitch with soft and loud voice intensity). For evaluating the degree of glottal closure, a modified classification of Södersten et al. was applied and the visibility of the anterior commissure was evaluated. The correlation of the three endoscopic techniques was assessed with Cohen and Fleiss’ kappa. Results: In even low loud phonation, the rigid-90° and rigid-70° endoscopies revealed a complete closure of the glottis in only 47.6% of subjects but with flexible endoscopy in 81%. The complete vocal fold length with anterior commissure was best visible with flexible endoscopy in 90.5% in low-soft and high-soft phonation. The rigid-90° endoscopy showed a slight agreement in comparison with the flexible endoscopy in regard to the types of vocal fold closure with a Cohen’s kappa coefficient k = 0.199. The rigid-90° endoscopy showed an almost perfect agreement with k = 0.84 when compared to the rigid-70° endoscopy. The flexible endoscopy compared to the rigid-70° endoscopy showed a fair agreement with k = 0.346. Conclusion: We found mainly corresponding results in both rigid-90° and rigid-70° endoscopic techniques which can be explained by the same transoral approach with the tongue pulled out, whereas the flexible transnasal endoscopy mainly gives a better view on the anterior commissure. The influence of transorally or transnasally guided endoscopic techniques needs to be considered in interpretation of laryngostroboscopic parameters like vocal fold closure and supraglottal hyperactivity.

The glottal closure is one of the most important assessment criteria when it comes to voice diagnostics. The degree of glottal closure is associated with a wide variety of perceptive and acoustic voice characteristics. It varies depending on voice pitch and loudness. The closure of the vocal folds is furthermore affected by secondary organic changes, age- or gender-specific anatomical differences.

According to the basic protocol of the European Laryngeal Society (ELS), laryngostroboscopy applies as a gold standard in evaluation of the vocal fold closure during phonation [1]. In 1960, the foundation stone was laid by Schönhärl for describing the vibration behavior of the vocal folds during phonation [2]. Besides glottal closure, those were the vibration amplitude, the phase progression, irregularities, and the mucosal wave.

Besides all these characteristics, the description of the vocal fold closure is essential in the clinical routine of laryngology and phoniatrics [3‒5]. It is important to differentiate between physiologic and pathologic insufficient vocal fold closure.

The basic idea of this study is that during rigid endoscopy, sticking out the tongue, which is held by the examiner, changes the tension in the tissues and muscles of the larynx. This could significantly influence the glottic closure. Every examination technique should assume physiological conditions as far as possible.

In general, one can differentiate between complete and incomplete glottal closure. But it is also clinically essential to distinguish between intermembranous and intercartilaginous glottal closure. In 1990, Södersten and Lindestad [6] classified the glottal closure during phonation. In this classification, type 1 represents a complete glottal closure, whereas the other types describe nine different types of incomplete closure (shown in Fig. 1). Type 1 corresponds to “complete closure all along the vocal folds,” type 2 “incomplete closure of the cartilaginous part,” type 3 “triangular incomplete closure reaching anterior to the vocal processes,” type 4 “triangular incomplete closure of the posterior third of the vocal folds,” type 5 “two thirds of the vocal folds,” and type 6 “all along the vocal folds.” Type A corresponds to “spindle shaped incomplete closure with closure at the vocal processes,” type B “spindle shaped incomplete closure at the posterior third of the membranous folds, closure at the vocal processes,” type C “spindle shaped incomplete closure at the anterior thirds of the vocal folds” and type D “spindle shaped incomplete closure at the posterior and anterior thirds of the vocal folds, closure at the vocal processes and at the middle of the membranous portion” [6].

Fig. 1.

Original classification of glottal closure during phonation by Södersten and Lindestad [6] in 1990 and modified classification used in this study: type 1 (complete closure), type 2 (incomplete closure in the cartilaginous part), type 3 (all closure types with a combined intercartilaginous and intermembranous insufficiency) and type 4 (all closure types with only intermembranous insufficiency but complete intercartilaginous closure during phonation).

Fig. 1.

Original classification of glottal closure during phonation by Södersten and Lindestad [6] in 1990 and modified classification used in this study: type 1 (complete closure), type 2 (incomplete closure in the cartilaginous part), type 3 (all closure types with a combined intercartilaginous and intermembranous insufficiency) and type 4 (all closure types with only intermembranous insufficiency but complete intercartilaginous closure during phonation).

