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. Author manuscript; available in PMC: 2024 Oct 15.
Published in final edited form as: J Voice. 2023 Apr 16;39(5):1419.e13–1419.e27. doi: 10.1016/j.jvoice.2023.03.011

Acoustic-aerodynamic voice outcome ratios identify changes in vocal function following vocal fold medialization for unilateral vocal fold paralysis

Latané Bullock 1,2, Laura E Toles 3, Robert E Hillman 1,2,4,5, Daryush D Mehta 1,2,4,5
PMCID: PMC10576834  NIHMSID: NIHMS1883597  PMID: 37068982

Abstract

Purpose:

This study aimed to determine whether ratio-based measures that combine acoustic (output) and aerodynamic (input) parameters detect post-operative change in vocal function following vocal fold medialization for unilateral vocal fold paralysis.

Method:

Pre- and post-operative acoustic and aerodynamic measures were analyzed retrospectively from 149 patients who underwent vocal fold medialization for unilateral vocal fold paralysis. A 2 × 2 repeated-measures analysis of variance was conducted for each of four acoustic-aerodynamic ratios—traditional vocal efficiency (VE), sound pressure level to aerodynamic power (SPL/AP), SPL to average airflow (SPL/AFLOW), and SPL to subglottal pressure (SPL/Ps)—to investigate the main effects and interaction of treatment stage and loudness level (comfortable and loud).

Results:

The patient group showed significant post-operative improvements in self reports of vocal function (Voice-Related Quality of Life) and clinical auditory-perceptual judgments of dysphonia (Consensus Auditory-Perceptual Evaluation of Voice). Main effects for both treatment stage and loudness level were statistically significant for all measures except SPL/Ps. There were interaction effects for VE and SPL/AP, suggesting that magnitude of the treatment effect differs based on loudness. SPL/AFLOW had medium-to-large effect sizes in both loudness conditions. There were post-operative changes in SPL/Ps that were dependent on the magnitude of the reduction in AFLOW; as expected, SPL/Ps increased post-operatively in a subgroup that had large post-operative reductions in AFLOW at the comfortable loudness level.

Conclusions:

Acoustic-aerodynamic ratios can aid in tracking changes in vocal function following vocal fold medialization. SPL/AFLOW exhibited the largest effect size, which is expected since a reduction in abnormally high AFLOW typically accompanies the increased modulation of glottal air flow associated with successful vocal fold medialization. Future study is needed to model physiological changes in acoustic-aerodynamic voice outcome ratios across different types of voice disorders.

INTRODUCTION

Unilateral vocal fold paralysis (UVFP) is a voice disorder that can cause substantial difficulty with voice production due to inadequate glottal closure (i.e., a gap extending throughout the membranous and cartilaginous glottis) and loss of vocal fold bulk and tone. UVFP may have iatrogenic origins, often due to surgical procedures operating in the laryngeal region, or can be idiopathic in nature (Rosenthal et al., 2007). After a vocal fold is paralyzed, individuals often experience alterations of voice quality and/or an increase in vocal effort (Richardson & Bastian, 2004; Spector et al., 2001). Outcome measurements that reflect voice production physiology are imperative to determine objective changes in vocal status following intervention for UVFP. Similar to many voice pathologies, UVFP impacts both how the voice sounds and how much effort is required to phonate (Kelchner et al., 2003). The respiratory-phonatory imbalance that occurs with UVFP suggests that objective voice outcome measures that are derived from combined assessments of laryngeal aerodynamics (vocal input) and voice acoustics (vocal output) should be the most promising. Thus, there is a need for objective measures that reflect both the vocal input and vocal output in individuals with UVFP.

Several studies have investigated changes in individual acoustic and aerodynamic voice parameters during voice production in individuals with UVFP. For example, following surgical treatment for UVFP (e.g., injection augmentation, thyroplasty), measures of phonatory airflow (AFLOW) have been found to decrease, and maximum phonation time has been shown to increase (Dastolfo et al., 2016; Desuter et al., 2018; Mes et al., 2022; Zeitels et al., 1999; Zeitels et al., 1998). Prior to treatment for UVFP, vocal function can be characterized by high AFLOW and low maximum phonation time compared to that of the vocally healthy population; surgical treatment can make measures approximate a normative range by addressing the insufficient glottal closure that leads to excessive air escape during phonation (e.g., Zeitels et al., 1998). Changes in subglottal pressure (Ps) during voice production, on the other hand, have been less conclusive as a marker of UVFP. Some studies have shown a significant decrease in Ps following clinical intervention for UVFP (Choi et al., 2008; Ryu et al., 2012; Zeitels et al., 1999), whereas others have not (Dastolfo et al., 2016; Desuter et al., 2018; Montgomery et al., 2000; Mortensen et al., 2009). This inconsistency in Ps characteristics across studies likely has to do with the varying degrees of glottal closure exhibited by patients, as well as interactions among Ps, AFLOW, and acoustic vocal intensity. For example, there is a known correlation between acoustic sound pressure level (SPL) and Ps to account for (Björklund & Sundberg, 2016; Fryd et al., 2016; Holmberg et al., 1994). In this study, it is further hypothesized that Ps measures might be a more salient measure for identifying treatment changes in the UVFP population when controlling for acoustic SPL and AFLOW.

