Dissociable Effects of Dysarthria Etiology and Severity on the Minimally Detectable Change of Speech Intelligibility

Purpose: The minimally detectable change (MDC) provides a threshold for interpreting changes in outcome measures that are outside of measurement error. There is emerging evidence that dysarthria etiology and speech severity impact the MDC of speech intelligibility; however, an investigation of these factors is needed to determine the unique effects of dysarthria etiology and speech severity. The purpose of the current study was to calculate and compare MDCs of speech intelligibility across five dysarthria etiology groups and four severity levels. Method: Speech Intelligibility Test recordings from 200 speakers were used as stimuli, including recordings from healthy controls ( n = 40) and individuals with neurodegenerative diseases such as amyotrophic lateral sclerosis ( n = 16), Huntington's disease ( n = 44), multiple sclerosis ( n = 60), and Parkinson's disease ( n = 40). Thirty inexperienced listeners provided orthographic transcriptions of 20 speakers each. MDCs were calculated, and speech severity levels were categorized based on empirically defined speech severity levels reported in Stipancic et al. (2021). Statistical analyses, including multiple linear regression, assessed the effects of etiology and severity on MDCs. Results: Speech severity was a significant predictor of MDCs, with greater impairments to speech intelligibility resulting in higher MDCs. Dysarthria etiology was not a significant predictor of MDCs. Conclusion: The impact of speech severity on MDCs of intelligibility in dysarthria underscores the need to calculate and provide MDCs across severity ranges to enhance the interpretation of patient outcomes.