Close modal

The types A-C can be found in patients with a sulcus vocalis, vocal fold atrophy, presbyphonia, or scaring after vocal fold surgery [7]. So far a posterior chink was commonly found in young euphonic women and was not considered as a pathologic finding if vocal constitution has been evaluated as vocally normal taking into account voice profile measurement [6, 8‒10].

The overview of the vocal folds including the anterior commissure depends to a large extent on the setting of the vocal tract. It is well known that just sticking out the tongue changes the view of the vocal folds considerably. To counteract this problem with the rigid examination technique, endoscopes were designed with 70° instead of 90° viewing angles. The technical improvement of the flexible techniques with chip-on-tip systems seems to revolutionize laryngostroboscopy regarding better laryngeal insight and less gagging despite still high acquisition costs. The aim of this study was to evaluate the glottal closure configuration and the visibility of the complete vocal fold dimensions including the anterior commissure using either conventional rigid-90° and rigid-70° laryngostroboscopy or flexible chip-on-tip laryngostroboscopy in euphonic adults.

Subjects

This study has been approved by the Ethics Committee of the Medical University of Vienna (EK 1546/2021). Written informed consent was obtained from all participants. To compare examination techniques using rigid-90°, rigid-70°, and flexible videolaryngoscopy, 21 subjects (10 male and 11 female) with a mean age of 34.6 ± 9.5 (mean age female = 33.6 ± 9.6 and mean age male 35.6 ± 9.8) were recruited at the ENT outpatient Department of the Medical University Hospital Vienna. Subjects with dysphonic voice sound were excluded from this study.

Investigation

For performing each of the examination techniques, following endoscope from XION medical GmbH (Berlin, Germany) was used a XION 90°-degree laryngoscope, a XION 70°-degree laryngoscope, and for flexible videolaryngoscopy a XION flexible video endoscope. To assess the three videolaryngoscopy techniques regarding the types of vocal fold closure during phonation, a modified classification of Södersten and Lindestad [6] was used (shown in Fig. 1). Type 1 stayed type 1 corresponding to complete closure all along the vocal fold. Type 2 comprises all closure types with an only intercartilaginous insufficiency. Type 3 describes all closure types with a combined intercartilaginous and intermembranous insufficiency. Type 4 summarizes all closure types with only intermembranous insufficiency but complete intercartilaginous closure during phonation.

The planning of sample size was based on the study of Schneider and Bigenzahn [9], whereafter the type 1 vocal fold closure at soft phonation should be expected in at least 10% of the examined subjects. Accordingly, cases of type 2 seen in soft and loud phonation and cases with changes from type 2 to type 1 from soft to loud phonation were each expected to be at around 40%, respectively. Cases of type 3 and 4 vocal fold closure were expected to be at around 10% for each type. Based on the expected percentage of each of the four types of vocal fold closure, it resulted in the sample size of at least 20 subjects for this study.

In this study, each subject was examined randomly using each of the three laryngostroboscopic techniques mentioned above. During the examination with rigid endoscopes, the sitting subject should slightly lean forward with the trunk. The neck should be stretched forward with the head placed slightly backward. With the rigid-70° endoscope, the head needs to be placed even more backward compared to rigid-90° endoscopy. To reduce the gag reflex, we aimed for only submaximal tongue protrusion in the rigid examination methods. During flexible transnasal endoscopy, a neutral sitting position is recommended. Each subject was instructed to perform soft and loud phonation of the vowel /i:/at habitual pitch and one octave above the habitual pitch each for about 3 s three times. Parallel to the examination, samples of each laryngostroboscopic examination were recorded for later analyses using Divas software version 2.8 by XION Medical GmbH (Berlin, Germany).

The vocal fold closure of each recording was assessed and allocated to one of the 4 types of vocal fold closure mentioned before accordingly. The visibility of anterior commissure of vocal folds during phonation has also been performed in each sample. In Figure 2, the closed phase of subject 12 during all three endoscopic techniques in low/high and soft/loud phonation is shown as an example.

Fig. 2.

Closed phase of all 3 endoscopic techniques during low/high and soft/loud phonation.

Fig. 2.

Closed phase of all 3 endoscopic techniques during low/high and soft/loud phonation.

