A separation of innate and learned vocal behaviors defines the symptomatology of spasmodic dysphonia

Abstract

Objective: Spasmodic dysphonia (SD) is a neurological disorder characterized by involuntary spasms in the laryngeal muscles. It is thought to selectively affect speaking; other vocal behaviors remain intact. However, the patients' own perspective on their symptoms is largely missing, leading to partial understanding of the full spectrum of voice alterations in SD.

Methods: A cohort of 178 SD patients rated their symptoms on the visual analog scale based on the level of effort required for speaking, singing, shouting, whispering, crying, laughing, and yawning. Statistical differences between the effort for speaking and the effort for other vocal behaviors were assessed using nonparametric Wilcoxon rank-sum tests within the overall SD cohort as well as within different subgroups of SD.

Results: Speech production was found to be the most impaired behavior, ranking as the most effortful type of voice production in all SD patients. In addition, singing required nearly similar effort as speaking, ranking as the second most altered vocal behavior. Shouting showed a range of variability in its alterations, being especially difficult to produce for patients with adductor form, co-occurring voice tremor, late onset of disorder, and familial history of dystonia. Other vocal behaviors, such as crying, laughing, whispering, and yawning, were within the normal ranges across all SD patients.

Conclusion: Our findings widen the symptomatology of SD, which has predominantly been focused on selective speech impairments. We suggest that a separation of SD symptoms is rooted in selective aberrations of the neural circuitry controlling learned but not innate vocal behaviors.

Level of evidence: 4 Laryngoscope, 129:1627-1633, 2019.

Keywords: Laryngeal dystonia; learned vocal behaviors; voice symptoms.

Figure 1.

(A) Density plot depicts the distribution of voice symptoms based on the ratings using a 10-point visual analog scale. Innate vocalizations are heavily skewed towards lower severity values, while voluntary vocalizations show the opposite trend. (B) The empirical cumulative distribution curves for each rating scale among all patients display differences in distribution between the voluntary and innate vocalization curves. Whispering, yawning and crying show a convex, left skew, with higher probabilities at lower rankings, while shouting, singing, and speaking show the opposite, concave, right skew, with higher probabilities associated with higher rankings.

Figure 2.

Density plots and empirical cumulative distribution curves show symptom ranges in different phenotypes and genotypes of SD, including (A) abductor and adductor forms; (B) SD with and without voice tremor; (C) SD with early and late onset of disorder; (D) familial and sporadic cases.

Figure 3.

Schematic distribution of SD symptomatology, with increasing symptom manifestation (left to right) from yawning (innate voiceless behavior) to speaking (learned, most complex voiced motor behavior). Gray shading (light to dark) indicates the severity of symptoms during the production of different vocal behaviors, which are grouped based on the organization of their neural control.