Spasmodic Dysphonia: a Laryngeal Control Disorder Specific to Speech

Spasmodic dysphonia (SD) is a rare neurological disorder that emerges in middle age, is usually sporadic, and affects intrinsic laryngeal muscle control only during speech. Spasmodic bursts in particular laryngeal muscles disrupt voluntary control during vowel sounds in adductor SD and interfere with voice onset after voiceless consonants in abductor SD. Little is known about its origins; it is classified as a focal dystonia secondary to an unknown neurobiological mechanism that produces a chronic abnormality of laryngeal motor neuron regulation during speech. It develops primarily in females and does not interfere with breathing, crying, laughter, and shouting. Recent postmortem studies have implicated the accumulation of clusters in the parenchyma and perivascular regions with inflammatory changes in the brainstem in one to two cases. A few cases with single mutations in THAP1, a gene involved in transcription regulation, suggest that a weak genetic predisposition may contribute to mechanisms causing a nonprogressive abnormality in laryngeal motor neuron control for speech but not for vocal emotional expression. Research is needed to address the basic cellular and proteomic mechanisms that produce this disorder to provide intervention that could target the pathogenesis of the disorder rather than only providing temporary symptom relief.