Dysarthria and Friedreich's ataxia: what can intelligibility assessment tell us?

Background: Friedreich's ataxia is one of the most common hereditary disorders of the nervous system. Dysarthria is a pervasive symptom of Friedreich's ataxia, yet the clinical presentation of speech symptoms remains poorly understood, leaving clinicians without the evidence required to develop therapy interventions.

Aims: The research reported herein had three aims: the first was to document the severity of the intelligibility deficit associated with Friedreich's ataxia dysarthria; the second was to document the phonetic profile of the intelligibility deficit; and the third was to use the results of the phonetic profile to estimate the motor control mechanisms and neurological substrates implicated in Friedreich's ataxia dysarthria.

Methods & Procedures: Eleven adult males with Friedreich's ataxia were recruited. Intelligibility was assessed using a single-word multiple-choice task completed by ten listeners.

Outcomes & Results: Intelligibility severity ratings ranged from mild to severe. The majority of subjects fell into the mild category. The phonetic errors revealed a distinctive profile, with word-final plosive voicing contrast representing the highest source of error. Involvement of cerebro-cerebellar loops was considered to be compatible with the profile results.

Conclusions: Friedreich's ataxia dysarthria has a distinctive phonetic profile and contains subgroups of different severity. High sources of error identified in the phonetic profile should be considered when developing targets for therapy intervention. Neurological substrates are considered to vary across individuals and to include involvement of cerebro-cerebellar loops. Further research involving acoustic analysis of final plosive voicing contrasts would provide further insights into the neurological substrates involved.