Primary Progressive Apraxia of Speech: A Distinct Disorder
Abstract & Commentary
By Michael Lin, MD, PhD, Associate Professor of Neurology and Neurosciences, Weill Cornell Medical College. Dr. Lin reports no financial relationships relevant to this field of study.
Synopsis: Primary progressive apraxia of speech has been well characterized as a distinct neurodegenerative disease, but underlying pathology and prognosis are uncertain in most cases.
Source: Josephs KA, et al. Characterizing a neurodegenerative syndrome: Primary progressive apraxia of speech. Brain 2012;135:1522-1536.
Apraxia of speech (AOS) is a disorder of planning or sequencing move- ments required for speech. Characteristics include trial and error articulatory groping (in which the mouth searches for the correct positions to create sounds, causing sounds to be repeated or distorted, or pauses to be inserted) and inconsistent articulation when asked to repeat the same phrase, particularly with utterances of increasing length or articulatory complexity. Patients are frequently aware of errors and correct themselves. AOS may co-occur with and be difficult to distinguish from aphasia and dysarthria, but isolated AOS is not associated with the errors in comprehension, grammar, or syntax seen in aphasia, and other dysarthrias are not characterized by trial and error articulatory groping or dependence on length of utterance. AOS is most frequently seen with left hemisphere stroke, but also has been described in neurodegenerative disorders, particularly frontotemporal degenerations. In their recent Brain article, Josephs and colleagues characterize primary progressive AOS as a neurodegenerative syndrome.
Over 1 year, 37 subjects with a neurodegenerative speech/language disorder were recruited at the Mayo Clinic. All subjects underwent detailed speech and language examination, neurologic evaluation, neuropsychologic testing, volumetric MRI, tensor-diffusion imaging, FDG PET, and Pittsburgh compound B (PiB) imaging. Dysarthria was permitted, but subjects with any evidence of aphasia were excluded, as were subjects with concurrent illnesses that could account for the speech deficit, including those meeting criteria for any neurodegenerative disease (Alzheimer's disease, Lewy body disease, frontotemporal degeneration, progressive supranuclear palsy, corticobasal degeneration, multisystem atrophy, or motor neuron disease).
Twelve subjects met their criteria for primary progressive AOS. The median age of onset was 73, and eight of the 12 were women. All subjects scored normally on tests for aphasia. There were no consistent neurologic or neuropsychologic findings, though five subjects had executive dysfunction, four had mild limb apraxia, and three had mild parkinsonism. Volumetric MRI showed focal atrophy of superior lateral premotor cortex and supplementary motor area. There was white matter loss underlying the same areas, extending to the inferior premotor cortex and corpus callosum. Tensor diffusion analysis also showed abnormalities in these white matter tracts, plus the premotor components of the superior longitudinal fasciculus. FDG-PET showed focal hypometabolism of the superior lateral premotor cortex and supplementary motor area. Amyloid burden assessed by PiB retention was increased in only one subject.
Commentary
This work defines primary progressive AOS as a clinically distinct neurodegenerative syndrome, different from primary progressive aphasia. The neuroanatomic substrate is focal atrophy and hypoactivity in superior lateral premotor cortex and supplementary motor cortex, consistent with their known function in planning and sequencing motor activity. There was no involvement of the insula, an area sometimes associated with AOS in stroke.
Clinical evolution and ultimate pathologic diagnoses remain to be determined. It is likely that different neurodegenerative diseases may present initially with primary progressive AOS. For example, several of their patients had features of frontotemporal degeneration, corticobasal degeneration, or progressive supranuclear palsy. Only one had PiB retention suggestive of Alzheimer's disease. The long-term clinical and pathologic follow-up of this cohort will be of interest.
Primary progressive apraxia of speech has been well characterized as a distinct neurodegenerative disease, but underlying pathology and prognosis are uncertain in most cases.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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