Spasmodic dysphonia

Clients with adductor spasmodic dysphonia (ADSD) tend to exhibit inter- and intraclient variability of signs and symptoms. This variability may result in inaccurate assessment of severity. Accurate assessment of severity requires knowledge concerning the factors that affect the expression of ADSD signs and symptoms. This study examined ADSD sign expression as a function of voicing and syntactic complexity. Fifteen ADSD participants and 15 control participants completed a task consisting of 30 sentences. ADSD signs were significantly more frequent in predominantly voiced sentences than in predominantly voiceless sentences, regardless of level of syntactic complexity. Center-embedded sentences comprising predominantly voiced consonants were found to evoke the greatest number of ADSD signs. These results have important implications for the assessment of ADSD.

Key Words: ADSD diagnostics, ADSD assessment, voiced plosives

Adductor spasmodic dysphonia (ADSD), a focal laryngeal dystonia, is a poorly understood voice disorder that predominantly strikes 30-60-year-olds. In general, clients diagnosed with ADSD exhibit a strained-strangled voice quality with intermittent disruptions/interruptions of voicing (Aminoff, Dedo, & Izdebski, 1978; Aronson, 1990; Aronson, Brown, Litin, & Pearson, 1968; Davidson & Ludlow, 1996; Izdebski, 1992; Reich & Till, 1983).

Numerous studies have provided evidence suggesting that ADSD may be a neurological disorder (e.g., Aminoff et al., 1978; Blitzer & Brin, 1992; Cohen et al., 1989; Feldman, Nixon, Finitzo-Hieber, & Freeman, 1984; Hall & Jerger, 1976; Pool et al., 1991; Robe, Brumlik, & Moore, 1960). More specifically, ADSD is believed to be a motor-control disorder that primarily affects volitional movement of the vocal folds (Ludlow, 1995; Ludlow & Connor, 1987). Persons who display the signs of ADSD may also display signs of one or more focal dystonias such as writer's cramp, blepharospasm, or torticollis (Jacome & Yanez, 1980; Rosenfield et al., 1990; Schweinfurth, Billante, & Courey, 2002). Cohen et al. (1989) found that ADSD speakers who have no visible signs of dystonia involving the eyes nonetheless display increased excitability of blink reflexes, a finding that is characteristic of generalized dystonia (Tolosa, Montserrat, & Bayes, 1988). Thus, ADSD is believed to be a movement disorder that falls within the general category of focal dystonia. Although the characteristics of ADSD and other focal dystonias support a hypothesis of brainstem involvement (Cohen et al., 1989), evidence supporting both brainstem (e.g., Hall & Jerger, 1976; Preibisch, Berg, Hofmann, SoIymosi, & Naumann, 2001) and cortical (e.g., Devous et al., 1990; Levy & Hallett, 2002) malfunction has been reported. However, no single area of neuropathology has been located.

Currently, diagnosis of ADSD is based on medical history, a perceptual voice evaluation, visualization of the anatomy and physiology of the vocal folds, and a general assessment of neurological control (Langeveld, Drost, Frijns, Zwinderman, & Baatenburg de long, 2000; Stewart et al., 1997). Procedures for conducting a perceptual voice examination vary greatly, but generally include informal assessment of the client's speech during conversation and reading. As mentioned by Langeveld and colleagues, an objective "gold standard" perceptual test is not available. However, progress toward a valid perceptual tool is being made (Langeveld et al., 2000; Stewart et al., 1997). Such perceptual tools must take into account interclient and intraclient variability in the expression of ADSD signs (Langeveld et al., 2000; Watson et al., 1991). Factors that are known to influence the expression of ADSD signs must be controlled such that variability is at a minimum, while sign expression is at a maximum, thereby increasing both the reliability and validity of these perceptual tools.

Factors that may result in an increase of ADSD signs include stress, public speaking, fatigue, strong emotions, upper respiratory tract infections, and prolonged use of the voice (Whyte & Darveniza, 1989). These factors must be considered when conducting perceptual evaluations of ADSD. Likewise, any suspected phonetic and/or linguistic factors that might affect the expression of ADSD clinical signs should be considered when developing speech stimuli for use in perceptual evaluations. It has been hypothesized that ADSD sign expression is affected by voiced-voiceless consonantal environment. There is also some evidence that ADSD sign expression may be affected by linguistic complexity.

