Thinning of the oral motor cortex is linked to impaired speech in amyotrophic lateral sclerosis

Most individuals with amyotrophic lateral sclerosis (ALS) develop bulbar impairment as their disease progresses. The ALS Functional Rating Scale-Revised (ALSFRS-R) bulbar subscore and neurological examination of upper (UMN) and lower motor neurons (LMN) are routinely used to assess this dysfunction but have inherent limitations. Speech‑derived measures have shown promise for capturing bulbar decline with greater sensitivity, but their neurobiological correlates remain unclear. This study examined the associations between quantitative speech measures and cortical thinning in ALS. Data from the Canadian ALS Neuroimaging Consortium were analyzed. Speech measures were extracted from audio recordings of the standardized "Bamboo Passage". Cortical thickness was calculated from T1‑weighted MRI scans. General linear models first compared cortical thickness between patients with ALS and healthy controls. Associations between the speech measures and cortical thickness were then assessed within the ALS group. Patients with ALS showed cortical thinning across bilateral frontotemporal regions, with the largest clusters in the bilateral motor cortices. Reduced speaking and articulation rates were associated with thinning in both oral motor cortices. In contrast, the ALSFRS-R bulbar subscore and UMN and LMN bulbar burden showed no significant associations. Measures of pausing behavior were negatively associated with frontal cortical regions. Thinning of the oral motor cortex in ALS was linked to reduced oral motor function, supporting speaking and articulation rate as sensitive markers of bulbar motor neuron degeneration. These measures demonstrated neuroanatomical associations that the ALSFRS-R bulbar subscore and neurological examination findings did not, highlighting their potential value for monitoring bulbar dysfunction in ALS.