Dementia with Lewy bodies and Parkinson disease dementia: More different than similar?

Dementia with Lewy bodies (DLB) is a relatively new diagnostic entity compared with Parkinson disease (PD) and associated dementia, with DLB consensus criteria first published in 1996.(1) In the initial and subsequent DLB consensus criteria, a key feature separating DLB from PD dementia (PDD) has been the order of motor vs cognitive symptoms. In PDD, individuals have a PD diagnosis first based on motor features and subsequently develop dementia. In DLB, dementia is present earlier on. To operationalize criteria for practical use, DLB diagnostic criteria suggested the 1-year rule, where DLB should be diagnosed if dementia emerges before or within 1 year of parkinsonian motor symptoms. This was consistent with commonly used PD criteria, in which early dementia was an exclusion criterion(2) and PDD was diagnosed in the context of established PD.(3) Diagnostic boundaries became more blurred and confusing in 2015, when the Movement Disorder Society published new PD criteria removing early dementia as an exclusion criterion, and subsuming individuals with DLB with parkinsonism into PD, with an optional DLB subtype designation.(4) This led to debates regarding whether DLB and PDD represent the same disease, distinct entities, or conditions on the same spectrum.(5,6) Discussions on this topic continue, and updated DLB criteria maintaining the 1-year rule were published in 2017,(7) while acknowledging that the 1-year marker was a largely arbitrary placeholder for operational use.