It is generally believed that the phonetic composition of the speech task may affect expression of the clinical signs of spasmodic dysphonia. Many clinicians use sets of voiced and voiceless sentences as tools to help them diagnose abductor spasmodic dysphonia (ABSD). The origin of this practice appears to stem from two landmark publications (Aronson, 1980; Dedo & Ship, 1980) in which the authors mentioned that, based on clinical observations, the signs of ABSD occur more frequently following voiceless consonants than following voiced consonants. Neither publication provided experimental evidence, nor did the authors discuss the role of consonant environment in the expression of ADSD. Several authors have mentioned that ABSD speakers often exhibit prolonged voice offset during voiceless consonant production (Ludlow, Nauton, Terada, & Anderson, 1991; Merson & Ginsberg, 1979; Wolfe & Bacon, 1976; Zwitman, 1979). However, it was not until much later that Barkmeier, case, and Ludlow (2001) provided support for the idea that ADSD signs occur more frequently following voiced consonants. Although their study was not specifically designed to address this question, Barkmeier et al. noted that expert listeners typically perceived voice breaks, a symptom of ADSD, in voiced environments, while typically perceiving breathy breaks, a symptom of ABSD, in voiceless environments; however, this hypothesis was not tested statistically.

Little is known concerning whether the linguistic content of the speech task might affect the expression of ADSD signs. However, converging evidence suggests that linguistic complexity can affect speech motor control in fluent (Bernstein Ratner & Sih, 1987; Pearl & Bernthal, 1980) and nonfluent children (Bernstein Ratner & Sih, 1987) and in adults who stutter (Hannah & Gardner, 1968; Kleinow & Smith, 2000; Wells, 1979). Results from two studies suggest linguistic complexity may affect ADSD sign expression; however, neither of these studies specifically tested such a hypothesis. In 1985, Bloch, Hirano, and Gould conducted a retrospective study designed to examine the effect of stimulus task on perceptual rating of ADSD severity. The authors collapsed 20 different stimuli into 10 categories of tasks, increasing in order of complexity from noncommunicative vocalizations, to communicative phonation using various modes of vocal fold vibration and prosody, to normal communicative phonation using choral reading and foreign language. The authors presented the results using the collapsed categories only. In their discussion, they concluded that the task severity ratings were dependent on the level of communicative difficulty and that severity increased with language complexity. However, without specific information concerning how linguistic complexity varied within each task, it is difficult to assess these conclusions.

Sapienza, Cannito, and Erickson (1996) examined the rate at which signs were exhibited by speakers with ADSD as they read the first and second paragraphs of "The Rainbow Passage" (Fairbanks, 1960) and found that the highest sign rates occurred in the most syntactically complex sentences. However, these sentences occurred in the second paragraph, were longer than less syntactically complex sentences in the reading, and contained less commonly occurring words. Thus, no specific conclusions could be drawn regarding the influence of syntactic complexity on ADSD sign rates.

Thus, although studies have suggested that voicing and syntactic complexity may influence the expression of ADSD signs, neither of these effects has been tested directly. Also, little is known concerning how these two effects might interact. The goals of this study were (a) to evaluate ADSD sign expression in voiced and voiceless consonant environments and (b) to evaluate the effects of syntactic complexity on the expression of ADSD signs.

Method

Participants

Fifteen adults diagnosed with ADSD (the ADSD group) and 15 adults with normal speech and language (the control group) served in this study. all potential ADSD participants were recruited from the Spasmodic Dysphonia Clinic at the Vanderbilt Voice Center in Nashville, TN. All potential participants in the ADSD group were evaluated by a team of two to four speech-language pathologists and an otolaryngologist. The evaluation consisted of the following: (a) a perceptual evaluation of voice quality and assessment of seventy; (b) an evaluation of other visible neurological signs including distracting sounds, facial movements, head movements, and respiratory patterns; (c) an assessment of the social consequences of the disorder; (d) a comprehensive assessment of the history of the disorder, including information on the date of onset, rate of onset, consistency of signs, coincident events, and prior help sought; and (e) visualization of anatomy and function of laryngeal structures through flexible endoscopy. All potential ADSD participants had either been referred by a neurologist or were subsequently referred to a neurologist for further evaluation. Potential participants were referred to the spasmodic dysphonia clinic at the Vanderbilt Voice Center for treatment of ADSD if the results of the perceptual, neurological, historical, and endoscopie examinations were consistent with ADSD.