Close modal

To analyze the observed types of vocal fold closure to the corresponding examination technique, the outcomes of examination techniques were compared pairwise using the Cohen’s kappa [11]. First, the outcomes of all subjects regarding the examination techniques were compared, and then they were compared separately by gender. To compare the outcomes of all three examination techniques, Fleiss’ kappa [12] was applied. The comparison was also applied to all subjects and to each gender separately. For the pairwise comparison of the visibility of anterior commissure of vocal folds by examination technique during phonation, the Cohen’s kappa has been used. The Fless’ kappa was performed for comparing the visibility of anterior commissure of vocal folds by all three examination techniques.

For the assessment of the results of Cohen’s and Fleiss’ kappa, following corresponding ranges of kappa [13] have been applied: Kappa range <0.00 = poor agreement, kappa range 0.00–0.20 = slight agreement, kappa range 0.21–0.40 = fair agreement, kappa range 0.41–0.60 = moderate agreement, kappa range 0.61–0.80 = substantial agreement, and kappa range 0.81–1.00 almost perfect agreement.

To compare the distribution of the types of vocal fold closure observed within each examination technique between male and female subjects, Mann-Whitney U test was performed. All statistical analyses were carried out using IBM SPSS 64-bit version 27 under Microsoft® Windows 10, 64-bit.

In Table 1, the descriptive data of the four glottal closure types during the three endoscopic techniques in low-soft or low-loud and high-soft or high-loud phonation and the visibility of the anterior commissure in regard to all subjects and gender specific are shown. The best vocal fold closure was achieved in all endoscopic techniques in low-loud phonation, however, with less frequency in rigid-90° and rigid-70° endoscopies (47.6%) and higher frequency in flexible endoscopy (81.0%).

Table 1.

Descriptive statistical results of three endoscopic techniques with the distribution of vocal fold closure types (type 1–4) and the visibility of the anterior commissure in regard to different pitch and intensity levels

