Research groups  Participants  Main findings 

Kang et al21  PD n=39 (drugnaïve patients), HC n=63  Decrease in PD vs HC 
Compta et al31  Baseline: PD n=27 (nondemented) 18 month followup: PD n=16 (nondemented), PD n=11 (dementia converters)  Decrease in dementia converters 
Bech et al32  PD n=22, PDD n=3, DLB n=11, MSA n=10, PSP n=20, CBD n=3  Decrease in DLB vs other disease groups 
Hall et al26  PD n=90, PDD n=33, DLB n=70, PSP n=45, CBD n=12, MSA n=48, AD n=48, controls n=107  Decrease in AD>DLB+PDD 
Schoonenboom et al33  DLB n=52, PSP n=20, CBD n=16, AD n=512, FTD n=144, VaD n=34, CJD n=6, controls n=275  Decrease in AD>VaD>DLB>CBD 
Parnetti et al27  PD n=38, DLB n=32, AD n=48, FTD n=31, controls n=32 

Andersson et al34  DLB n=47, PDD n=17, AD n=150  Decrease in DLB vs PDD 
Shi et al22  Discovery cohort: PD n=126, MSA n=32, AD n=50, controls n=137 Validation cohort: PD n=83  Slight decrease in PD and MSA vs controls 
Montine et al28  PD n=41, PDD n=11, AD n=49, HC n=150  Decrease in PDD vs HC 
Süssmuth et al29  PSPRS n=20, PSPP n=7, MSAP n=11, MSAC n=14, PD n=23, controls n=20 

Alves et al23  PD n=109, AD n=20, HC n=36  Decrease in PD vs HC 
Ohrfelt et al30  PD n=15, DLB n=15, AD n=66, controls n=55  Decrease in AD+DLB vs controls and PD 
Compta et al24  PD n=20, PDD n=20, HC n=15  Decrease in PDD>PD vs HC 
Parnetti et al25  PD n=20, PDD n=8, DLB n=19, AD n=23, HC n=20  Decrease in DLB>PDD>PD 
AD, Alzheimer's disease; CBD, corticobasal degeneration; CJD, Creutzfeldt–Jakob disease; CSF, cerebrospinal fluid; DLB, dementia with Lewy bodies; FTD, frontotemporal dementia; HC, healthy controls; MSA, multiple system atrophy; MSAC, multiple system atrophy cerebellar type; MSAP, multiple system atrophy parkinsonian type; PD, Parkinson's disease; PDD, Parkinson's disease dementia; PSP, progressive supranuclear palsy; PSPP, progressive supranuclear palsy–parkinsonism; PSPRS, progressive supranuclear palsy–Richardson's syndrome; VaD, vascular dementia.