Acoustic-aerodynamic voice outcome ratios combining individual acoustic or aerodynamic metrics have the potential to be sensitive to the multifaceted voice production changes that occur in individuals with UVFP and could be robust to capture changes following intervention for UVFP. Previous studies that have combined Ps and acoustic SPL measures to assess the impact of surgical interventions on treating UVFP have involved small numbers of patients and been limited to small sample sizes of up to ten patients (Zeitels et al., 1999; Zeitels et al., 1998). Results from those studies demonstrate the promise of documenting post-surgical aerodynamic changes in a two-dimensional visualizion of Ps versus SPL. Aerodynamic metrics of SPL-compensated Ps migrated (decreased) toward but not within the expected normative range (Holmberg et al., 1994).

A recent study by our group investigated the utility of three acoustic-aerodynamic ratios to identify post-operative changes in patients who were surgically treated for bilateral phonotraumatic lesions (Toles et al., 2022). In that study, we found that the traditional measurement of vocal efficiency (VE; the ratio of acoustic power to aerodynamic power) was not a sensitive post-operative measure. However, the alternative ratios of SPL to aerodynamic power (SPL/AP) and SPL/Ps both significantly increased post-operatively (reflecting improvements in self-reports and auditory-perceptual judgments of vocal function for the patient group) within two loudness conditions (typical and loud levels) with moderate effect sizes. The SPL/Ps measure performed similarly to the SPL/AP measure but with less overall variability, so we concluded that SPL/Ps might be a more stable measure that can be used more reliably both clinically and in ambulatory settings for individuals with phonotrauma. Thus, there is a need for a more comprehensive assessment of how well these previously identified acoustic-aerodynamic measures characterize vocal function and treatment effects in groups of UVFP patients that are large enough to support robust statistical testing.

The current work applies a similar framework to the study of patients treated for UVFP. For individuals with UVFP, however, it is likely that the AFLOW component of acoustic-aerodynamic ratios may be more influential on the overall aerodynamic mechanism due to the nature of the disorder. Thus, we hypothesized that the larger pre-operative glottal gap in UVFP patients (relative to gaps caused by phonotraumatic lesions) would lead to a large pre- to- post-treatment effect size for AFLOW and Ps. It is currently unclear which combination of measures would be best to include in an acoustic-aerodynamic ratio for assessing patients with UVFP, and in what circumstances it would be better to use one over the other. In this study, we aimed to determine whether acoustic-aerodynamic ratios were able to detect a positive change in vocal function in a group of patients with UVFP following an injection laryngeal medialization procedure. We then aimed to identify which acoustic-aerodynamic ratios were more effective at identifying a post-medialization change.

METHODS

Participants

UVFP presents as immobility of one of the vocal folds, usually due to peripheral nerve damage (Kelchner et al., 1999; Kupfer & Meyer, 2014; Tucker, 1980) and is associated with asymmetric vocal fold motion, incomplete glottal closure (glottal incompetence), and increased vocal effort during voicing (Gillespie et al., 2014; Kelchner et al., 2003). This condition is typically treated surgically with temporary or permanent procedures that reposition the paralyzed vocal fold at the glottal midline (Barbu et al., 2015; Friedman et al., 2010; Mohanty et al., 2011; Parker et al., 2015; Zeitels et al., 2003). For the current study, data were gathered from a retrospective patient chart review of the clinical database at the Massachusetts General Hospital Center for Laryngeal Surgery and Voice Rehabilitation (MGH Voice Center). This retrospective study was approved by the Institutional Review Board at Mass General Brigham (Protocol #: 2008P000616, most recently approved on November 14, 2022). Diagnoses were based on a complete team evaluation by laryngologists and speech-language pathologists at the Massachusetts General Hospital Voice Center that included (1) a complete case history, (2) endoscopic imaging of the larynx (Mehta & Hillman, 2012), (3) aerodynamic and acoustic assessment of vocal function (Patel, Awan, Barkmeier-Kraemer, Courey, Deliyski, Eadie, Paul, Svec, et al., 2018), (4) patient-reported voice-related quality of life (V-RQOL) questionnaire (Hogikyan & Sethuraman, 1999), and (5) clinician-administered consensus auditory-perceptual evaluation of voice (CAPE-V) assessment (Kempster et al., 2009).

