Salient Voice Symptoms in Primary Muscle Tension Dysphonia

Abstract

Introduction.

Patients with primary muscle tension dysphonia (pMTD) commonly report symptoms of vocal effort, fatigue, discomfort, odynophonia, and aberrant vocal quality (eg, vocal strain, hoarseness). However, voice symptoms most salient to pMTD have not been identified. Furthermore, how standard vocal fatigue and vocal tract discomfort indices that capture persistent symptoms—like the Vocal Fatigue Index (VFI) and Vocal Tract Discomfort Scale (VTDS)—relate to acute symptoms experienced at the time of the voice evaluation is unclear. Finally, correlations between voice symptoms and acoustic vocal quality are poorly understood. As such, the objectives of this study were to: (1) identify the most salient pMTD symptoms, (2) correlate the VFI and VTDS with acute voice symptom severity, and (3) better understand relationships between voice symptom experiences and vocal acoustics.

Methods.

Thirty subjects (15 pMTD, 15 controls) rated their vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and vocal quality on separate 100 mm visual analog scales (VAS) and completed the VFI-Part1 and VTDS. Cepstral peak prominences (CPP) were obtained from voice recordings of sustained /i/ and the all-voiced consensus of auditory-perceptual evaluation of voice (CAPE-V) sentence.

Results.

Patients with pMTD reported significantly higher severities of vocal effort (P < 0.0001), fatigue (P = 0.001), and discomfort (P = 0.0008) and scored significantly higher on both vocal indices (VFI-Part1: P < 0.0001; VTDS: P = 0.0056) compared to vocally healthy controls. Both indices had medium-to-high correlations with acute symptom severities in both groups. However, there were no significant differences between groups on odynophonia severity (P = 0.349), acoustic vocal quality (/i/: P = 1.00, CAPE-V: P = 0.228) or self-perceptual vocal quality (P = 0. 141). There were also no significant relationships between vocal acoustics and patient symptoms or between vocal acoustics and standard vocal fatigue and vocal tract discomfort indices (P’s > 0.05).

Conclusion.

Somatosensory symptoms of vocal effort, vocal fatigue, and vocal tract discomfort are the most salient clinical features in patients with pMTD. Standard voice indices that capture more chronic symptoms can also reliably capture symptoms experienced at the time of the voice evaluation in patients with pMTD. Although odynophonia and aberrant acoustic vocal quality may be present in some patients with pMTD, they do not appear to be symptoms central to the condition. Because somatosensory experiences are more salient than vocal quality and vocal acoustics in pMTD, somatosensory symptoms of vocal effort, fatigue, and discomfort should hold more weight during the voice evaluation when pMTD is suspected.

INTRODUCTION

Primary muscle tension dysphonia (pMTD) is a functional voice disorder thought to be the result of excessive muscle tension and hyperfunction in and around the larynx that changes the larynx’s position and alters vocal function.^1,2 However, there is little experimental evidence and a lack of robust study designs with well-vetted physiologic metrics and vocally healthy comparison cohorts to test and support this theory. More recent studies even suggest individuals with pMTD and vocally healthy individuals have similar muscle tension, contraction, and movement patterns in the laryngeal system.^3–7 Interestingly, across these studies, patients with pMTD still consistently reported significantly higher vocal effort and vocal tract discomfort compared to vocally healthy controls.^3–7 These significant somatosensory features across studies parallel common symptoms attributed to pMTD, including increased vocal effort, vocal fatigue, vocal tract discomfort, and odynophonia, as well as changes in vocal quality (eg, hoarseness, roughness, strain, voice breaks).⁸

Individuals with pMTD also tend to score higher on self-perceptual measures, including the Vocal Handicap Index (VHI-10) and Vocal Fatigue Index (VFI).^8,9 Vocal fatigue has also been shown to be significantly higher in patients with pMTD compared to patients diagnosed with benign vocal fold lesions (eg, nodules).¹⁰ Because symptoms of increased vocal effort, fatigue, discomfort, and odynophonia may be more prevalent in patients with pMTD and may be more severe compared to other voice disorders, self-reported measures of these somatosensory experiences may be valuable in forming the diagnostic profile of pMTD.