Distribution of type of vocal fold closure (%)Visibility of anterior commissure (%)
examination techniquesexamination conditionssubjectscomplete closureincomplete closure in the cartilaginous part /anterior to the vocal processesincomplete closure of posterior third /two-thirds /all along the vocal foldsspindle-shaped incomplete closure of vocal folds type A to Dvisiblenot visible
Rigid-90° endoscopy Low Soft All 9.5 52.4 28.6 9.5 33.3 66.7 
Male 20.0 70.0 10.0 0.0 10.0 90.0 
Female 0.0 36.4 54.5 9.1 54.5 45.5 
Loud All 47.6 38.1 4.8 9.5 19.0 81.0 
Male 70.0 20.0 10.0 0.0 10.0 90.0 
Female 27.3 54.5 9.1 9.1 27.3 72.7 
High Soft All 9.5 19.0 42.9 28.6 33.3 66.7 
Male 10.0 30.0 20.0 40.0 20.0 80.0 
Female 9.1 9.1 63.6 18.2 45.5 54.5 
Loud All 19.0 52.4 14.3 14.3 28.6 71.4 
Male 20.0 50.0 30.0 0.0 20.0 80.0 
Female 18.2 54.5 27.3 0.0 36.4 63.6 
Rigid-70° endoscopy Low Soft All 4.8 57.1 28.6 9.5 42.9 57.1 
Male 10.0 80.0 10.0 0.0 40.0 60.0 
Female 0.0 36.4 54.5 9.1 45.5 54.5 
Loud All 47.6 38.1 4.8 9.5 33.3 66.7 
Male 60.0 30.0 10.0 0.0 20.0 80.0 
Female 36.4 45.5 9.1 9.1 45.5 54.5 
High Soft All 14.3 14.3 42.9 28.6 52.4 47.6 
Male 10.0 30.0 20.0 40.0 30.0 70.0 
Female 18.2 0.0 63.6 18.2 72.7 27.3 
Loud All 28.6 42.9 14.3 14.3 42.9 57.1 
Male 20.0 50.0 30.0 0.0 20.0 80.0 
Female 36.4 36.4 27.3 0.0 63.6 36.4 
Flexible endoscopy Low Soft All 38.1 38.1 19.0 4.8 90.5 9.5 
Male 60.0 30.0 10.0 0.0 80.0 20.0 
Female 18.2 45.5 36.4 0.0 100.0 0.0 
Loud All 81.0 19.0 0.0 0.0 81.0 19.0 
Male 100.0 0.0 0.0 0.0 70.0 30.0 
Male 63.6 36.4 0.0 0.0 90.9 9.1 
High Soft All 4.8 28.6 42.9 23.8 90.5 9.5 
Male 40.0 20.0 40.0 0.0 80.0 20.0 
Female 9.1 18.2 63.6 9.1 100.0 0.0 
Loud All 61.9 19.0 9.5 9.5 81.0 19.0 
Male 80.0 10.0 10.0 0.0 70.0 30.0 
Female 45.5 27.3 18.2 9.1 90.9 9.1 
Distribution of type of vocal fold closure (%)Visibility of anterior commissure (%)
examination techniquesexamination conditionssubjectscomplete closureincomplete closure in the cartilaginous part /anterior to the vocal processesincomplete closure of posterior third /two-thirds /all along the vocal foldsspindle-shaped incomplete closure of vocal folds type A to Dvisiblenot visible
Rigid-90° endoscopy Low Soft All 9.5 52.4 28.6 9.5 33.3 66.7 
Male 20.0 70.0 10.0 0.0 10.0 90.0 
Female 0.0 36.4 54.5 9.1 54.5 45.5 
Loud All 47.6 38.1 4.8 9.5 19.0 81.0 
Male 70.0 20.0 10.0 0.0 10.0 90.0 
Female 27.3 54.5 9.1 9.1 27.3 72.7 
High Soft All 9.5 19.0 42.9 28.6 33.3 66.7 
Male 10.0 30.0 20.0 40.0 20.0 80.0 
Female 9.1 9.1 63.6 18.2 45.5 54.5 
Loud All 19.0 52.4 14.3 14.3 28.6 71.4 
Male 20.0 50.0 30.0 0.0 20.0 80.0 
Female 18.2 54.5 27.3 0.0 36.4 63.6 
Rigid-70° endoscopy Low Soft All 4.8 57.1 28.6 9.5 42.9 57.1 
Male 10.0 80.0 10.0 0.0 40.0 60.0 
Female 0.0 36.4 54.5 9.1 45.5 54.5 
Loud All 47.6 38.1 4.8 9.5 33.3 66.7 
Male 60.0 30.0 10.0 0.0 20.0 80.0 
Female 36.4 45.5 9.1 9.1 45.5 54.5 
High Soft All 14.3 14.3 42.9 28.6 52.4 47.6 
Male 10.0 30.0 20.0 40.0 30.0 70.0 
Female 18.2 0.0 63.6 18.2 72.7 27.3 
Loud All 28.6 42.9 14.3 14.3 42.9 57.1 
Male 20.0 50.0 30.0 0.0 20.0 80.0 
Female 36.4 36.4 27.3 0.0 63.6 36.4 
Flexible endoscopy Low Soft All 38.1 38.1 19.0 4.8 90.5 9.5 
Male 60.0 30.0 10.0 0.0 80.0 20.0 
Female 18.2 45.5 36.4 0.0 100.0 0.0 
Loud All 81.0 19.0 0.0 0.0 81.0 19.0 
Male 100.0 0.0 0.0 0.0 70.0 30.0 
Male 63.6 36.4 0.0 0.0 90.9 9.1 
High Soft All 4.8 28.6 42.9 23.8 90.5 9.5 
Male 40.0 20.0 40.0 0.0 80.0 20.0 
Female 9.1 18.2 63.6 9.1 100.0 0.0 
Loud All 61.9 19.0 9.5 9.5 81.0 19.0 
Male 80.0 10.0 10.0 0.0 70.0 30.0 
Female 45.5 27.3 18.2 9.1 90.9 9.1 

The anterior commissure was best visible with flexible endoscopy in low-soft phonation (90.5%) and in high-soft phonation (81.0%), whereas the anterior commissure with rigid-70° endoscopy was most frequently seen in high-soft phonation (52.4%) and with rigid-90° endoscopy during low-soft (33.3%) and high-soft phonation (33.3%). Considering all four phonation conditions, the anterior commissure can be seen more frequently with flexible endoscopy as with rigid endoscopy.

In Table 2, the results of comparisons of glottal closure type and visibility of the anterior commissure during the three endoscopic techniques are demonstrated, pairwise using the Cohen’s kappa and all three techniques together with Fleiss’ kappa, in regard of all subjects and gender specific.

Table 2.

