Dementia with Lewy bodies (DLB) and Parkinson’s disease dementia (PDD) not only differ for the time of onset of cognitive deficits but also present variability in affected functions which are relevant in understanding underlying pathology. Cognitive performance of two global cognitive screening scales, the Mini-Mental State Examination (MMSE) and the Montreal Cognitive Assessment (MoCA), as well as of a neuropsychological test battery, was evaluated in 18 DLB and 21 PDD patients. Feasibility for each cognitive test was investigated. Both MMSE and MoCA are feasible assessments in PDD and DLB patients. MoCA was more sensitive in discriminating groups as higher number of DLB patients showed pathological performances on the Digit Span Forward subitem (p = 0.049). The Stroop test in PDD and the Trail Making Tests—A and B, and the Benton’s judgment of line orientation tests in both groups were considered not feasible. Among feasible cognitive tests in at least one group, Rey–Osterrieth complex figure test copy (p = 0.013) and semantic fluency (p = 0.038) are sensitive in discriminating DLB from PDD cognitive profile. Trail Making Tests—A and B, the Benton’s judgment of line orientation and the Stroop tests are not feasible for assessing patients with frank dementia. Longitudinal studies should not include those tasks to reduce the risk of missing data once disease progresses and dementia develops. DLB patients present more severe and widespread cognitive dysfunction than PDD, particularly in attentive, visuospatial, and language domains.