Although a variety of potential somatosensory experiences and vocal quality features are commonly reported as symptoms in pMTD, it is unclear which of these symptoms are the most salient (ie, prominent, central, and key to the diagnosis). As such, quantifying the severity of these voice symptoms and comparing symptom severity to vocally healthy control participants can help better define the condition. Identifying key features that are most salient to pMTD is especially important considering that patients with pMTD may not always exhibit more muscle tension and hyperfunction in and around the laryngeal system^11–13 but seem to present with greater somatosensory experiences (ie, sensory feelings of vocal effort, fatigue, discomfort, and pain with voice use).¹⁴ Previous work also suggests acoustic and aerodynamic measurements are unreliable in distinguishing patients with pMTD from non-pMTD patients with voice disorders,¹⁵ and “hyperfunctional” supraglottic patterns (eg, mediolateral and anterolateral compression) have also consistently been shown to occur in individuals with and without pMTD.^3,6,16–18 Furthermore, not everyone with pMTD experiences issues with the sound quality of their voices, although the majority will describe issues with the feel of their voice (eg, increased vocal discomfort).^15,19 Determining the accuracy and reliability of voice metrics to define a voice disorder like pMTD can help clarify how much weight should be placed on these metrics during the voice evaluation.²⁰

Finally, the current standardized self-perceptual indices that are typically used in voice clinics, such as the VHI-10, VFI, and Vocal Tract Discomfort Scale (VTDS), evaluate patient experiences more globally as they relate to vocal impact and dysfunction across multiple weeks.¹ Standardized assessments of voice symptoms, however, do not include severity metrics of patient experiences at the time of the voice evaluation. As such, it is difficult to determine whether these standardized indices that assess symptoms experienced over a period of time (eg, trait vocal fatigue) represent the symptoms patients experience at the time of their voice evaluation (eg, state vocal fatigue).^21,22

The first objective of this study was to determine which voice symptoms reported in the pMTD literature are the most salient (eg, central) to patients with pMTD. The second objective was to study relationships between standardized vocal fatigue and vocal tract discomfort indices that capture sensory experiences across multiple-week time spans and acute symptom severities assessed at the time of the voice evaluation. Finally, because acoustic vocal output is typically weighed heavily in the voice evaluation, the third objective was to determine relationships between voice symptoms and acoustic vocal quality in patients with pMTD.

METHODS

Participants

Thirty subjects (15 vocally healthy controls and 15 patients diagnosed with pMTD) were recruited to participate in the study at UT Southwestern Center for Voice Care (see Table 1 for participant demographics). All study procedures were approved by the UT Southwestern Institutional Review Board. Participants included in the experimental group were diagnosed with pMTD by one of three board-certified laryngologists and one of six voice-specialized speech-language pathologists at the UT Southwestern Center for Voice Care, determined based on case history and self-report of vocal deficits in the context of the absence of structural (eg, vocal fold lesions) or neurogenic (eg, vocal fold paralysis) voice disorders observed on laryngoscopic evaluation. Participants were recruited into the vocally healthy control group if they had no structural or neurological laryngeal abnormalities during laryngoscopic evaluation and self-reported no issues with their voice over the past year. They also needed to score less than 7 out of 40 on the VHI-10 to confirm there were no vocal deficits.

TABLE 1.

Participant Demographics

Group	Gender	Age in years (SD)	Race	Ethnicity
pMTD	33% men	39.4 (14.52)	88% Caucasian	6% Hispanic/Latino
	67% women		6% Asian	94% Non-Hispanic/Latino
			6% Black of African American
Controls	15% men	31.33 (16.52)	88% Caucasian	0% Hispanic/Latino
	85% women		6% Asian	100% Non-Hispanic/Latino
			6% Black or African American

