Characterizing Speech Errors Across Primary Progressive Apraxia of Speech Subtypes

Apraxia of speech (AOS) is a motor speech disorder affecting articulatory planning and speech programming. When AOS is the sole manifestation of neurodegeneration, it is termed primary progressive apraxia of speech (PPAOS). Recent work has shown that there are distinct PPAOS subtypes: phonetic, prosodic, and those that do not clearly align with either (mixed). PPAOS subtypes differ with respect to the predominating motor speech difficulties, as well as disease progression and underlying pathology. Because past studies have determined PPAOS subtype based on clinical impression, the goal of the present study was to quantitatively determine the distribution of speech error types across PPAOS subtypes in a word repetition task and to investigate how word complexity affects the type and number of speech errors across PPAOS subtypes.

Forty-five patients with PPAOS (13 phonetic, 23 prosodic, and nine mixed) and 45 healthy controls produced multiple repetitions of words that varied in phonetic complexity. Sound additions, deletions, and substitutions/distortions (phonetic errors) and within-word segmentations (prosodic errors) were calculated.

All three PPAOS groups produced significantly more errors than controls, but the total number of errors was comparable among subtypes. The phonetic group produced more phonetic-type errors compared to the prosodic group but comparable to the mixed group. The prosodic group produced more segmentations compared to the phonetic and mixed PPAOS groups. As word complexity increased, the total number of errors increased for PPAOS patients. The phonetic and prosodic groups were more likely to produce phonetic- and prosodic-type errors, respectively, as word complexity increased.

This study provides novel quantitative data showing that PPAOS subtype can be supported by the type and distribution of speech errors in a word repetition task. This may facilitate earlier, more reliable differential diagnosis and aid in disease prognosis, as PPAOS subtypes have distinct disease trajectories.