Word Count Matters: Features of Written Language Production in Progressive Apraxia of Speech with and without Agrammatism

Introduction: Apraxia of speech (AOS) is a motor speech disorder characterized by sound distortions, substitutions, deletions, and additions; slow speech rate; abnormal prosody; and/or segmentation between words and syllables. AOS can result from neurodegeneration, in which case it can be accompanied by the primary agrammatic aphasia (PAA), which when presenting together are called AOS+PAA. AOS can also be the sole manifestation of neurodegeneration, termed primary progressive AOS (PPAOS). Together these form the agrammatic-apractic spectrum disorders. Recent work has shown that agrammatic-apractic spectrum patients show reduced quantity of written language production on a picture description task versus controls. However, no study to date has investigated if there are differences in quantity (amount of writing) and quality (grammaticality) in the written language production between PPAOS and AOS+PAA patients, which was the aim of this study. Methods: Twenty-four AOS+PAA patients, 24 PPAOS patients, and 24 typical controls performed the Western Aphasia Battery (WAB) written picture description task. The total number of words and sentences, as well as the type-token frequency, mean length of utterance, proportion of nouns, and function words, and overall sentence grammaticality were compared among groups. Results: The PPAOS group showed significantly reduced number of words (beta = -44.2, p < 0.0001) and sentences (beta = -4.04, p < 0.0001) compared to typical controls, and the AOS+PAA group showed significantly reduced number of words compared to both PPAOS patients (beta = -17.0, p = 0.02) and controls (beta = -61.20, p < 0.0001), as well as reduced number of sentences compared to controls (beta = -4.33, p < 0.0001). AOS+PAA patients also showed grammatical deficits consistent with their concomitant aphasia diagnosis. Conclusions: This study provides novel quantitative data showing that agrammatic-apractic spectrum disorder patients show decreased written language output on a written picture description task compared to controls, even when there is no overt evidence of aphasia (i.e., PPAOS). Furthermore, these data show that controls, PPAOS patients, and AOS+PAA patients can all be distinguished based on the quantity of information and grammatical errors in a written picture description task. Future studies will explore sources beyond language, such as motoric impairment, that may result in reduced written quantity in agrammatic-apractic spectrum disorders.