Protocol

Vocal symptom severity ratings

Participants were coached on completing separate 100 mm visual analog scales (VAS) for each of the following symptoms: perceived severity of vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and vocal quality (Figure 1). Vocal effort was described as the “physical exertion it takes to get the voice out.” Vocal fatigue was described as “how tired the voice feels when you use it.” Vocal tract discomfort was described as the “unpleasantness you may feel in your throat when using your voice.” Odynophonia was described as “how much pain there is when you talk.” And vocal quality was described by “how clear your voice sounds to you.” When completing the VAS scales, participants were asked to complete each scale representing their symptoms at the time of their visit and to place a vertical line on each VAS 100 mm horizontal line representing each of the five voice symptoms (vocal effort, fatigue, discomfort, odynophonia, vocal quality). 0 and 100 represented basement and ceiling anchors, where 0 represented no symptoms or a typical/normal sounding and feeling voice, and 100 represented maximum symptoms or “the worst cold imaginable.” Additionally, participants completed Part 1 of the Vocal Fatigue Index (VFI-Part1)^4,23 and the VTDS²⁴ to capture their severity of symptoms over the “past two weeks.” VFI-Part1 was used because it has previously been shown to be the most reliable subscale to differentiate typical experiences with vocal fatigue due to increased vocal demands from pathological levels of vocal fatigue.^4,23

FIGURE 1.

100 mm VAS. Example 100 mm visual analog scale (VAS) representing quantification of self-perceived symptoms of vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and vocal quality on separate lines.

Acoustic voice samples

Acoustic voice samples were recorded utilizing a condenser microphone placed around the head and arranged at a 45°−90° angle and 4–10 cm away from the corner of the lips, per standard procedures.²⁰ Participants were then asked to produce three sustained /i/ vowels for 3 to 5 seconds in length at a conversational volume and modal pitch. Participants also recited the consensus of auditory-perceptual evaluation of voice (CAPE-V) sentences. Voice samples were recorded in Praat (version 6.3.10) and saved as .wav files for subsequent acoustic analysis.

Data processing and analysis

Vocal symptom severity ratings

A 100 mm ruler was used to measure the vertical tick marks placed on each horizontal 100 mm VAS consisting of vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and vocal quality severities. The severity number representing each tick mark on the 100 mm line was recorded in an Excel spreadsheet. VFI-Part1 and VTDS sum scores were also inputted into an Excel spreadsheet.

Vocal acoustics [Cepstral peak prominence (CPP)]

Each acoustic voice sample for each of the three sustained /i/ vowel utterances was cropped to produce one 5–10 seconds long uninterrupted voice sample for each subject. The all-voiced CAPE-V sentence “We were away a year ago” was also cropped. CPPs for the sustained vowels and all-voiced CAPE-V sentence were obtained for each acoustic voice sample using a customized script in batchmode (Phonanium CommV). CPP was used as the primary acoustic parameter because it has previously been shown to be the most robust metric to distinguish patients with pMTD from vocally healthy controls.^6,25

Statistical analysis

To determine group differences for each of the five voice symptoms on the 100 mm VAS (vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and vocal quality), standard voice indices (VFI-Part1, VTDS), and acoustic vocal metrics (CPP), independent t tests were run with Bonferroni correction set to 0.006 to account for multiple comparisons. Pearson correlations were used to compare (1) the strength of relationships between state measures of vocal fatigue severity (100 mm VAS) and trait fatigue severity via standardized questionnaires (VFI-Part1), (2) relationships between state vocal tract discomfort severity on the 100 mm VAS and trait discomfort severity on the VTDS, and (3) relationships between voice symptoms (VAS, VFI-Part1, VTDS) and acoustic metrics of vocal quality (CPP), for both the pMTD and vocally healthy control groups. All statistical analyses were run in Python (version 3.11.8).

RESULTS

Objective 1: Determine voice symptoms most salient to patients with pMTD

Vocal effort (t = 6.216, P < 0.0001), vocal fatigue (t = 4.496, P = 0.001), and vocal tract discomfort (t = 4.558, P = 0.0008) were all significantly higher in the pMTD group compared to the vocally healthy control group. Patients with pMTD also rated their vocal fatigue and vocal tract discomfort symptoms as significantly higher on the VFI-Part 1 (t = 6.670, P < 0.0001) and the VTDS (t = 3.856, P = 0.0056). However, there were no significant group differences on odynophonia (t = −2.168, P = 0.349) or vocal quality (t = −2.580, P = 0.141) VAS severity measures.