Pairwise comparison of three endoscopic techniques with Cohen’s kappa and comparison of all three endoscopic techniques with Fleiss’ Kappa in regard to different pitch and intensity levels

Paired comparison of the examination techniquesRigid-90° endoscopy versus flexible endoscopyRigid-90° endoscopy versus rigid-70° endoscopyFlexible endoscopy versus rigid-70° endoscopy
 examination items types of vocal folds closure 
examination conditions all low high all low high all low high 
soft loud soft loud soft loud soft loud soft loud soft loud 
Cohen’s kappa coefficients Subjects all 0.199 0.325 0.385 0.378 0.306 0.840 0.921 0.846 0.931 0.860 0.346 0.257 0.385 0.456 0.400 
male −0.042 0.091 na 0.412 0.079 0.815 0.756 0.804 1.000 1.000 0.057 −0.014 na 0.412 0.079 
female 0.353 0.429 0.421 0.179 0.494 0.831 1.000 0.857 0.833 0.725 0.554 0.429 0.548 0.353 0.735 
 examination items visibility of anterior commissure 
examination conditions all low high all low high all low high 
soft loud soft loud soft loud soft loud soft loud soft loud 
 Subjects all 0.075 0.100 −0.033 0.100 0.172 0.640 0.200 0.640 0.438 0.696 0.000 0.146 −0.105 0.208 0.125 
male 0.054 0.054 −0.212 0.118 0.194 0.615 0.286 0.615 0.211 1,000 0.118 0.286 −0.129 0.194 0.194 
female 0.072 na 0.072 na 0.108 0.621 0.098 0.621 0.476 0.492 −0.185 na −0.185 na −0.170 
Paired comparison of the examination techniquesRigid-90° endoscopy versus flexible endoscopyRigid-90° endoscopy versus rigid-70° endoscopyFlexible endoscopy versus rigid-70° endoscopy
 examination items types of vocal folds closure 
examination conditions all low high all low high all low high 
soft loud soft loud soft loud soft loud soft loud soft loud 
Cohen’s kappa coefficients Subjects all 0.199 0.325 0.385 0.378 0.306 0.840 0.921 0.846 0.931 0.860 0.346 0.257 0.385 0.456 0.400 
male −0.042 0.091 na 0.412 0.079 0.815 0.756 0.804 1.000 1.000 0.057 −0.014 na 0.412 0.079 
female 0.353 0.429 0.421 0.179 0.494 0.831 1.000 0.857 0.833 0.725 0.554 0.429 0.548 0.353 0.735 
 examination items visibility of anterior commissure 
examination conditions all low high all low high all low high 
soft loud soft loud soft loud soft loud soft loud soft loud 
 Subjects all 0.075 0.100 −0.033 0.100 0.172 0.640 0.200 0.640 0.438 0.696 0.000 0.146 −0.105 0.208 0.125 
male 0.054 0.054 −0.212 0.118 0.194 0.615 0.286 0.615 0.211 1,000 0.118 0.286 −0.129 0.194 0.194 
female 0.072 na 0.072 na 0.108 0.621 0.098 0.621 0.476 0.492 −0.185 na −0.185 na −0.170 
Comparison of all examination techniquesRigid-90° endoscopy versus rigid-70° endoscopy versus flexible endoscopy
examination items types of vocal folds closure  visibility of anterior commissure 
examination conditions all low high low high  all low high low high 
 soft loud soft loud  soft loud soft loud 
Fleiss’ kappa coefficients Subjects all 0.467 0.339 0.314 0.468 0.538 0.587 0.493  0.042 −0.029 0.174 0.036 −0.029 0.149 0.238 
male 0.286 0.020 0.293 0.136 0.298 0.610 0.283  −0.005 −0.200 0.100 0.050 −0.200 0.050 0.282 
female 0.563 0.325 0.560 0.598 0.600 0.448 0.640  0.022 0.022 0.083 −0.091 0.022 0.083 0.083 
Comparison of all examination techniquesRigid-90° endoscopy versus rigid-70° endoscopy versus flexible endoscopy
examination items types of vocal folds closure  visibility of anterior commissure 
examination conditions all low high low high  all low high low high 
 soft loud soft loud  soft loud soft loud 
Fleiss’ kappa coefficients Subjects all 0.467 0.339 0.314 0.468 0.538 0.587 0.493  0.042 −0.029 0.174 0.036 −0.029 0.149 0.238 
male 0.286 0.020 0.293 0.136 0.298 0.610 0.283  −0.005 −0.200 0.100 0.050 −0.200 0.050 0.282 
female 0.563 0.325 0.560 0.598 0.600 0.448 0.640  0.022 0.022 0.083 −0.091 0.022 0.083 0.083 

na, not available

The comparison of rigid-90° endoscopy and flexible endoscopy regarding the types of vocal fold closure showed a slight agreement with a Cohen’s kappa coefficient k = 0.199. The rigid-90° endoscopy compared to the rigid-70° endoscopy showed an almost perfect agreement with k = 0.84. The flexible endoscopy compared to the rigid-70° endoscopy showed a fair agreement with k = 0.346.