Chart review search criteria included a diagnosis of unilateral vocal fold immobility (complete or partial), inpatient surgical procedure of medialization laryngoplasty (with or without cricothyroid subluxation and/or adduction arytenopexy) (Isshiki et al., 1975; Zeitels et al., 1999), and the availability of acoustic and aerodynamic measures of vocal function at pre- and post-surgical outpatient clinical visits. Transcervical medialization laryngoplasty in the operating room was the primary surgical option for this patient group with UVFP in which an implant material (Gore-Tex) is used to reposition the membranous portion of the immobile vocal fold to restore glottal closure during phonation (Parker et al., 2015; Zeitels et al., 2003). The persistence of an interarytenoid gap and/or differences in the positioning of the vocal folds was addressed by performing an adduction arytenopexy in which the entire arytenoid was moved into a better position (Zeitels et al., 1998). Persistent flaccidity of the paralyzed vocal fold was addressed by performing a cricothyroid subluxation procedure, which served to lengthen and stiffen the vocal fold (Zeitels et al., 1999). The current study focused on measures before and after inpatient surgical procedures, rather than outpatient procedures in the clinic such as injection medialization (Crumley, 1994; Friedman et al., 2010).

The data set resulting from these search criteria consisted of 149 patients. The demographics for this study cohort are summarized in Figure 1. The cohort consisted of 81 female patients and 68 male patients. The mean age of the cohort was 59 years old (stdev = 13, min = 17, max = 88). Most patients (n = 133) had pre-treatment visits within two months of their surgical procedure; similarly, most patients (n = 122) had post-treatment visits within two months of their surgical procedure. Pre-treatment visits tended to occur closer to treatment date relative to post-treatment visits; the average time between pre-treatment visit and treatment was 20.3 days, whereas average time between treatment and post-treatment visit was 31.6 days. Patients were treated between 2005 and 2021.

Figure 1.

Figure 1.

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Demographic summary of the studied patient cohort diagnosed with unilateral vocal fold paralysis.

Figure 2 illustrates the treatment pathway for each patient in the cohort by plotting treatments (whether inpatient or outpatient) that each patient underwent and their pre- and post-treatment clinical visits. Clinical visits with acoustic and aerodynamic voice assessments (for voice outcome ratio computation) are plotted as circles; for completeness, visits with missing V-RQOL, CAPE-V, or maximum phonation time (MPT) data are plotted as open circles. MPT was obtained as a secondary measure of vocal function that can be compared with normative values in the literature (Maslan et al., 2011). Outpatient procedures are indicated as triangles. The length of time between events is represented by the presence or absence of connecting lines: if two events occurred within 180 days of each other, they are connected with a grey line. Patients are ordered by the number of treatments they received for visualization. For patients with multiple inpatient treatments, only the first treatment was included in our dataset (corresponding to time 0 in Figure 2).

Figure 2.

Figure 2.

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Visualization of assessment and treatment pathway for each patient in the study. Colored circles at integer timepoints indicate the occurrence of treatment, whether it was an inpatient or outpatient procedure. Time 0 refers to the time of the first treatment. Gray filled and open circles indicate times of a clinical visit during which acoustic, aerodynamic, and/or perceptual voice assessments were performed.

The V-RQOL and CAPE-V subjective ratings were collected to document the severity level of each patient’s voice disorder before and after treatment. V-RQOL scores were derived from ordinal ratings between 1 and 5 provided by the patient in response to ten questions regarding the severity and frequency of their voice problem over the previous two weeks (Hogikyan & Sethuraman, 1999). The V-RQOL instrument consists of a social-emotional subscale (probed by four of the questions), physical functioning subscale (probed by six of the questions), and an overall total score (computed from all ten questions). Standardized V-RQOL scores for the two subscales and the total score lie between 0 and 100, with higher scores indicating a higher quality of life related to an individual’s voice production. The CAPE-V scores are auditory-perceptual assessments of overall voice disorder severity, pitch, loudness, and voice quality on visual analog scales (Kempster et al., 2009). The CAPE-V was administered for each patient by a speech-language pathologist (often, the treating clinician) during a routine clinical evaluation of sustained vowel samples, prompted sentences, and spontaneous speech. Each dimension was scaled from 0 to 100, with 0 indicating typical and 100 indicating severely deviant.

Table 1 reports the group-wide statistics for the standardized clinical scores from the V-RQOL and CAPE-V. Patient-wise changes in the V-RQOL Total Score and CAPE-V Overall Severity are reported in Figure 3. As expected, the patient group exhibited large effect sizes in terms of post-operative improvements in the V-RQOL Total Score (Cohen’s d = 1.26) and CAPE-V Overall Severity (d = −1.15), which provided evidence that the surgical procedures resulted in successful treatment outcomes.

Table 1.

Clinical ratings of self-reported quality of life using the Voice-Related Quality of Life (V-RQOL) subscales and dimensions of the clinician-judged Consensus Auditory-Perceptual Evaluation of Voice (CAPE-V). Mean (standard deviation) reported for before and after surgical treatment for unilateral vocal fold paralysis.