Objective 2: Determine the relationship between acute (state) and chronic (trait) vocal fatigue and vocal tract discomfort

Pearson correlation coefficients between acute measures of vocal fatigue on 100 mm VAS (state vocal fatigue) and the VFI-Part 1 (trait vocal fatigue) were moderate yet statistically significant for both the pMTD (r = 0.623, P = 0.013) and vocally healthy control groups (r = 0.525, P = 0.045). Pearson correlations between acute measures of vocal tract discomfort on 100 mm VAS (state vocal tract discomfort) and the VTDS (trait vocal tract discomfort) were also statistically significant for both groups, with the vocally healthy control group showing a strong correlation (r = 0.875, P < 0.0001) and the pMTD group showing a moderately strong correlation (r = 0.601, P = 0.018).

Objective 3: Determine relationships between voice symptoms and acoustic vocal quality

There were no significant differences between groups on CPP values for the sustained /i/ vowel (t = 1.212, P = 1.00) or the all-voiced CAPE-V sentence (t = 2.368, P = 0.228). There were also no significant correlations between voice symptoms and vocal quality in either the pMTD (Table 2) or vocally healthy control (Table 3) groups.

TABLE 2.

pMTD Group Pearson’s Correlations Between Voice Symptoms (VAS, VFI-Part1, VTDS) and Vocal Acoustics (CPP)

	CPP	CPP CAPE-V
Parameter	sustained /i/	sentence
Vocal effort VAS	−0.118	−0.061
Vocal fatigue VAS	0.248	0.253
Vocal tract discomfort VAS	−0.118	−0.051
Odynophonia VAS	0.226	0.152
Self-perceived vocal quality VAS	−0.177	−0.239
VFI-Part 1	−0.090	−0.111
VTDS	0.026	0.071

Abbreviations: VAS, 100 mm visual analog scale; VFI-Part 1, Part 1 of the Vocal Fatigue Index; VTDS, Vocal Tract Discomfort Scale; CAPE-V Sentence, ”We were away a year ago.”

TABLE 3.

Control Group Pearson’s Correlations Between Voice Symptoms (VAS, VFI-Part1, VTDS) and Vocal Acoustics (CPP)

	CPP	CPP CAPE-V
Parameter	sustained /i/	sentence
Vocal effort VAS	−0.320	−0.097
Vocal fatigue VAS	−0.284	0.078
Vocal tract discomfort VAS	−0.440	−0.243
Odynophonia VAS	−0.384	−0.261
Self-perceived vocal quality VAS	0.406	0.192
VFI-Part 1	−0.086	0.070
VTDS	−0.336	−0.073

Abbreviations: VAS, 100 mm visual analog scale; VFI-Part 1, Part 1 of the Vocal Fatigue Index; VTDS, Vocal Tract Discomfort Scale. CAPE-V Sentence, ”We were away a year ago”

DISCUSSION

Patients with pMTD have a range of possible voice symptoms that can be reported during an evaluation, including vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and aberrant vocal quality. However, it is unclear which of these symptoms are the most salient to this clinical population. The first objective of the study was to identify the voice symptoms most relevant to patients with pMTD using self-perceived VAS scales of vocal effort, vocal fatigue, vocal tract discomfort, odynophonia, and vocal quality, as well as standardized indices of vocal fatigue and vocal tract discomfort. Patients diagnosed with pMTD reported significantly higher vocal effort, vocal fatigue, and vocal tract discomfort across all self-perceptual measurements (VAS, VFI-Part 1, VTDS) compared to vocally healthy controls. However, no significant group differences were found for odynophonia or perceived levels of aberrant vocal quality.

The second objective of the study was to compare acute perceptions of vocal fatigue and vocal tract discomfort VAS scores to chronic perceptions of vocal fatigue and vocal tract discomfort via the VFI and VTDS scales. There was a moderate, yet significant, correlation between the vocal fatigue VAS and VFI-Part 1 and between the vocal tract discomfort VAS and VTDS. The third objective was to determine relationships between voice symptoms and vocal acoustics. There were no significant group differences on voice acoustic parameters (CPP) or significant correlations between voice symptoms and self-reported vocal quality in either the vocally healthy or pMTD groups.