Participants selected for inclusion in the ADSD group were free from any other neurological or speech disorder. all participants had been treated previously via injection of botulinum toxin (BTX) into the thyroarytenoid muscle. All participants reported achieving a normal voice subsequent to BTX injection. Such results support a likely diagnosis of ADSD rather than ABSD, but do not rule out the possibility of mixed SD (signs of both ADSD and ABSD). Potential participants with ADSD were excluded from the study if they reported that their voices were normal at the time of data collection. Data for participants were collected immediately prior to a repeat BTX injection, the date of which was self-selected by the participant. Detailed information describing participants with ADSD is presented in Table 1.

Participants in the control group were recruited from faculty and staff in the Department of Audiology and Speech Pathology at the University of Tennessee, Knoxville. These participants reported no history of speech or language disorder and had never received speech or language therapy. all participants in the control group were evaluated by a licensed speech-language pathologist and found to have normal speech. Participants in the control group were age matched (+ or -5 years) and sex matched to participants in the ADSD group.

All participants in both groups completed high school and most (14 ADSD and 11 control) had completed at least 1 year of college-level coursework. The mean number of college years completed was 2.93 (SD = 2.29) for the ADSD group and 3.03 (SD = 3.18) for the control group, with no significant differences between groups, i(28) = 0.099, p = .922.

Speech Material

Ten sentences at each of three levels of syntactic complexity were developed, for a total of 30 sentences (see the Appendix). Five sentences at each level of syntactic complexity consisted of words containing mostly voiced consonants and 5 consisted of words containing mostly voiceless consonants. Each sentence contained 17 to 19 syllables. all words in the stimulus sentences had at least 100 occurrences in the American Heritage Institute database (Carroll, Davies, & Richman, 1971). Order effects were eliminated by using three randomly ordered lists of sentences. The three levels of syntactic complexity used for the stimuli were as follows: (a) simple active affirmative declaratives within prepositional phrases (e.g., The dog buried the dusty bone in the dense garden behind the gate.), (b) right embedded complex sentences (e.g., The girls remembered that their bikes were in the cabinet of the garage.), and (c) center-embedded complex sentences-subject-object (e.g., The new bike that he backed over belonged to the little girl from downstairs.). Mean voiced/ voiceless consonant ratio and mean sentence length for each level of syntactic complexity are reported in Table 2.

Reading of the first paragraph of "The Rainbow Passage" also was included to allow for comparison of results from the sentences to results obtained from standard clinical material. Because such a comparison was not the focus of this study, statistical results from "The Rainbow Passage" are not formally presented in this article.

Procedures

Participants in the ADSD group were recorded on DAT using a Sony digital Walkman and a Sony condenser microphone placed approximately 2 in. from the corner of the mouth. These participants were recorded in a sound-treated treatment room at the Vanderbilt Voice Center. Participants in the control group were recorded on a Sony PCM-R500 digital tape recorder using a Sennheiser MD-441-U microphone placed approximately 12 in. from the lips. These participants were recorded at the University of Tennessee, Knoxville, in an Acoustic Systems RE-144 single-walled sound booth. Recording procedures were slightly different due to the available equipment at each site and the constraints of recording in a clinic environment. The differences in recording procedure between the two groups likely had no effect on the outcome of this study because all recordings were of high quality and perceptual measures (counts of voice signs) were easily obtained under both recording conditions.

