Hoehn and Yahr Scale

This scale was used to classify participants according to the stage of progression of their disease, following the criteria proposed by Hoehn and Yahr [¹⁰]. It evaluates symptom severity on a scale from 0 to 5, where 0 corresponds to asymptomatic patients; 1 indicates unilateral motor involvement; 2 represents bilateral involvement without balance impairment; 3 indicates mild to moderate bilateral involvement with some postural instability but physical independence; 4 corresponds to severe disability, though the person is still able to stand or walk without assistance; and 5 represents patients who are confined to a wheelchair or bed or who are bedridden [⁸,²⁶]. This scale is widely used in research on PD [²⁷].

Voice Handicap Index (VHI-30)

This instrument was used to assess the perceived impact of voice on the physical, functional, and emotional domains. According to Núñez et al. [²⁸], the VHI-30 demonstrates high test-retest reliability (r = 0.822; p < 0.001) and excellent internal consistency (α = 0.93). Its use has been extensively validated in national and international voice research [²⁹-³⁰].

Praat Software (Version 6.1.08)

Praat was used for acoustic voice analysis, allowing quantitative data processing, audio signal manipulation, and observation of vocal emission parameters [³¹-³²].

GRBAS Scale

This scale was used for auditory-perceptual evaluation of the voice and is one of the most widely recognized tools worldwide. Focused on laryngeal-level assessment, it demonstrates a high degree of reliability [³³-³⁴] and is frequently used in clinical practice due to its ease of application. Ratings are based on an ordinal numerical scale ranging from 0 to 3 [³⁵].

Disease Progression Stages

To classify patients according to the progression of PD, the Hoehn and Yahr staging system [¹⁰] was used. This scale is one of the most widely applied and referenced tools in the literature. Its selection was based on its simplicity and its ability to categorize disease progression based on motor symptoms, an aspect particularly relevant for analyzing how these symptoms may influence the vocal alterations observed in this study.

When examining the distribution of participants across disease stages, most were concentrated in stages 2 and 3, with a notable decrease in representation at the initial and final stages. This trend may be explained by the limited manifestation of perceptible symptoms in the early stages and the substantial motor restrictions characteristic of advanced stages, both of which reduce the likelihood of participation in research studies.

Individuals in stage 5 typically present severe motor impairment, complete loss of autonomy, and total dependence for mobility, often being bedridden or wheelchair-bound. This condition substantially limits their access to healthcare centers and, therefore, their inclusion in research. This pattern has been consistently reported in studies conducted both in Chile and internationally, which have documented an absence or paucity of participants in advanced stages of PD due to the challenges associated with attending outpatient evaluations [¹²,²⁰].

Acoustic Voice Parameters

Regarding fundamental frequency, the men in the sample presented a mean value of 163 Hz, which is higher than the normal range described as being between 120 and 140 Hz [⁴⁴-⁴⁶]. In contrast, the women showed a mean of 172 Hz, which is lower than the reference values established around 200 Hz [⁴⁴-⁴⁶]. These findings align with previous research [19], which reported a slight increase in fundamental frequency in men and a decrease in women with PD. Possible explanations include laryngeal and respiratory muscle rigidity, as well as reduced mobility of the pharyngolaryngeal tract [¹³,¹⁵,¹⁸,⁴⁷-⁴⁹].

Other authors have suggested that the increase in F0 in men may be related to generalized increased muscle tone, particularly in the laryngeal musculature. In contrast, the decrease in women may be associated with deficits in intonation control [¹³]. It is essential to consider that the study population consisted of older adults, who naturally experience anatomical and physiological changes linked to aging, including modifications to the phonatory system and, consequently, to vocal characteristics. Nevertheless, Landazuri et al. [¹⁹], in a study of acoustic parameters in 10 individuals with PD, reported differences between older adults with and without PD.

However, the specific contribution of aging, PD itself, and individual compensatory mechanisms to vocal quality remains unclear. Additionally, contradictory results have been reported in studies such as Romero et al. [²⁵], where most participants with PD did not exhibit F0 alterations and remained within normal ranges. This may be attributed to the small sample size and the fact that all participants were under 60 years of age.

With respect to formants, a reduction in F1 was observed in both sexes, which is physiologically related to jaw opening. These findings are consistent with previous studies that have also reported a decrease in F1 [⁵⁰]. Variations in F2 and F3 further suggest alterations in jaw opening and tongue positioning, influenced by the motor symptoms of the disease. In Chile, research on formants in this population is limited; however, the work of Martínez and Soto [²⁰] stands out. They analyzed the phonetic-acoustic performance of Spanish vowels in speakers with PD and reported significant differences in F1 values for /e/ and /u/ in men with and without PD. The authors attributed these changes to reduced jaw opening resulting from diminished movement amplitude and muscular rigidity, both characteristic features of PD [²⁰].