Vocal effort, vocal fatigue, and vocal tract discomfort are the most salient voice symptoms in patients with pMTD

Findings from the present study suggest the most salient voice symptoms in patients with pMTD are increased vocal effort, vocal fatigue, and vocal discomfort. Recent studies have echoed these findings,^3–7 demonstrating patients with pMTD report significantly higher ratings of vocal effort and vocal tract discomfort compared to vocally healthy controls. Other papers have reported odynophonia and aberrant vocal quality are common to patients with pMTD as well.^2,26,27 However, there were no group differences on the latter two voice symptoms found in the present paper.

There are several potential reasons underlying why symptoms of vocal effort, fatigue, and discomfort may be more prevalent in patients with pMTD. First, it is necessary to define each construct to fully paint the picture of why we are seeing these heightened symptoms in pMTD. Effort is characterized by the perceived exertion of mental or physical energy necessary to complete a specific task.^21,22,28 An individual’s perception of effort is also directly correlated with the amount of work expended to achieve the desired outcome.^21,22,28 Fatigue is the state of mental or physical exhaustion caused by continued effort, activity, or stress. It typically presents as “performance fatigue,” which is the decreased ability to perform physiological tasks or maintain performance.²² Fatigue is multifactorial and may result from a wide variety of causes, such as prolonged stress or sustained effort. Discomfort is the experience of unpleasant symptoms within the body.²⁴ The VTDS specifically qualifies vocal tract discomfort by the following symptoms: burning, tightness, dryness, aching, tickling, soreness, and lump in the throat.²⁴ Therefore, the sensation of discomfort experienced in pMTD is likely not severe enough to qualify as pain but is instead an unwanted, irritating sensation.

The first potential reason for the prevalence of heightened somatosensory symptoms in patients with pMTD is how they appraise their vocal effort and vocal fatigue levels compared to vocally healthy controls. Effort can be assessed either objectively (eg, amount of physical exertion within the laryngeal muscles or level of muscle hypercontraction) or subjectively (eg, cognitive or psychological effort required to phonate). In some cases, an individual’s perception of effort may be directly correlated to the amount of physical effort exerted (eg, higher vocal demands result in higher perceived effort).²⁸ Multiple studies have observed this direct correlation between level of vocal demand and reported vocal effort levels in both vocally healthy controls²⁹ and patients with pMTD.^3–6,16,25 Individuals may also perceive their levels of effort in relation to their cognitive capacity (eg, psychological effort) rather than physical effort exerted.²⁸ Two individuals may perform a task with similar physical effort but experience drastically different amounts of effort based on their cognitive appraisal of the vocal effort.

This second phenomenon aligns with what this study and other studies have observed in patients with pMTD. In these studies, vocally healthy controls and patients with pMTD perform the same vocal tasks (eg, CAPE-V sentences, sustained vowels) and present with the same laryngeal muscle patterns, but the pMTD group reports higher levels of perceived vocal effort.^4,5 Patients with pMTD could be appraising their vocal effort within the framework of cognitive effort, suggesting that these voice symptoms the pMTD population experiences may have more to do with lower perceptual effort thresholds and less to do with performance fatigue. Stated differently, a decreased cognitive threshold level for vocal effort could possibly explain why patients with pMTD experience increased vocal effort despite similar vocal demand and laryngeal muscle patterns when compared to vocally healthy controls. However, future work is needed to elucidate this theory.