Sign Coding

Three syllable-level ADSD signs have been identified by researchers as being key to the classification of the ADSD voice: pitch shift (Ludlow, Naunton, Sedory, Schulz, & Hallett, 1988; Sapienza, Walton, & Murry, 1999, 2000), phonatory break (Aminoff et al., 1978; Davidson & Ludlow, 1996; Langeveld et al., 2000; Ludlow et al., 1988), and aperiodic segment within the syllable (Ludlow et al., 1988; Sapienza et al., 1999, 2000). These three syllable-level signs have been shown to differentiate ADSD speakers from control speakers (Sapienza et al., 1999) and also to differentiate ADSD speakers from those with muscular tension dysphonia (MTD; Sapienza et al., 2000). Glottal fry has been shown to help characterize ADSD speakers as well (Davidson & Ludlow, 1996; Langeveld et al., 2000). Thus, for this study, severity of ADSD was assessed via a procedure in which the following four syllable-level signs of ADSD were tallied: (a) sudden pitch change, (b) phonatory break within the syllable, (c) aperiodic segment within the syllable, and (d) sudden and inconsistent production of glottal fry. A licensed speech-language pathologist trained at the Vanderbilt Voice Center spasmodic dysphonia clinic with experience in the assessment of over 200 individuals with ADSD tallied audible signs from audio recordings of the 30 sentences and "The Rainbow Passage."

Results

Reliability

To assess intra- and interrater reliability, the data for 6 randomly selected participants (3 from the ADSD group and 3 from the control group) were receded approximately 2 months after the initial coding by the original rater and were also coded by a second rater, a graduate student in speech-language pathology who had been trained to recognize the signs of spasmodic dysphonia but was blind to the purpose of the study. For sentences produced by the ADSD group, interrater reliability was 92%, and intrarater reliability was 90%. For sentences produced by the control group, interrater reliability was 99% and intrarater reliability was 97%.

Syntactic Complexity and Voicing Effects

ADSD sign rates for the sentence battery are presented graphically in Figure 1. As expected, almost no voice signs were perceived in the control group recordings. By contrast, the mean number of signs perceived in the ADSD group recordings ranged from approximately 7 to more than 15 per 100 syllables. More signs were perceived in the predominantly voiced than the predominantly voiceless sentences. For the predominantly voiced sentences produced by ADSD speakers, an increasing number of signs were perceived as syntactic complexity increased. This was not the case in the predominantly voiceless sentences, where syntactic complexity seemingly did not affect the expression of ADSD signs. Such results suggest both main effects of voicing and syntactic complexity as well as an ordinal interaction between voicing and syntactic complexity (see Howell, 1992; Keppel, 1991).

To investigate the ordinal interaction between voicing and syntactic complexity, simple-effects analyses were conducted for voiced and voiceless sentences. For voiced sentences, a significant large effect was found for syntactic complexity level at the .05 level, F(2, 28) = 5.,726, p = .008 ([eta]^sup 2^ = .290). Post hoc Sidak comparisons using a family-wise alpha level of .05 revealed that sign rates were significantly higher in center-embedded sentences than in simple sentences (p = .046) or in right-embedded sentences (p = .024). For voiceless sentences, there was no significant simple effect of syntactic complexity.

Discussion

Voicing Effects

Voicing environment was found to affect the expression of ADSD signs. This finding is consistent with the observations made by Barkmeier et al. (2001). More signs were perceived in the predominantly voiced sentences than in the predominantly voiceless sentences, regardless of level of syntactic complexity. If voicing environment, without regard to specific phonetic content, is a major factor in the expression of ADSD, then there should be a strong correspondence between the voiced/voiceless consonant ratio and ADSD sign expression. The mean voiced/voiceless consonant ratio for the tasks decreased in the following order: predominantly voiced sentences (4.1), first paragraph of "The Rainbow Passage" (2.2; reported here as an informal clinical comparison), and predominantly voiceless sentences (0.8). Yet "The Rainbow Passage" evoked ADSD signs at a rate similar to the predominantly voiceless sentences (7.79 signs per 100 syllables). Lack of correspondence between voiced/voiceless consonant ratio and sign rate suggests that if voiced/voiceless environment was a factor affecting sign expression differences between "The Rainbow Passage" and the sentence task, it was not the only factor.