Perceptual assessment scale Pre-surgery Post-surgery Change
V-RQOL
 Social-Emotional 56.9 (26.2) 82.1 (22.1) +25.9 (28.2)
 Physical Functioning 42.2 (19.0) 74.7 (22.5) +33.5 (25.4)
 Total Score 47.0 (20.5) 77.8 (21.5) +31.4 (25.0)
CAPE-V
 Overall Severity 55.8 (22.9) 28.4 (19.3) −28.6 (25.1)
 Roughness 36.2 (23.2) 24.0 (17.3) −12.0 (25.9)
 Breathiness 45.9 (26.2) 19.5 (18.7) −34.7 (26.6)
 Strain 32.0 (19.3) 17.8 (13.6) −14.8 (21.7)
 Pitch 35.9 (22.3) 21.5 (14.9) −17.9 (24.6)
 Loudness 35.6 (25.0) 22.5 (19.3) −24.6 (28.1)
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Figure 3.

Figure 3.

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Visualization of per-patient changes in pre- to post-treatment values for the self-reported Voice-Related Quality of Life Total Score and clinician-administered Clinical Auditory-Perceptual Evaluation of Voice Overall Severity rating. V-RQOL scores lie between 0 and 100, with higher scores indicating a higher quality of life. CAPE-V scores are visual analog scale ratings that range from 0 to 100, with zero indicating normality and 100 indicating severe deviance of the specified voice quality characteristic.

Instrumental acoustic and aerodynamic voice assessment

Acoustic and aerodynamic measures were collected utilizing the Phonatory Aerodynamic System (KayPENTAX, Montvale, NJ), which consisted of an airflow head combined with a face mask for measurements of oral airflow, a pressure transducer connected to an intraoral tube for air pressure measurements, and a microphone that is at a fixed mouth-to-microphone distance of 15 cm for SPL measurement. Each patient was instructed to produce a monotonic string of consonant-vowel syllables (/pa:pa:pa:pa:pa/) at a comfortable loudness level and a louder-than-comfortable level. Following clinical guidelines for instrumental voice assessment (Patel, Awan, Barkmeier-Kraemer, Courey, Deliyski, Eadie, Paul, Švec, et al., 2018), these two loudness conditions were gathered to determine the changes in voice outcome measures that might occur when attempting to phonate at different SPL. The syllable string was repeated three times, resulting in three values that were averaged to yield a single value for each voice outcome measure within each treatment stage (pre- and post-surgery) and each loudness condition (comfortable and loud).

Three acquired acoustic and aerodynamic measures were obtained as averages over from the middle three syllables: average SPL (dB SPL) during phonation; average phonatory airflow (AFLOW; L/s) during phonation; and average subglottal air pressure (Ps; cm H2O) as indirectly estimated from the equilibrated intraoral pressure during the consonant (Rothenberg, 1973). Subsequently, the acquired measures were used to compute derived measures to yield four acoustic-aerodynamic voice outcome ratios. The traditional vocal efficiency measure was calculated using the equation provided in the Phonatory Aerodynamic System software and was a unitless value reported in terms of parts per million (ppm):

Vocal Efficiency=1.4137×10−7×10SPL100.09806×Ps×AFLOW.

The acoustic-aerodynamic ratio of SPL to aerodynamic power (AP) was calculated as the ratio of average SPL and the product of the average AFLOW and average Ps and was in units of (dB SPL)/(cm H2O × L/s) (Grillo & Verdolini, 2008):

SPL/AP=SPLPs×AFLOW.

Two simplified acoustic-aerodynamic ratios were also computed. The ratio of SPL to Ps was calculated as the ratio of average SPL and average Ps, in units of (dB SPL)/(cm H2O):

SPL/Ps=SPLPs,

and the ratio of SPL to AFLOW was calculated as the ratio of average SPL to average AFLOW, in units of (dB SPL)/(L/s):

SPL/AFLOW=SPLAFLOW.

Statistical Analysis

Data from the group of patients (n = 149) were subject to separate two-way repeated-measures analyses of variance (ANOVAs) for each acquired (SPL, Ps, AFLOW) and derived (VE, SPL/AP, SPL/AFLOW, SPL/Ps) vocal parameter since each voice outcome measure was on a different scale. To adhere to ANOVA normality assumptions, we removed patients with at least one voice outcome measure below the lower quartile or above the upper quartile of the measurement’s distribution. For each 2 × 2 ANOVA, the dependent variable was one of the seven voice outcome measures. The independent variables were treatment stage (pre- or post-surgery) and loudness condition (comfortable or loud). An interaction term (treatment stage × loudness condition) was included in each model to determine if any differences found within treatment stage were affected by loudness condition. The strengths of each main effect and interactions were judged using the partial η2 statistic (small = 0.01, medium = 0.06, large = 0.14) (Cohen, 1988, p. 287). For statistically significant interaction effects, a paired-samples t-test was conducted to compare changes in pre- to post-surgical values within each loudness condition. Outliers (again determined by upper and lower quartiles) were removed for each measure individually. A Bonferroni correction was applied to the alpha level of significance to correct for multiple tests (α = 0.025). Cohen’s d effect sizes for repeated measures were calculated to interpret the strength of the relationship (small = 0.20, medium = 0.50, large = 0.80) (Cohen, 1988, p. 25).

