Differential effects of velopharyngeal dysfunction on speech intelligibility during early and late stages of Amyotrophic Lateral Sclerosis

The detrimental effects of velopharyngeal dysfunction (VPD) on speech intelligibility in persons with progressive motor speech disorders are poorly understood. In this study, we longitudinally investigated the velopharyngeal and articulatory performance of 142 individuals with varying severities of amyotrophic lateral sclerosis (ALS). Our goal was to determine the mechanisms that underlie the effects of VPD on speech intelligibility during early and late stages of ALS progression. We found that during the early stages of the disease, the effect of VPD on intelligibility was partially mitigated by an increase in articulatory (e.g., lower lip and jaw) movement speed. This apparent articulatory compensation eventually became unavailable during the late stages of disease progression, which led to rapid declines of speech intelligibility. The transition across the early and late stages was characterized by the slowing of the composite movement of lower lip and jaw below 138 mm/s, which indicated the onset of precipitous speech decline and thus, may provide important timing information for helping clinicians to plan interventions.