Some researchers believe that ADSD is a movement-control disorder that affects vocal fold adduction for phonation onset (e.g., Ludlow & Connor, 1987; Reich & Till, 1983). Ludlow and Connor suggested that speech tasks requiring rapid laryngeal adjustment would be most impaired. If this is true, more ADSD signs should occur following voiced plosives, the production of which requires the most rapid adduction of the vocal folds, than following consonant productions that require less rapid adduction of the vocal folds. Such was the case in this study. While reading sentences, ADSD speakers were far more likely to exhibit signs either at onset of voicing following voiced plosives or during the vowel following voiced plosives than following any other consonant. ADSD signs were observed following 15% of voiced plosives, but were observed 7% or less of the time following any other category of consonant. Similar results were observed in the first paragraph of "The Rainbow Passage," where ADSD signs followed 10% of voiced plosives. Given this finding, higher proportions of voiced plosives should evoke higher sign rates than lower proportions. This also proved to be true. Voiced plosives composed 31% of all consonants found in the predominantly voiced sentences, whereas voiced plosives composed only 10% of the consonants found in the predominantly voiceless sentences and only 13% of the consonants found in the first paragraph of "The Rainbow Passage." Thus, it may be that the predominantly voiceless sentences and "The Rainbow Passage" evoked ADSD signs at similar rates because they both provided far fewer opportunities for such signs to occur.

Syntactic Complexity and Voicing

ADSD sign rates increased with syntactic complexity, but only when the stimulus sentences contained predominantly voiced consonants. Syntactic complexity did not affect sign expression in the predominantly voiceless sentences produced by participants with ADSD. This finding could not be explained by differences in sentence length, word frequency, or fatigue, as these variables were controlled in the design of the study. There were slight variations in voiced/voiceless consonant ratio and in the distribution of voiced plosives throughout the three levels of syntactic complexity for the predominantly voiced sentences. Nevertheless, these variations did not explain the differences in the syntactic complexity findings between the predominantly voiced and predominantly voiceless sentences.

It might have been that the center-embedded sentences had a higher mean voiced/voiceless ratio than did the simple or right-embedded sentences (thus evoking a higher sign rate in the ADSD speakers); however, this was not the case (see Table 2). Simple sentences had the highest voiced/voiceless ratio (4.62), while right-embedded and center-embedded sentences had a lower voiced/voiceless ratio (3.80).

It also might have been that while the center-embedded sentences had the lowest voiced/voiceless consonant ratio of the three sentence types, they nonetheless contained a higher number of consonants requiring rapid laryngeal adjustments (i.e., voiced plosives), thereby evoking higher sign rates in the ADSD speakers. In the predominantly voiced sentences, voiced plosives composed 32% percent of consonants in simple sentences, 30% of consonants in right-embedded sentences, and 31% of consonants in center-embedded sentences. Given such an equal distribution of voiced plosives across the three levels of syntactic complexity, it seems unlikely that this factor accounted for the observed increase in sign rate as a function of syntactic complexity.

Assuming that the effect of syntactic complexity on the expression of ADSD signs in the predominantly voiced sentences is real, several hypotheses can be postulated to explain why this effect was not present in the voiceless sentences. First, it is possible that syntactic complexity mediates ADSD sign expression in a nonlinear manner. Specifically, it might be necessary for ADSD sign expression to exceed a minimum threshold before the effect of syntactic complexity emerges. If such a model were true, then syntactic complexity should have little or no effect in the predominantly voiceless sentences because they do not evoke ADSD signs at a rate above the threshold. second, it also is possible that there may be a ceiling effect operating in the voiceless sentences, obscuring the effects of syntactic complexity. Speech sounds requiring rapid adjustments of the vocal folds (e.g., voiced plosives) might provide high-probability opportunities for the occurrence of ADSD signs. If so, then the predominantly voiced sentences provide many opportunities for such events to occur, allowing the effects of syntactic complexity to be realized. On the other hand, the voiceless sentences provided few such high-probability opportunities, all of which may have been exhausted in the simple sentences, allowing no room for increased sign rates with increasing syntactic complexity.

Although this study does not provide the type of data necessary to suggest specific neural mechanisms that might explain increased ADSD sign rates with increased syntactic complexity, it is possible that this effect might be explained in terms of resource allocation theory. Numerous competing models of resource allocation have been developed, but generally all such models are based on the idea that resource demands when two tasks are performed simultaneously are greater than the sum of the resource demands when the same two tasks are performed separately (Eysenck & Keane, 1991) and therefore place new demands on coordination and avoidance of interferences. Also central to these models is the notion that resources are limited (Norman & Bobrow, 1975). The two most important resource allocation models are the central capacity theory (Kahneman, 1973) and the theory of multiple resources (Wickens, 1984a, 1984b). Either of these two theories can be used to explain increased sign rates with increasing syntactic complexity in ADSD speakers.

