Persistent Speech Sound Disorder: Characteristics and Course in a Multigenerational Family

Abstract

Limited research exists on persistent SSDs that affect speech intelligibility. This leaves people with significant impairments, their families, and clinicians under-informed about a disorder that has lifelong consequences. This thesis studied the characteristics, course, and outcomes for individuals with significant Speech Sound Disorders persisting into the school years, adolescence, and adulthood. This was achieved through the comprehensive phenotyping of a unique, large family (the PM family) with a high aggregation of severe, multigenerational SSD persisting beyond 9 years of age. This thesis begins with a critical review of the persistent SSD literature including definition, prevalence, classification, cause, comorbidity, risk factors, characteristics, course, and outcomes. Study 1 described the phenotype in a nuclear family (n=11). Speech, language, literacy, phonological processing, numeracy, cognition, oro-motor, and manual motor skills were assessed. A core phenotype distinguished persistent from resolved SSD cases that was characterised by a multiple verbal trait disorder, including Childhood Apraxia of Speech (CAS). Study 2 examined the emergence, course, and persistence of speech errors into adulthood in family members with persistent CAS (n=7: the father & his twin, 5 offspring) using retrospective and prospective data. This study proposed the majority of persistent errors reflected an impaired ability to rapidly coordinate multi-articulator movements across multiple segments and syllables. Support included the presence of uniquely described segmental and transitional distortion errors, and bi-directional errors. Study 3 analysed the course of persistent CAS across the lifespan; and the speech, language, educational, socio-emotional, and vocational outcomes in the most severely affected family member. The findings of this thesis add rich information on a neglected area of study, and form the basis for future phenotype-genotype and neuroimaging studies.