Pairwise scatterplots and Pearson’s correlation coefficients were computed to compare the post-operative changes in each of the voice outcome measures. This would help us answer, for example, whether a patient’s AFLOW measure decreasing pre- to post-treatment would be correlated with subglottal pressure increasing or decreasing.

RESULTS

ANOVA main effects and interaction for each measure

Table 2 summarizes the results of each of the seven ANOVA models that statistically evaluated the relationship between treatment stage, loudness condition, and the voice outcome measurement of interest. An interaction term was included to capture Treatment × Loudness effects. For all acquired (SPL, AFLOW, and Ps) and derived (VE, SPL/AP, SPL/AFLOW, SPL/Ps) measures, we found a main effect for treatment and loudness, except for SPL/Ps, which was not sensitive to treatment. AFLOW was the only acquired measure with a Treatment × Loudness interaction effect. VE and SPL/AP were the only derived measures with a Treatment × Loudness interaction effect.

Table 2.

Results for each ANOVA modeling the relationship between treatment stage (Treatment), loudness condition (Loudness), and the voice outcome measurement of interest. An interaction term (Interaction) captured was included to capture the potential Treatment × Loudness effect. Degrees of freedom (df), F-value (F), p-value (p), and eta-squared values (ηp2) are reported for each measure.

Effect df F p ηp2
SPL
 Treatment 1 30.036 < 0.001 0.174
 Loudness 1 546.696 < 0.001 0.793
 Interaction 1 0.531 0.4676 0.004
AFLOW
 Treatment 1 78.021 < 0.001 0.373
 Loudness 1 89.322 < 0.001 0.405
 Interaction 1 11.903 0.0008 0.083
Ps
 Treatment 1 7.888 0.0057 0.055
 Loudness 1 389.5 < 0.001 0.741
 Interaction 1 1.318 0.253 0.010
VE
 Treatment 1 40.95 < 0.001 0.271
 Loudness 1 83.547 < 0.001 0.432
 Interaction 1 8.318 0.0047 0.070
SPL/AP
 Treatment 1 35.284 < 0.001 0.216
 Loudness 1 132.445 < 0.001 0.509
 Interaction 1 8.077 < 0.001 0.059
SPL/AFLOW
 Treatment 1 129.495 < 0.001 0.517
 Loudness 1 31.616 < 0.001 0.207
 Interaction 1 0.013 0.9085 0.000
SPL/Ps
 Treatment 1 0.091 0.7635 0.001
 Loudness 1 390.527 < 0.001 0.747
 Interaction 1 0.864 0.3544 0.006
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The following sections elaborate on these ANOVA results by inspecting each measurement in more detail. For measures that were statistically different pre- to post-treatment, we inspected these measures more closely with post-hoc t-tests (Table 3) and patient-wise visualizations (see Figure 4 and Figure 5 for comfortable condition and Figure A.1 and Figure A.2 for loud condition).

Table 3.

Paired t-tests quantifying pre-treatment to post-treatment changes in voice outcome measures. Mean (standard deviation) values across the patient cohort are reported for each measure. Cohen’s d represents the effect size of treatment as estimated by a paired t-test.

Measure Comfortable Loud
Presurgery Postsurgery d Presurgery Postsurgery d
SPL 75.6 (5.8) 78.9 (5.7) 0.51 82.4 (7.9) 85.5 (6.4) 0.36
AFLOW 0.336 (0.214) 0.200 (0.098) −0.75 0.459 (0.292) 0.279 (0.174) −0.70
Ps 8.7 (3.1) 9.5 (3.8) 0.22 13.7 (5.4) 15.1 (6.0) 0.25
VE 23.1 (22.5) 70.1 (65.2) 0.73 51.0 (58.4) 151.2 (137.6) 0.76
SPL/AP 34.6 (26.9) 48.9 (32.9) 0.47 18.0 (15.2) 27.1 (21.1) 0.45
SPL/AFLOW 249 (145) 391 (182) 0.82 190 (112) 345 (192) 0.88
SPL/Ps 9.0 (3.2) 8.9 (3.2) −0.03 6.3 (2.5) 6.0 (2.1) −0.11
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Figure 4.

Figure 4.