Appraisal of vocal fatigue may also fall under the same paradigm. Unlike effort and discomfort, fatigue may be a whole body, systemic experience rather than isolated to a specific region. It is possible that patients with pMTD have more intense vocal demands. However, it is more likely that patients with pMTD evaluate their vocal fatigue more severely than vocally healthy controls (many of whom had equally high demands in this study). Smeltzer et al³⁰ found that individuals with functional voice disorders, especially pMTD, may be inaccurate in their ability to perceive sensory feedback and may process their sensory experiences differently.³⁰ Additionally, this study found that patients with functional voice disorders presented with lower attentional regulation. These findings could suggest patients may not tune into or attend to their symptoms until they are severe, at which point it prompts them to present to the voice clinic with more severe symptoms and vocal impact (which would be represented in standard indices like the VHI-10 and VFI-Part 1).³⁰

A second reason could involve laryngeal motor function in individuals with pMTD. Patients with pMTD have also been found to have greater motor imprecision in the paralaryngeal region (eg, infrahyoid muscles), even when there were no muscle tension or hypercontraction patterns observed in these same regions.⁷ These motor imprecisions could suggest that the paralaryngeal muscles are exerting more effort to support the larynx during phonation, which may result in increased sensations of vocal effort and in turn, causing higher levels of vocal fatigue and discomfort as well. For example, if a golfer has an imprecise and ineffective swing, it will require more physical effort to obtain the same results of a more precise swing. If the swing is not corrected, it can lead to limb fatigue and discomfort more quickly. Like the imprecise golfer, an individual with pMTD may require more physical effort to perform the same vocal tasks due to their motor imprecision. This in turn may cause the individual with pMTD to report higher levels or earlier onset of vocal fatigue and vocal tract discomfort, especially with changes in vocal demands.

A third reason could be that individuals with pMTD have altered somatosensory feedback systems. Shembel et al, 2024 found patients with pMTD appraised laryngeal sensations more intensely, though there were no differences in peripheral laryngeal responses to tactile stimulation between vocally healthy individuals and patients with pMTD.¹⁴ The perception of vocal effort, fatigue, and discomfort may not directly correlate to the amount of measured muscle tension. An altered sensory feedback system may cause a person to feel as though they are exerting more effort, even when objective signs of muscle tension are absent. A heightened perception of effort exerted may then contribute to an increased perception of discomfort and fatigue as well.

Additionally, the study reported that despite similar odds ratios of the presence of laryngeal motor airway protection responses to laryngeal stimulation (eg, laryngeal adductor reflexes), there were not only greater perceptions of tactile stimulation severity with the same level of stimuli in the pMTD group but also significantly higher vocal effort, fatigue, and discomfort in the pMTD group compared to vocally healthy controls.¹⁴ This study and several other studies suggests there may be a difference in the sensory processing of laryngeal sensations and the brain’s perception of physiological exertion (eg, vocal effort, fatigue, discomfort) between individuals with pMTD and vocally healthy controls.^14,30 For example, Smeltzer et al³⁰ found that patients with functional voice disorders had a decreased ability to attend to body sensations and found that those with reduced sensory attention and regulation had higher VHI-10 and VFI-Part1 scores. These findings may indicate that individuals with functional voice disorders, including pMTD, have divergent sensory processing and perception of physiological voice symptoms as compared to vocally healthy controls.³⁰ pMTD may also be more appropriately categorized as a central sensitivity disorder like irritable larynx syndrome.^31,32 However, future studies are necessary to elucidate the higher order sensory processing in patients with pMTD.

The lack of significant prevalence of odynophonia and aberrant vocal quality in patients with pMTD may be attributed to several potential reasons. Odynophonia could be caused by increased muscle tension and hypercontraction in the laryngeal muscles.^2,27 However, there is growing literature acknowledging there is little to no muscle tension or laryngeal muscle pattern differences in patients with pMTD compared to vocally healthy controls,^3–7 which may explain why significant global increases in odynophonia were not observed in this study. No group differences in pain or pressure thresholds, both globally or in the paralaryngeal region, have been observed either, further suggesting odynophonia is not an axiomatic symptom to pMTD.¹⁴

In general, pain acts as an alarm system to notify the body of potential injury or damage, thus urging the individual to avoid specific behaviors (eg, vocalization) which cause the pain. Yet, there is no “damage” occurring with vocalizations in persons with pMTD, so the body does not alert the individual to the pain. Several studies, including vocal loading protocols, found no physiological tension or hypercontraction differences in the muscles within and around the larynx pre-post vocal loading in both vocally healthy and pMTD groups.^4,11 Although not statistically significant, patients with pMTD still experienced higher levels of odynophonia. These reports of vocal pain may be the result of central pain sensitivity in some individuals with pMTD. These individuals could still receive erroneous pain signals that tells the person a false narrative about what is occurring in their body. However, further research is needed to understand the underlying mechanisms leading to the various phenotypes of pMTD, including pain perceptions in this clinical population.

