Speech Fluency in Frontotemporal Lobar Degeneration

Impaired speech fluency is a central feature of progressive language disorders, but the cognitive and neural bases for impaired fluency are unclear. Proposed causes of fluency impairment include deterioration of syntactic competence, interfering with the construction of well-formed utterances; a semantic deficit that slows lexical access; a deficit in executive function affecting working memory, planning, mental search, and other processes; or a motor apraxia affecting the articulation of words. We studied speech fluency (words per minute) and associated contributing factors in patients with frontotemporal lobar degeneration (FTLD) in order to assess these components of fluency in three subgroups of FTLD: progressive nonfluent aphasia (PNFA), semantic dementia (SemD), and a social and executive disorder without obvious aphasia (Soc/Exec). Speech fluency was assessed in a semi-structured speech sample, and we analyzed other language characteristics of this speech sample as well as neuropsychological measures of language and executive functioning. Voxel-based morphometry analyses of high resolution MRI were used to identify the neuroanatomic correlates of impaired fluency. We found that impaired speech fluency in PNFA is due in part to deficits of grammar and verb use, but not to speech errors or executive difficulty, and this correlates with left frontal atrophy. SemD patients exhibit reduced speech fluency correlated with aspects of word meaning. Soc/Exec patients, though not aphasic, also have impaired speech fluency, related to broad executive limitations, and this correlates with bilateral atrophy in frontal cortical regions.