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Changes observed for acquired measures pre- to post-treatment at the patient level in the Comfortable condition. (Left) Distribution of measures pre-treatment (light grey) and post-treatment (dark grey). Every data point corresponds to one patient. Above each plot is Cohen’s d corresponding to a paired t-test. (Right) Histograms of post-treatment minus pre-treatment values. The vertical red line denotes zero, or no change, pre- to post-treatment. Statistical significance is notated as: *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).

Figure 5.

Figure 5.

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Changes observed for derived measures pre- to post-treatment at the patient level in the Comfortable condition. (Left) Distribution of measures pre-treatment (light grey) and post-treatment (dark grey). Every data point corresponds to one patient. Above each plot is Cohen’s d corresponding to a paired t-test. (Right) Histograms of post-treatment minus pre-treatment values. The vertical red line denotes zero, or no change, pre- to post-treatment. Statistical significance is notated as: *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).

Acquired measures: AFLOW best indicated treatment stage

Acquired measures all changed significantly pre- to post-surgery in the expected directions, with variable effect sizes (Table 3): AFLOW decreased from 0.336 (0.214) to 0.200 (0.098) L/s with a large effect size of 0.75, SPL increased from 75.6 (5.8) to 78.9 (5.7) dB SPL with a medium effect size of 0.51, and Ps increased from 8.7 (3.1) to 9.5 (3.8) cm H2O for a small effect size. Across the board, the Loud condition paralleled the Comfortable condition. We visualized the effect of treatment on each measure by plotting every individual (after removing of outliers as defined in Methods) pre-treatment and post-treatment in the Comfortable condition, connecting the two with a line (Figure 4, left column). We extended this by also visualizing the distribution of ‘delta’ pre- to post-surgery values; each individual was converted to a single data point indicating relative improvement or decline (Figure 4, right column). The large effect sizes of the AFLOW measure can be observed by the steep downward slope of the grey connecting lines and the leftward bias away from zero in the histogram.

Derived measures: VE and SPL/AFLOW best indicated treatment stage

The same analyses and visualizations were used for derived measures as for the acquired measures (Figure 5). As suggested by the ANOVA, we found that SPL/AFLOW, VE, SPL/AP, and were significantly different pre- to post-treatment when phonating comfortably, with Cohen’s d effect sizes of 0.82 (large), 0.73 (medium), and 0.47 (medium). respectively. Again, the Loud condition largely paralleled the Comfortable condition. Whereas SPL/AFLOW drastically increased post-surgically for most patients, Figure 5 revealed variability in this trend; some patients’ SPL/AFLOW decreased after treatment, represented by negative values on the histogram on the right panel). SPL/Ps was the only non-significant measure of treatment stage.

Pairwise correlations between measures

Figure 6 displays scatterplots and Pearson’s correlation coefficients for each pair of objective (acquired and derived) and perceptual (V-RQOL and CAPE-V) voice outcome measures. For each pair of measures, we plotted measurement 1 pre- to post-treatment delta on the x-axis and measurement 2 pre- to post-treatment delta on the y-axis. Pearson’s correlation coefficient between the deltas is indicated above each plot. Correlations among acquired measures (SPL, AFLOW, Ps, MPT) were low (r ~ .3 or below), except for the Ps-AFLOW pair that was positively correlated (r = .39). This indicated that post-operative changes in subglottal pressure and post-operative changes in glottal airflow were correlated with each other; a patient with a large decrease in airflow was likely to exhibit a decrease in subglottal pressure. Although the linear correlations were low between outcome ratios and self-report measures, importantly the directionality of the change in these measures from before to after surgery agreed in the majority of cases. For example, the direction of change (positive or negative direction) of the acoustic-aerodynamic ratio measure VE agreed with the direction of change of the V-RQOL total score for 80% of the patients. Correlations among derived measures were statistically significant both to each other and to the acquired measures they depended on, as expected.

Figure 6.

Figure 6.

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Correlations among pre- to post-surgery change for every pair of voice outcome measure, including perceptual ratings (CAPE-V and V-RQOL), acquired measures (SPL, AFLOW, Ps, MPT), and derived acoustic-aerodynamic ratios (VE, SPL/AP, SPL/Ps, SPL/AFLOW). In each plot, every grey circle represents one patient’s change pre- to post-treatment from two clinical visits. Lines indicate center of gravity for the variable. Above each plot is Pearson’s correlation coefficient between the two measures, along with statistical significance of the correlation (***p < 0.001, **p < 0.01, *p < 0.05).

DISCUSSION

The purpose of this study was to determine whether acoustic-aerodynamic ratios can detect positive group-based change in vocal function following vocal fold medialization for UVFP and to identify which ratios were the most effective at detecting that change. Results found that several of the ratios that were tested—VE, SPL/AP, and SPL/AFLOW—were effective at identifying phonatory changes post-medialization. Of those, VE and SPL/AP had interaction effects between treatment stage and loudness condition, suggesting that the magnitude of treatment effect differed based on loudness; i.e., to be useful in the clinic, these measures have to take loudness into account, which is a known property of vocal efficiency measures (Titze, 1992). SPL/AFLOW detected post-treatment change with the largest effect but did not demonstrate an interaction with loudness. Therefore, SPL/AFLOW can identify UVFP treatment changes without accounting for how loudly the individual is attempting to phonate.