Discomfort could be a more prevalent symptom than odynophonia due to the less intense nature of the symptom. While pain is a signal for damage, discomfort is a signal for slight distress or unease. It does not require imminent injury to alert the brain, suggesting that discomfort could be more aligned with a psychological or sensory experience. Discomfort warns the body of unideal circumstances to motivate the individual to adjust their behavior accordingly (eg, shifting seating position, leaving an awkward social situation). Additionally, since there is no “damage” in or around the larynx,^4,5 there may not be cause for aberrant vocal quality to be present in individuals with pMTD. Furthermore, the primary complaints across the pMTD literature refer to the feel of the voice rather than the sound quality of the voice.¹⁹ Patients with pMTD appear to be more concerned about their sensory experiences, which may result in decreased perceptions of aberrant vocal quality in some patients. However, there were no significant group differences on perceptual or acoustic vocal quality between individuals with and without pMTD, suggesting this clinical vocal pattern may be a phenotype of pMTD and not central to the diagnosis.

Acute symptoms of vocal fatigue and vocal tract discomfort significantly correlate with the VFI and VTDS, respectively

The moderate to strong correlations between state VAS and trait standardized indices, both for vocal fatigue and vocal tract discomfort constructs, suggest commonly used standardized indices in the voice clinic, like the VFI and VTDS, can also accurately capture symptoms patients perceive at the time of their voice evaluation. The positive relationship between state and trait vocal fatigue and vocal tract discomfort is integral to understanding the vocal health of patients, especially for patients with pMTD who often have high vocal demands.³³ The significant correlations between state and trait symptoms may be the result of multiple reasons. Individuals who present with high trait vocal fatigue and vocal tract discomfort may be more likely to experience state vocal fatigue and discomfort,^34,35 especially if they have high vocal demands. Alternately, successive periods of acute (state) vocal fatigue may result in or reinforce chronic (trait) vocal fatigue, where the repetitive cycle of acute fatigue episodes creates a susceptibility to trait fatigue.³⁵ Patients with high trait fatigue or discomfort may also require less vocal loading to experience higher levels of state vocal tract discomfort or vocal fatigue.

No significant relationship between voice symptoms and acoustic vocal quality in patients with pMTD

There were no significant relationships between somatosensory voice symptoms and vocal acoustics, nor significant acoustic analysis differences found between participants with and without pMTD. These findings parallel other studies in patients with and without pMTD that found acoustics are a poor biomarker for patients with pMTD³⁶ in general and that acoustics correlate poorly with perceptual measures in individuals with and without pMTD.^3,13,25 These findings also echo previous studies that have shown no relationships between patient-reported symptoms and severity of dysphonia in patients with pMTD.⁸ While some studies found that patients with pMTD have higher fundamental frequencies and lower CPP compared to vocally healthy controls,^37,38 other studies have found that vocal acoustics are not reliable in distinguishing group differences.^36,39,40 The mixed results across studies likely has to do with heterogeneity in clinical presentations of pMTD and the possibility of different sub-cohorts or phenotypes within the heterogeneous spectrum of pMTD. Depending on the snapshot of the cohort captured within each study, the result may or may not present differences in acoustic parameters between groups with and without pMTD, suggesting aberrant vocal quality may be a phenotype and not a central characteristic of pMTD.

For example, certain pMTD phenotypes could involve clinical reporting of concerns with the sound of the voice, while other pMTD phenotypes may not share the same concerns. This is demonstrated in the lack of overall group differences on the vocal quality VAS parameter seen in the present study. While a subset of pMTD patients complain about the sound of their voice, the majority of patients with pMTD report concerns about the feel of their voice.^13,19,36 This aspect of pMTD remains elusive to clinicians, as pMTD patients may present with symptoms typically associated with aberrant vocal quality (eg, increased vocal effort, fatigue, and discomfort) but sound normal. These clinical observations are supported by studies demonstrating persons with pMTD report greater improvement in the feel of their voice following therapy compared to other voice disorders.¹⁹ The pMTD group also noted improvements in the sound of their voice commensurate to the other groups, despite a lack of significant changes in acoustic measures.¹⁹ These findings further support the theory that pMTD is not the direct result of hyperfunctional motor patterns but rather the individual’s perception and appraisal of vocal effort, fatigue, and discomfort. Therefore, perceptual voice symptoms may be the key diagnostic indicator for pMTD rather than acoustics or other measures of vocal quality.

