Acoustic Vowel Metrics as Correlates of Dysphagia and Dysarthria in Brainstem Neurodegenerative Diseases.

Background/Objectives: Swallowing and speech rely on shared brainstem circuits coordinating oropharyngeal motor functions. In neurodegenerative diseases affecting the brainstem-such as progressive supranuclear palsy (PSP), amyotrophic lateral sclerosis (ALS), and multiple system atrophy (MSA)-bulbar dysfunction often impairs tongue propulsion and motility, affecting both swallowing (dysphagia) and phonation (dysarthria). This study aimed to investigate whether vowel-based acoustic features are associated with swallowing severity in brainstem-related disorders and to explore their potential as surrogate markers of bulbar involvement. Methods: This was a cross-sectional observational study. Thirty-one patients (13 PSP, 12 ALS, 6 MSA) underwent clinical dysarthria assessment, acoustic analysis of the first (F1) and second (F2) formants during sustained phonation of /a/, /i/, /e/, and /u/, and swallowing evaluation using standardized clinical scales (DOSS, FOIS, ASHA-NOMS) and fiberoptic endoscopic evaluation (Pooling Score, Penetration-Aspiration Scale). The vowel space area (tVSA, qVSA) and Formant Centralization Ratio (FCR) were computed. Results: Significant correlations emerged between acoustic vowel metrics and dysphagia severity, especially for liquids. The FCR showed strong correlations with DOSS (ρ = -0.660, p < 0.0001), FOIS (ρ = -0.531, p = 0.002), ASHA-NOMS (ρ = -0.604, p < 0.0001), and instrumental scores for liquids: the Pooling Score (ρ = 0.538, p = 0.002) and PAS (ρ = 0.630, p < 0.0001). VSA measures were also associated significantly with liquid swallowing impairment. F2u correlated with dysarthria severity and all liquid-related dysphagia scores. Conclusions: Vowel-based acoustic parameters, particularly FCR and F2u, reflect the shared neuromotor substrate of articulation and swallowing. Acoustic analysis may support early detection and monitoring of bulbar dysfunction, especially where instrumental assessments are limited.