All three endoscopic technics compared showed a Fleiss’ kappa coefficient k = 0.467 which comply with a moderate agreement.

The comparison of rigid-90° endoscopy and flexible endoscopy regarding the visibility of the anterior commissure showed a slight agreement with a Cohen’s kappa coefficient k = 0.075. The rigid-90° endoscopy compared to the rigid-70° endoscopy showed a substantial agreement with k = 0.64. The flexible endoscopy compared to the rigid-70° endoscopy showed a slight agreement with k = 0. The visibility of the anterior commissure compared throughout all endoscopic technics with all loudness and pitch grades showed a slight agreement with Fleiss’ kappa k = 0.042.

Table 3 demonstrates the results of the types of vocal fold closure and the visibility of the anterior commissure considering both examination technique and sex of the subjects using Mann-Whitney U test. In two cases, a significant difference was found with rigid-90° and rigid-70° endoscopy during low-soft phonation in both cases.

Table 3.

Mann-Whitney-U test comparing both genders on vocal fold closure and visibility of the anterior commissure

Examination techniquesExamination itemsExamination conditionsMann-Whitney U test±SEp value
Rigid-90° endoscopy Types of vocal fold closure Low Soft Male 85.5±13.0 0.0295 
Female 
Loud Male 77±13.0 0.1321 
Female 
High Soft Male 54±13.4 0.9725 
Female 
Loud Male 50±13.1 0.7564 
Female 
Rigid-70° endoscopy Low Soft Male 83.5±12.6 0.0430 
Female 
Loud Male 68±13.0 0.3867 
Female 
High Soft Male 52±13.4 0.8633 
Female 
Loud Male 43±13.4 0.4262 
Female 
Flexible endoscopy Low Soft Male 79.5±13.4 0.0850 
Female 
Loud Male 75±9.7 0.1734 
Male 
High Soft Male 47±13.4 0.6047 
Female 
Loud Male 73±12.3 0.2230 
Female 
Rigid-90° endoscopy Visibility of anterior commissure Low Soft Male 30.5±11.6 0.0845 
Female 
Loud Male 45.5±9.7 0.5116 
Female 
High Soft Male 41.0±11.6 0.3494 
Female 
Loud Male 46.0±11.1 0.3494 
Female 
Rigid-70° endoscopy Low Soft Male 52±12.2 0.8633 
Female 
Loud Male 41.0±11.6 0.3494 
Female 
High Soft Male 31.5±12.3 0.0986 
Female 
Loud Male 31.0±12.2 0.0986 
Female 
Flexible endoscopy Low Soft Male 44.0±7.2 0.4679 
Female 
Loud Male 43.5±9.7 0.4262 
Female 
High Soft Male 44.0±7.2 0.4679 
Female 
Loud Male 43.5±9.7 0.4262 
Female 
Examination techniquesExamination itemsExamination conditionsMann-Whitney U test±SEp value
Rigid-90° endoscopy Types of vocal fold closure Low Soft Male 85.5±13.0 0.0295 
Female 
Loud Male 77±13.0 0.1321 
Female 
High Soft Male 54±13.4 0.9725 
Female 
Loud Male 50±13.1 0.7564 
Female 
Rigid-70° endoscopy Low Soft Male 83.5±12.6 0.0430 
Female 
Loud Male 68±13.0 0.3867 
Female 
High Soft Male 52±13.4 0.8633 
Female 
Loud Male 43±13.4 0.4262 
Female 
Flexible endoscopy Low Soft Male 79.5±13.4 0.0850 
Female 
Loud Male 75±9.7 0.1734 
Male 
High Soft Male 47±13.4 0.6047 
Female 
Loud Male 73±12.3 0.2230 
Female 
Rigid-90° endoscopy Visibility of anterior commissure Low Soft Male 30.5±11.6 0.0845 
Female 
Loud Male 45.5±9.7 0.5116 
Female 
High Soft Male 41.0±11.6 0.3494 
Female 
Loud Male 46.0±11.1 0.3494 
Female 
Rigid-70° endoscopy Low Soft Male 52±12.2 0.8633 
Female 
Loud Male 41.0±11.6 0.3494 
Female 
High Soft Male 31.5±12.3 0.0986 
Female 
Loud Male 31.0±12.2 0.0986 
Female 
Flexible endoscopy Low Soft Male 44.0±7.2 0.4679 
Female 
Loud Male 43.5±9.7 0.4262 
Female 
High Soft Male 44.0±7.2 0.4679 
Female 
Loud Male 43.5±9.7 0.4262 
Female 