Clinical implications

Self-reported metrics of somatosensory voice symptoms experiences, especially those that assess vocal effort, vocal fatigue, and vocal tract discomfort (eg., VAS, VFI, and VTDS), are a valuable key addition to diagnosing and assessing the severity of pMTD. Considering pMTD is currently largely a diagnosis of exclusion, including voice metrics central to the diagnosis that can be used as inclusionary criteria can provide clinicians with more confidence in diagnosing pMTD. These metrics can also be used during voice therapy to track session-to-session progress and provide more objective data for voice therapy outcomes. These data are especially relevant to therapeutic interventions which largely focus on alleviating voice symptoms and often focus on the feel of the voice (eg, somatosensory discrimination in resonant voice therapy).

In contrast, the results of this study bring into question the clinical utility of acoustics in diagnostic protocols for suspected pMTD. Our findings and previous work suggest that the current standardized acoustic analysis utilized in vocal evaluations does not accurately capture the experiences of all pMTD patients and therefore, should not be as strongly emphasized as a diagnostic indicator of pMTD. The significant group differences on vocal effort, vocal fatigue, and vocal tract discomfort suggest perceptual measures that assess these voice symptoms may be more salient to this clinical population. Stated differently, these findings indicate that vocal effort, fatigue, and discomfort should hold more weight during voice evaluations, and the increased perceptions of these voice symptoms should be added to the inclusion criteria for pMTD.

Limitations

There are several limitations of this study worth mentioning. The first limitation is that even though we could determine the most salient symptoms within a small cohort of patients with pMTD compared to vocally healthy controls, our sample size was not large enough to determine cutoff scores for diagnosis. The second limitation is that only relationships between vocal acoustics and self-perceptual voice symptom severity metrics were evaluated in this study. However, there are various other components of the voice evaluation process, including aerodynamics and laryngeal imaging. The relationships between these other voice evaluation metrics and the severity of somatosensory voice symptoms patients with pMTD experience also require elucidation.

Future directions

Future studies should aim to determine somatosensory VAS cutoff scores to improve diagnostic sensitivity and specificity for pMTD using much larger sample sizes. This is especially important in order to account for the heterogeneity in clinical presentations that can occur across patients with pMTD. Cutoff numbers will provide more accurate diagnostic profiles for patients with pMTD and can begin to more accurately elucidate criteria for inclusion, stepping away from current diagnostic paradigms that are largely exclusionary in nature. Additionally, considering aerodynamic assessment is another key component of gold-standard voice evaluations, and relationships between aerodynamics and somatosensory experiences in patients with pMTD are poorly understood, studies should focus on elucidating relationships between aerodynamic instrumentation and self-perceptual voice symptom severity metrics, in addition to relationships between acoustics and self-perceptual measures of voice symptoms.

CONCLUSION

The most salient voice symptoms in patients with pMTD are vocal effort, vocal fatigue, and vocal tract discomfort, and as such, these voice symptoms should hold considerable importance during voice evaluations and when determining pMTD diagnostic profiles. The significantly higher ratings of self-perceived vocal somatosensory symptoms compared to vocally healthy controls support the growing literature that patients with pMTD perceive their symptoms differently even when motor patterns and measures of vocal quality are comparable between groups. A moderate to strong correlation between state and trait somatosensory voice symptom measures confirms that current standardized self-reporting measures accurately capture state and trait voice symptom severity symptoms at the time of evaluation. Finally, there is no relationship between VAS symptoms and acoustic vocal quality, suggesting that somatosensory symptoms should hold more weight than acoustic analysis during voice evaluations.