In the present study, F2 and F3 values for the vowel /a/ differed between sexes: in men, F2 remained within normal limits, whereas in women it was reduced. Conversely, F3 was reduced in men but remained within normal limits in women. For F4, both men and women showed increased values compared to average normative data for individuals without vocal pathology [⁴⁴]. F1 and F2 vary according to jaw opening and tongue position, whereas F3 and F4 depend primarily on the length and width of the vocal tract. Thus, considering these mechanisms, the alterations observed in individuals with PD may be explained by reduced muscular activity caused by hallmark symptoms of the disease, such as hypokinesia, bradykinesia, and rigidity, which manifest as limited jaw opening, restricted and slowed tongue movements, and rigidity with shortening of the vocal tract [¹³,²⁰].

Finally, regarding spectral slope (Alpha ratio), typical values range between 6 and 12 dB per octave depending on phonation type [³⁵]. In this study, mean values were −3.54 for men and −3.63 for women. Although no PD-specific studies on spectral slope have been identified, research conducted on individuals with normal and dysphonic voices reports negative values in the latter group, associated with breathier and more hypofunctional voices, features frequently observed in patients with PD. The literature describes that these patients often exhibit reduced vocal fold adduction, resulting in hypophonic voice quality [¹³,³⁸]. These findings are consistent with those reported previously by Jara et al. [¹²] in a pilot sample of 10 individuals with PD, where a visual analysis through laryngeal nasofibroscopy revealed vocal fold hypotonia and fusiform glottic gaps in most participants.

Self-Perceived Vocal Disability

Regarding the self-perception of vocal disability, assessed using the VHI-30 questionnaire, 81% of participants (n = 34) reported a mild level of disability. The remaining eight subjects were evenly distributed between moderate and severe disability. This trend may be explained by the fact that most of the sample was in the earlier stages of the disease, whereas only 33% were in stages 4 and 5. This finding aligns with a previous study in which 90% of individuals with PD reported mild impairment [²⁵].

In the more advanced stages of the disease, the most significant motor alterations occur, which supports the hypothesis that greater motor impairment is associated with increased self-perceived vocal disability. Some earlier studies found no significant association between vocal disability and disease severity; however, these results have been attributed to reduced sample sizes and the inclusion of participants predominantly in the early stages of PD [¹⁷,⁵¹]. When analyzing perceived disability by sex, men reported greater difficulties, which is consistent with other studies [¹⁷,¹⁸]. Nevertheless, this may be related to the fact that men nearly doubled the number of women in the sample, and PD is more prevalent in men.

Another relevant issue concerns the actual awareness that individuals with PD have regarding their vocal difficulties. A study by Majdinasab et al. [¹⁸] compared self-perceived vocal difficulties using the VHI-30 with the perceptions of a family member or caregiver. Their findings revealed differences in the scores, showing that the impact of the voice problem was perceived to be greater by the family member than by the individual with PD. This raises the possibility of a perceptual deficit in individuals with PD regarding their own vocal loudness and speech characteristics. This aspect is particularly relevant in the present study, as the disability ratings were provided exclusively by the participants themselves. Therefore, it is worth questioning whether the results in this section truly reflect the actual disability experienced by individuals with PD or rather their subjective perception of it.

Similar results were recently reported in another study conducted in Chile, which compared the vocal difficulties perceived by individuals with PD and their relatives. Regardless of the stage of disease progression, relatives perceived the vocal difficulties as more severe than the patients themselves, particularly regarding functional aspects, indicating that individuals with PD tend to become more introverted over time [⁵²].

In this context, several studies support this interpretation, suggesting that individuals with PD exhibit a somatosensory deficit that limits their ability to perceive their own vocal difficulties, particularly in the physical domain (e.g., sensations of effort, tension, or air loss). A greater awareness of such difficulties often emerges only when swallowing becomes compromised or when depressive symptoms are more pronounced [⁵³,⁵⁴].

Additionally, it is important to consider whether the VHI-30 is a valid and reliable instrument for assessing the specific types of vocal difficulties experienced by individuals with PD. Although it is widely used to measure self-perceived vocal difficulties, it was not designed to capture the unique vocal challenges associated with this population. Therefore, it is possible that certain vocal difficulties in people with PD may not be fully captured by this instrument.

Notably, adaptations of the VHI already exist for transgender populations, pediatric populations, and singers, among others, underscoring the importance of tailoring assessment tools to the specific characteristics and needs of different groups.

Perceptual Voice Evaluation

The auditory-perceptual evaluation of participants’ voices, conducted by three external judges using the GRBAS scale, showed that the most frequently altered vocal parameters were roughness, followed by breathiness and asthenia. This is consistent with the literature [¹³-¹⁴,⁴⁸,⁵⁵], which reports that individuals with PD may present: (1) roughness, attributed to compensatory mechanisms in response to generalized body and laryngeal rigidity; (2) hoarseness, potentially associated with rigidity of the cricothyroid muscle; (3) asthenia, resulting from inadequate respiratory support and incomplete glottal closure; and (4) breathiness, due to insufficient vocal fold adduction [¹³-¹⁴].