The aim of this descriptive study was to observe the glottal closure in euphonic voices with three different laryngostroboscopic techniques including a rigid-90° endoscope, a rigid-70° endoscope, and a flexible endoscope with chip-on-tip technology. Our main question of interest was how the use of different endoscopic techniques might impact the glottal closure configuration during phonation as well as the visibility of the complete vocal fold length and the anterior commissure. A complete glottal closure is usually considered important in evaluation of voice constitution and vocal fitness for especially voice professionals [14]. A complete glottal overview is the precondition for exact evaluation of possible organic or phonation-associated vocal fold alterations. If the glottal overview is incomplete, lesions in the anterior region of the vocal folds can be easily overlooked.

We found corresponding results in both rigid endoscopic techniques independent of the camera angle of 90° or 70° concerning glottal fold closure which can be explained by the same transoral approach with the tongue pulled out. The flexible transnasal endoscopy seems to provide a better view on the complete vocal fold length and the anterior commissure with less influence on the physiological anatomical conditions. By comparing the gender, a significant difference could be found only in low-soft phonation with more complete glottal closure in men. These results match with the results from Södersten et al. in 1995 [7]. Several studies in the past revealed an incomplete closure of the posterior glottis more often in women as a sex-related physiological finding, some of them using rigid and some using flexible fiber-optic endoscopes [6, 8‒10].

In 2003, Schneider and Bigenzahn [9] examined 520 young women with euphonic voices with regard to glottal closure configuration with rigid-2D endoscopes. During low soft phonation, only 12.5% showed a complete closure of the glottis. During loud phonation, the authors found a complete closure in 41.4%. In 35.9%, a posterior chink was found even at loud phonation. In 11.3%, the glottis was not sufficient visible or alterations of the vocal folds regarding to phonation were found.

In 2019, Cielo et al. [8] examined 56 women, aged 20–30 years, with a rigid endoscope and also found a posterior glottal gap as a predominant finding without any impact on acoustic voice quality. In 1995, Södersten et al. [10] showed similar findings but using flexible fiber-optic endoscopes. A posterior glottal chink, if only in the intercartilaginous or extending to the intermembranous part, was stated to be a normal physiologic finding in young women for no increased breathiness was detected.

These findings can mainly be explained by gender-specific anatomical differences. Friedrich and Lichtenegger [15] (1996) described the gender-specific differences from the anatomical aspect by sizing 50 larynx specimens. Among other results, they found that the average male thyroid cartilage showed an angle of 90° and the average female thyroid cartilage showed an angle of 120°. The gender-specific differences in length of the vocal folds are decisive for the extent of voice pitch, register, and the natural speech pitch. In average, the posterior glottis occupies 45% of the whole length of the glottis in women and 40% in men. So the gender-specific differences appear to have definitely an impact on the type of glottal closure mainly caused by gender-specific anatomical differences.

The influence of the different endoscopes on glottal closure and visibility of anterior commissure needs to be considered even in other laryngeal examination techniques like high-speed laryngeal imaging, although it has not found its way into clinical routine yet. As shown by Kendall [16] in 2009, the advantage of high-speed imaging is in visualizing aperiodic vocal fold vibration pattern for classic stroboscopic imaging depending on periodic vibration of the vocal folds.

In the past years, the use of flexible endoscopy for description of phonation pattern failed due to pure light and image quality [17, 18]. With the new chip-on-the-tip technology, which hosts the image sensor in the tip of the endoscope, flexible laryngostroboscopy enters the next stage and offers far better quality than fiber-optic techniques. This is a big advantage for patients with a strong gag reflex or anatomical difficulties like a high tongue base. In 2005, Low et al. [19] showed a well correlation with tongue base level, using the Mallampati classification, and the visibility of the larynx with a rigid-70° endoscope.