According to the central capacity theory (Kahneman, 1973), a common, limited resource pool must be distributed between sensory, cognitive, and motor tasks. Thus, syntactic planning, phonological planning, and motor execution draw from a central resource pool. Increasing the difficulty of one task may result in decreased performance on another. In the ADSD speaker, increases in syntactic complexity would require a redistribution of resources, possibly resulting in increased ADSD signs.

The theory of multiple resources (Wickens, 1984a, 1984b) provides a more refined model of resource allocation where resources are defined not only in terms of the amount of available resources, but also in terms of the types of available resources. Here, resources are organized into pools according to (a) the encoding model, (b) the encoding type, (c) the different stages of the encoding processing, and (d) the type of response. Tasks that require resources from the same pool cause interferences, resulting in decreased performance on one or more tasks. Syntactic and phonological planning are drawn from the same resource pool. Thus, it may be that in the ADSD speaker, increased syntactic complexity may interfere with resource allocation, resulting in increased ADSD signs.

It is important to note that resource allocation models are theoretical constructs and that each theory is not without its detractors. The data from this study cannot be used to support any one model. Yet, resource allocation in general seems a plausible explanation for increased ADSD signs with syntactic complexity.

Directions for Future Research

The sentence battery used in this study was necessarily lengthy and would not be viable as a diagnostic tool in a clinical setting. Future research is needed to determine whether a small subset of syntactically complex, predominantly voiced sentences evoke clinically useful increases in ADSD signs.

A common diagnostic difficulty that faces voice clinicians is whether an individual has ADSD or some other disorder. Little is known concerning how the interaction between voicing and syntactic complexity might affect the expression of signs in other voice disorders such as MTD or in ADSD's sister disorder ABSD. Future research should focus on the ability of this or similar sentence batteries to aid in the differential diagnose of ADSD versus other voice disorders.

Clinical Implications

Perceptual voice evaluation remains an important part of the assessment process for ADSD. During such evaluation, factors that are known to influence the expression of ADSD signs must be controlled such that variability is at a minimum while sign expression is at a maximum, thereby increasing both the reliability and validity of these perceptual tools. When designing stimulus materials, clinicians should recognize that both the phonetic content and the syntactic complexity of these materials could affect sign expression in ADSD speakers. Great care should be taken to control for both these variables.

Accurate assessment of ADSD signs is important in the process of treatment. Typically, a speech-language pathologist, in conjunction with an otolaryngologist, assesses the client's severity immediately prior to BTX injection. The perceived level of severity is assessed along with other information such as the length of breathy period and the time since last injection to determine the dosage of the next injection. Stimulus materials that evoke maximum signs allow for more accurate assessment of severity and more appropriate determination of dosage. Thus, syntactically complex tasks comprising mostly voiced consonants with a large percentage of voiced plosives might be important tools in the process of dosage assessment.

Summary and Conclusions

This study has shown that consonant voicing and level of syntactic complexity during production of predominantly voiced sentences affect the expression of syllable-level ADSD signs in individuals with ADSD. For these individuals, ADSD signs were significantly more frequent in predominantly voiced sentences than in predominantly voiceless sentences, regardless of level of syntactic complexity. Center-embedded sentences comprising predominantly voiced consonants were found to evoke the greatest number of ADSD signs. These results suggest that syntactically complex sentences comprising mostly voiced consonants, with an emphasis on voiced plosives, could be an important part of a diagnostic battery for ADSD.

Acknowledgments

This research was supported by a Professional Development Award from the University of Tennessee, Knoxville. I must extend my deepest appreciation to the faculty and staff at the Vanderbilt Voice Center for allowing the collection of these data during very busy clinic hours. Particular thanks must be extended to Ed Stone, Cheryl Ballante, Melissa Kirby, Mark Courey, Gaelyn Garret, and James Neterville, all of whom provided great assistance and demonstrated infinite patience during the data collection process.

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Received August 26, 2002

Accepted February 25, 2003

DOI: 10.1044/1058-0360(2003/087)

Molly L. Erickson

University of Tennessee, Knoxville

Contact author: Molly L. Erickson, Audiology and Speech Pathology, University of Tennessee, 578 South Stadium Hall, Knoxville, TN 37996. E-mail: merickso@utk.edu

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