Each of the component measures that comprise the ratios demonstrated a treatment effect in both loudness conditions. AFLOW demonstrated the largest effect size, which was expected given previous studies that have shown a significant reduction in AFLOW following successful vocal fold medialization in groups of patients with UVFP (Montgomery et al., 2000). The reduction in post-operative AFLOW is a result of the medialization procedure reducing or eliminating the glottal gap (lack of complete glottal closure during phonation) and increasing modulation of the glottal air flow. One downside to using AFLOW as a primary outcome measure is that it interacted with loudness condition, suggesting that the treatment effect differed based on how loudly the patient was attempting to phonate. Fortunately, the ratio of SPL/AFLOW, which is able to identify changes following medialization with a medium-to-large effect size, accounts for loudness and eliminates the interaction effect with loudness condition. Thus, SPL/AFLOW might be the preferred outcome measure to collect in the clinic to determine functional changes despite how loudly the patient is attempting to phonate.

Medialization also resulted in higher SPL with a medium effect size, regardless of attempted loudness condition, which was expected since increasing vocal volume is also a common targeted outcome for medialization procedures. Additionally, Ps increased on average following the medialization procedure with mean (standard deviation) of Ps changing from 8.7 (3.1) cm H2O to 9.5 (3.8) cm H2O, with small effect sizes (d = 0.26–0.28). These results compare with a previous investigation finding patients with UVFP increased their mean (standard deviation) Ps from 8.0 (3.4) cm H2O before surgery to 10.2 (2.5) cm H2O after surgery (Dastolfo et al., 2016). Measures of Ps relative to SPL showed no statistical change following medialization, which is indicative of potential variability of Ps across participants. Ps, however, demonstrated a small-to-medium correlation with AFLOW (r = .39), suggesting that there might be an effect if we controlled for AFLOW. To investigate this, the participants were separated into three equal groups using AFLOW as a proxy to estimate the size of glottal gap at baseline. A large reduction of AFLOW was estimated to be associated with a larger glottal gap at baseline. We compared the lower third (i.e., smallest change in AFLOW) and the upper third (large reduction of AFLOW). This revealed more insight on treatment changes. The group who had a larger post-treatment reduction of airflow demonstrated a statistically significant decrease in Ps in the comfortable condition post-medialization with a moderate-to-large effect size (d = 0.44). In the same group, the ratio of SPL/Ps demonstrated a statistically significant increase (d = 0.59) in the comfortable condition. Neither Ps alone nor SPL/Ps demonstrated statistically significant changes in the loud condition in this group post-medialization. The group who demonstrated smaller post-treatment reductions of airflow demonstrated no statistically significant changes in either Ps alone or in SPL/Ps. Based on these follow-up analyses, we propose that SPL/Ps could be helpful to identify treatment changes, but the magnitude of change in AFLOW and loudness condition must be taken into consideration.

One limitation of this study is that it only investigated the use of acoustic-aerodynamic outcomes before and after medialization procedures. Though voice therapy is not first-line treatment for vocal fold paralysis, it can serve a secondary role of optimizing vocal function to reduce overall vocal effort and maximize acoustic-aerodynamic balance (Miller, 2004). A handful of studies have investigated the effects that voice therapy has on vocal outcomes in patients with vocal fold paralysis, many of which have found voice therapy to be beneficial in this population (Walton et al., 2017). Miyata and colleagues (2020) found that Ps decreased following voice therapy in a group of patients with unilateral vocal fold paralysis. No studies have yet investigated how acoustic-aerodynamic ratios change as a function of complementary voice therapy in patients who have undergone medialization for unilateral vocal fold paralysis. Since AFLOW is more likely to be affected by the medialization procedure itself (i.e., reducing the size of the glottal gap), we suspect that Ps and SPL/Ps will be more subject to change following the more subtle impact of post-operative voice therapy. This expectation is supported by previous work that has pointed out inherent limitations in the stability of the AFLOW measure due to other factors (e.g., variation in the size of the interarytenoid/cartilagenous posterior glottal opening while maintaining closure of the membranous glottis during phonation) that can impact AFLOW but have minimal impact on voice quality (Belsky et al., 2020; Gartner-Schmidt et al., 2015; Hillman et al., 1989). This means that AFLOW may only be sensitive to relatively large changes in vocal function, such as those often associated with vocal fold medialization. Future work should investigate how these measures respond to post-operative voice therapy intervention.

