Syndromes dominated by apraxia of speech show distinct characteristics from agrammatic PPA

We assessed whether clinical and imaging features of subjects with apraxia of speech (AOS) more severe than aphasia (dominant AOS) are more similar to agrammatic primary progressive aphasia (agPPA) or to primary progressive AOS (PPAOS).

Sixty-seven subjects (PPAOS = 18, dominant AOS = 10, agPPA = 9, age-matched controls = 30) who all had volumetric MRI, diffusion tensor imaging, F18-fluorodeoxyglucose and C11-labeled Pittsburgh compound B (PiB)-PET scanning, as well as neurologic and speech and language assessments, were included in this case-control study. AOS was classified as either type 1, predominated by sound distortions and distorted sound substitutions, or type 2, predominated by syllabically segmented prosodic speech patterns.

The dominant AOS subjects most often had AOS type 2, similar to PPAOS. In contrast, agPPA subjects most often had type 1 (p = 0.01). Both dominant AOS and PPAOS showed focal imaging abnormalities in premotor cortex, whereas agPPA showed widespread involvement affecting premotor, prefrontal, temporal and parietal lobes, caudate, and insula. Only the dominant AOS and PPAOS groups showed midbrain atrophy compared with controls. No differences were observed in PiB binding across all 3 groups, with the majority being PiB negative.

These results suggest that dominant AOS is more similar to PPAOS than agPPA, with dominant AOS and PPAOS exhibiting a clinically distinguishable subtype of progressive AOS compared with agPPA.