Many studies evaluating the glottal closure often use rigid endoscopes so far. Rigid endoscopes with 70° view angle should solve the problem with better evaluation of the anterior commissure. Using a rigid-70° endoscope, the patient needs to lean forward and elevate the chin resulting in even more laryngeal tension [20]. In this study, the rigid-70° laryngoscopy did not reveal a better glottal closure configuration but on average a better visibility of the anterior commissure compared with the rigid-90° laryngoscopy.

The visibility of the anterior commissure is mainly limited by position of the epiglottis. If the patient phonates a proper /i:/in different loudness levels, the epiglottic position slightly changes. Overall in our study, the anterior commissure was best seen in soft phonation in all three endoscopic techniques.

In the last few years, the transnasal flexible endoscopes found its way in the laryngology and voice diagnostics. Flexible transnasally introduced endoscopes allow a better overall view of the larynx and the glottis. Another advantage is the reduced gag reflex with less impact on vocal fold tension and supraglottal contraction. In rigid laryngostroboscopy, the patient is instructed to stick out the tongue which is held by the examiner. Then the rigid endoscope is inserted through the mouth and the patient has to produce an /i:/vowel. It is obvious that with flexible endoscopy through the nose a much more physiological starting position exists regarding phonation. In this way, not only a held /i:/but also other vowels and even continuing speech and singing can be observed. Although not commonly used in clinical practice, the maneuver of rotating the flexible endoscope by 180° allows additional dimension concerning visibility of the ventricles of Morgagni, the ventricular folds, the anterior commissure, and, partially, the subglottis. This maneuver was not included in this study as not routinely performed. Taking our study results into account the flexible laryngostroboscopy provides more representative data of glottal closure configuration during phonation and of glottal morphology, which depict the physiological conditions more truthfully.

Chandran et al. [21] (2010) examined 52 patients with euphonic voices and no pathological findings on the vocal folds with both rigid and CCD-flexible endoscopes in regard to a posterior glottal chink. In comparison between the two endoscopic technics, no difference was found in men, but it was more likely to find a posterior chink with the rigid laryngoscope in women.

Nospes et al. [22] (2011) examined 52 patients and 47 vocally healthy subjects with functional dysphonia also with both rigid and flexible endoscopes. The authors also found a better glottal closure using a flexible endoscope as shown in our study. Similar results were already obtained by Södersten and Lindestad [23] in 1992 comparing flexible fiber-optic endoscopy to the rigid endoscopy. They pointed out further the impact of loudness for with rising loudness the degree of incomplete glottal closure decreased in both flexible and rigid techniques. Sulter and Albers [14] stated that during loud phonation at least 90% of glottal closure should be achieved. If that is not the case, the larynx was suggested to be less robust.

Limitations

The data of this study consider vocally healthy female and male subjects in middle adulthood. It has not been proofed for younger or older age-groups. It was not investigated whether the results found were due to the body posture, the tongue position, tongue holding, pressure on the tongue with the rigid endoscope, or any other alteration caused by the endoscope. Future studies are necessary to compare anterior commissure visibility and glottal configuration during phonation with flexible endoscopes and the patient in different positions.

Taking into consideration, the results presented in this study and in previous studies, it can be pointed out that the type of endoscopic technique used in voice diagnostics, either rigid or flexible, has an inevitable impact on evaluation of vocal fold closure and visibility of anterior commissure with clinically relevant effects on interpretation of voice function. An incomplete glottal closure diagnosed with rigid endoscopy should not be overrated as vocal limitation. Even the invisibility of the anterior commissure should not be misinterpreted as a sign of supraglottic anteroposterior constriction. When using rigid laryngostroboscopes, a rigid-70° endoscope is favorable over a rigid-90° endoscope because of better visibility on the anterior commissure. The modern flexible laryngostroboscopy with chip-on-tip should become standard in clinical setting, although despite still high acquisition costs.

This study has been approved by the Ethics Committee of the Medical University of Vienna (EK 1546/2021). Written informed consent was obtained from all participants.

The authors have no conflicts of interest to declare.

There was no funding.

Roland Paulus: wrote the paper, conceived and designed the analysis, collected the data, and recruitment of subjects. Matthias Leonhard: recruitment of subjects, substantial contributions to conception and design, or acquisition of data, or analysis and interpretation of data. Guan-Yuh Ho: statistics, analysis, and interpretation of data. Annabella Kurz: recruitment of subjects. Berit Schneider-Stickler: involved in drafting the manuscript or revising it critically for important intellectual content, recruitment of subjects.

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.

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