Unifying Divergent Conceptions in Nonfluent/Agrammatic and Semantic Primary Progressive Aphasia

The nonfluent/agrammatic variant of primary progressive aphasia (nfav-PPA) and primary progressive apraxia of speech (PPAOS) are neurodegenerative syndromes that raise diagnostic challenges related to several issues. First, there are two divergent conceptions, one stipulating that (i) nfav-APP and PPAOS are distinct entities, and the other (ii) that PPAOS has to be integrated into the nfav-APP spectrum. A second related issue concerns the consideration of phonological dimensions, lying at the language interface with speech, which could potentially help overcome the nfva-PPA/PPAOS controversy. Third, there is a lack of internationally validated clinical tests assessing apraxia of speech and syntactic abilities with sufficient specificity and sensitivity. This narrative review discusses these issues taking into account clinical, neurocognitive and neurobiological dimensions. It proposes a conceptual-integrative framework conciliating competing nfav-APP/PPAOS accounts while suggesting a graded continuum with subdivisions, related to neurodegenerative expansion throughout language/speech production systems, ranging from syntactic to phonological to phonetic-articulatory impairments. A second controversy in the field of PPA arises from divergent conceptions of semantic PPA (sv-PPA), defined by primary damage to verbal semantics, and of semantic dementia (SD) characterized by multimodal semantic impairments. The current consensus criteria of PPA have deconstructed the initial SD conception by absorbing it into sv-PPA, hence leaving mixed and some non-verbal semantic phenotypes nosologically orphaned. Again, the article proposes a conceptual and integrative model, built on findings from clinical research and cognitive neuroscience, suggesting a graded continuum with subdivisions spanning from verbal to different non-verbal semantic impairments including social-semantic/behavioral phenotypes.