Cholinergic depletion and basal forebrain volume in primary progressive aphasia

Primary progressive aphasia (PPA) is a heterogeneous syndrome with various neuropathological causes for which no medical treatment with proven efficacy exists. Basal forebrain (BF) volume loss has been reported in PPA but its relation to cholinergic depletion is still unclear. The primary objective of this study was to investigate whether cholinergic alterations occur in PPA variants and how this relates to BF volume loss. An academic memory clinic based consecutive series of 11 PPA patients (five with the semantic variant (SV), four with the logopenic variant (LV) and two with the nonfluent variant (NFV)) participated in this cross-sectional in vivo PET imaging study together with 10 healthy control subjects. Acetylcholinesterase (AChE) activity was quantitatively measured in the neo- and allocortex using N-[¹¹C]-Methylpiperidin-4-yl propionate (PMP)-PET with arterial sampling and metabolite correction. Whole brain and BF volumes were quantified using voxel-based morphometry on high-resolution magnetic resonance imaging (MRI) scans. In the PPA group, only LV cases showed decreases in AChE activity levels compared to controls. Surprisingly, a substantial number of SV cases showed significant AChE activity increases compared to controls. BF volume did not correlate with AChE activity levels in PPA. To conclude, in our sample of PPA patients, LV but not SV was associated with cholinergic depletion. BF atrophy in PPA does not imply cholinergic depletion.