Though this study presents outcomes that can be easily and reliably collected in the clinical setting, it also has implications for identifying vocal function changes in daily life. Advances in ambulatory voice monitoring over the past several years have resulted in an increased ability to estimate a variety of vocal function measures in daily life (Carullo et al., 2013; Lee et al., 2019; Mehta et al., 2015; Popolo et al., 2005). Specifically, estimates of Ps are now feasible to collect using an accelerometer placed on the subglottal portion of the anterior neck (Lin et al., 2019; Marks et al., 2020; Marks et al., 2019). Recent work has also described methods for estimating environmental SPL in daily life (Whittico et al., 2020). Considering these advances, ambulatory estimates of SPL/Ps during activities of daily living in individuals with unilateral vocal fold paralysis could provide new insights into vocal function and its relation to vocal effort and fatigue.

The retrospective scope of this study allowed for analysis of a large number of patients, but there are obvious limitations to this design. We were unable to control for data collection methods, which could introduce potential error into the data. However, the clinical voice evaluation protocol at the MGH Voice Center is standardized, so all patients are given similar instructions and tasks are completed in the same order. To further decrease bias in the results, we identified and removed outliers.

Conclusions

Acoustic-aerodynamic ratios identified changes in vocal function following vocal fold medialization. SPL/AFLOW exhibited the largest effect size, which is expected since a reduction in abnormally high AFLOW typically accompanies the increased modulation of glottal air flow associated with successful vocal fold medialization. SPL/AFLOW also did not interact with loudness condition, suggesting that it would be a useful outcome to identify vocal function changes after medialization without having to account for how loudly the patient attempted to phonate. The traditional vocal efficiency measure and the ratio of SPL/AP were also able to identify treatment effects, but the effect differed based on loudness condition. SPL/Ps was able to identify treatment effects with a medium effect size at comfortable volume levels when accounting for the magnitude of the reduction of AFLOW.

ACKNOWLEDGMENTS

The authors would like to thank the clinical speech-language pathologists at the MGH Voice Center for performing the evaluations of the patients included in this study, the laryngologists for meticulous documentation of the surgical procedures performed, and Anatoly Goldstein for clinical database management and queries. This work was supported by the National Institutes of Health National Institute on Deafness and Other Communication Disorders (T32 DC000038 training grant funding for Latané Bullock awarded to Gwenaelle Geleoc, R01 DC019083 awarded to Daryush Mehta, and P50 DC015446 awarded to Robert Hillman). The article’s contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Institutes of Health.

APPENDIX

Figure A.1.

Figure A.1.

Open in a new tab

Changes observed for acquired measures pre- to post-treatment at the patient level in the Loud condition. (Left) Distribution of measures pre-treatment (light grey) and post-treatment (dark grey). Every data point corresponds to one patient. Above each plot is Cohen’s d corresponding to a paired t-test. (Right) Histograms of post-treatment minus pre-treatment values. The vertical red line denotes zero, or no change, pre- to post-treatment. Statistical significance is notated as: *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).

Figure A.2.

Figure A.2.

Open in a new tab

Changes observed for derived measures pre- to post-treatment at the patient level in the Loud condition. (Left) Distribution of measures pre-treatment (light grey) and post-treatment (dark grey). Every data point corresponds to one patient. Above each plot is Cohen’s d corresponding to a paired t-test. (Right) Histograms of post-treatment minus pre-treatment values. The vertical red line denotes zero, or no change, pre- to post-treatment. Statistical significance is notated as: *** (p < 0.001), ** (p < 0.01), and * (p < 0.05).

Footnotes

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Disclosure: Drs. Robert Hillman and Daryush Mehta have a financial interest in InnoVoyce LLC, a company focused on developing and commercializing technologies for the prevention, diagnosis and treatment of voice-related disorders. Dr. Hillman’s and Dr. Mehta’s interests were reviewed and are managed by Massachusetts General Hospital and Mass General Brigham in accordance with their conflict-of-interest policies.

DATA AVAILABILITY STATEMENT

Mass General Brigham and Mass General are not allowed to give access to data without the Principal Investigator (PI) for the human studies protocol first submitting a protocol amendment to request permission to share the data with a specific collaborator on a case-by-case basis. This policy is based on very strict rules dealing with the protection of patient data and information. Anyone wishing to request access to the data must first contact Ms. Sarah DeRosa, Program Coordinator for Research and Clinical Speech-Language Pathology, Center for Laryngeal Surgery and Voice Rehabilitation, Massachusetts General Hospital: sederosa@partners.org.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

Mass General Brigham and Mass General are not allowed to give access to data without the Principal Investigator (PI) for the human studies protocol first submitting a protocol amendment to request permission to share the data with a specific collaborator on a case-by-case basis. This policy is based on very strict rules dealing with the protection of patient data and information. Anyone wishing to request access to the data must first contact Ms. Sarah DeRosa, Program Coordinator for Research and Clinical Speech-Language Pathology, Center for Laryngeal Surgery and Voice Rehabilitation, Massachusetts General Hospital: sederosa@partners.org.

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