According to the Yanagihara classification of hoarseness, 83% of the sample fell within grades 1 and 2, which may be related to most participants being in the early stages of the disease.

Integration of Findings

When jointly analyzing the acoustic measures, perceptual evaluation, and self-perceived vocal disability, a pattern consistent with the pathophysiology of PD becomes evident. Rigidity, hypokinesia, and reduced articulatory amplitude generate alterations in formant frequencies (F1-F4) and produce a more negative spectral slope, which perceptually manifests as roughness, breathiness, and asthenia. However, these objective alterations are not always reflected in higher self-reported vocal disability, which aligns with evidence suggesting a possible somatosensory deficit characteristic of PD that may limit patients’ awareness of their own vocal difficulties.

The absence of statistically significant differences across disease stages suggests that vocal impairments may emerge early and remain relatively stable as PD progresses. This could explain the similar acoustic and perceptual patterns observed in stages 2 and 3. Additionally, the reduced sample size within each disease stage likely decreased the statistical power needed to detect differences. Similar findings were reported by Majdanisab et al. [¹⁸], who also found no association between disease severity and vocal disability in a sample composed primarily of individuals in the early years of disease progression.

Internationally, the study by Cavazos [⁵⁶] is noteworthy for including 47 individuals with PD and assessing videostroboscopy, acoustic parameters, and self-perception of voice. Although no significant associations among the evaluated parameters were found, all measures were altered relative to normative values from individuals without vocal pathology, a finding that is consistent with the results of the present study.

The clinical heterogeneity of PD may also explain variability among studies. Several authors [⁵⁷-⁵⁸] have suggested the existence of distinct disease subtypes, considering differences in clinical presentation, the presence of age-related comorbidities, and variability in motor symptom progression. Some studies describe a rapid progression of motor symptoms during Hoehn and Yahr stages 1 and 2, followed by stabilization in stages 3 and 4 [⁵⁹-⁶¹]; however, other studies, like that of Scollo et al. [⁵⁸], have not confirmed this pattern, partly due to the limited number of participants in advanced stages.

Regarding the absence of sex-related differences, the findings suggest that the impact of PD on phonatory function may be sufficiently broad to attenuate baseline anatomical and physiological differences between male and female larynges. This aligns with the observations of Gnerre et al. [⁶²], who reported no sex differences in jitter, shimmer, or harmonic-to-noise ratio among individuals with PD, unlike in healthy controls. Furthermore, biomechanical studies have shown that variables such as fundamental frequency, maximum phonation time, and glottal closure tend to be affected similarly in both sexes [²⁵]. This phenomenon may also help explain the lack of differences in self-perceived vocal difficulties across sex and disease stage.

Finally, Chiaramonte and Bonfiglio [¹³] noted that many acoustic studies in PD rely on small samples and heterogeneous methodologies, which limit the ability to detect subtle effects or sex-related differences. This widely documented methodological limitation may also contribute to the lack of significant associations in the present study’s comparative analyses.

Despite these challenges, the present findings hold important scientific and clinical implications. Having a sample of 42 participants represents a valuable contribution, given the well-known difficulty of recruiting individuals with PD for research studies. The results provide a multidimensional understanding of the impact of PD on voice by integrating acoustic, perceptual, and self-perceptual measures, and they offer insight into how individuals with PD experience and recognize their vocal difficulties. Clinically, these findings reinforce the need to continue developing and validating PD-specific assessment protocols, as well as to further explore the roles of age, disease progression, and individual variability in the mechanisms underlying vocal alterations in PD.

Limitations

One of the main limitations when studying specific populations, such as individuals with PD, is the difficulty in obtaining an adequate sample size, particularly in the early and late stages of the disease.

Additionally, the sample was composed of participants from a single city who attended the same local PD association, which does not reflect the diversity of individuals with PD across different regions. Therefore, future studies should consider recruiting participants from various geographic locations and diverse contexts. Another limitation was the lack of a PD-specific self-assessment scale for vocal function validated in Chile, which hampers the ability to accurately measure self-perceived vocal difficulties among this population.

A potential source of bias stems from collecting vocal samples at only one time of day. The voice is known to fluctuate in PD even during the same day, largely influenced by medication intake. Previous research has shown that fundamental frequency (F0) may vary depending on whether the individual is in the “on” or “off” medication state (Pinho et al., 2018). This variability may partly explain why it is so challenging to characterize a pathognomonic vocal profile for PD or to distinguish vocal characteristics across disease stages.