Phenotype-Related Characteristics of Tongue Motor Function in Amyotrophic Lateral Sclerosis: A Retrospective...

This study aimed to characterize lingual motor function in patients with amyotrophic lateral sclerosis (ALS) by analyzing static acoustic indices obtained from sustained vowels and dynamic indices derived from sentence reading. The following aspects were examined: (1) the independence of static and dynamic indices, (2) the relationship between F2 transition duration and F2 excursion, (3) differences in vowel formant frequencies across ALS phenotypes, (4) associations between static indices and speech intelligibility, and (5) acoustic elements contributing to static indices.

Sixty-eight patients with ALS were enrolled (42 men and 26 women). The main analyses focused on the men (n = 42), categorized as Spinal-onset (n = 21), Spinal-onset with bulbar involvement (Spinal + Bulbar) (n = 9), and Bulbar-onset (n = 12). From sustained vowels (/a/-/o/), F1 and F2 values were extracted. The triangular vowel-space area (tVSA) and formant centralization ratio (FCR) were calculated using F1 and F2 of /a/, /i/, and /u/. Dynamic measures were obtained from the diphthong /ai/ within /taiyo/ during sentence reading, yielding F2 transition duration and excursion.

Static indices (tVSA and FCR) correlated with F2 excursion only in the Spinal-onset group (r = 0.458, p = 0.049), whereas no associations were observed in the other phenotypes or in the overall sample. This finding suggests that static and dynamic indices may reflect partially distinct aspects of lingual motor control. tVSA and FCR showed a negative correlation across all phenotypes, confirming that they are complementary indicators of vowel-space expansion and centralization. Regarding time-space control, Bulbar-onset ALS showed no relationship between F2 transition duration and excursion, suggesting that spatial limitations of tongue movement may not necessarily parallel changes in articulatory speed. Phenotype comparisons revealed significant differences in F1 of /e/ and F2 of /a/, /i/, /u/, and /o/. These findings indicate phenotype-related differences in vertical and anterior-posterior tongue movement characteristics. In Bulbar-onset ALS, tVSA correlated with speech intelligibility (visual analog scale: VAS), and FCR showed a trend-level association (p = 0.075). Acoustic element analysis indicated that tVSA was primarily associated with F1 of /a/ and F2 of /i/, whereas FCR was closely associated with F2 of /i/.

These findings suggest that combining static (articulatory position) and dynamic (articulatory transition) acoustic indices enables multifaceted evaluation of phenotype-related characteristics of lingual motor dysfunction in ALS. Bulbar-onset ALS is characterized by restricted anterior-posterior movement and impaired vertical tongue adjustment, both of which may contribute to reduced intelligibility.