Objective: Adductor spasmodic dysphonia (ADSD) is characterized by a strained-strangled voice quality, whose diagnosis relies exclusively on auditory-perceptual features. However, muscle tension dysphonia (MTD) -- a functional voice disorder -- can mimic ADSD thereby contributing to diagnostic confusion. Unlike MTD, ADSD has been described as 'task-specific', implying that certain vocal tasks such as sentences loaded with predominantly voiced consonants will provoke greater sign expression. This investigation examined the diagnostic value of variable sign expression based upon phonetic loading as a means to disambiguate ADSD and MTD.
Patients and methods: Five listeners, who were blinded to the purpose of the study, used a 10-cm visual analog scale to rate the dysphonia severity of two sentences (one all-voiced and one containing primarily voiceless consonants) produced by participants with ADSD (n = 29) or MTD (n = 33).
Results: A mixed-design ANOVA, with Group (ADSD vs. MTD) as the between-subjects variable and Sentence Type (all-voiced vs. voiceless) as the within-subjects variable, confirmed a significant Group-by-Sentence Type interaction effect (p = 0.0002). In ADSD, ratings of dysphonia severity for the all-voiced sentence were significantly more severe than for the voiceless sentence (p < 0.0001), whereas in MTD no significant difference was observed (p = 0.9981). The ROC curve confirmed that differences in dysphonia severity between voiced and voiceless sentences represented a highly specific (88-100%), but only 48% sensitive diagnostic marker.
Conclusions: Phonetic loading influences sign expression in ADSD, and assists in discriminating ADSD from MTD.