Spectrum of Lewy body dementias: relationship of Parkinson's disease dementia to dementia with Lewy bodies
Clive Ballard, Sara Garcia-Ptacek, Inger van Steenoven, and Dag Aarsland
Dementia with Lewy bodies (DLB) and Parkinson’s disease (PD) with dementia (PD-D) are characterized by parkinsonism and a dementia syndrome typically dominated by attentional, visuospatial, and executive dysfunction and relatively well-preserved memory. Additional key symptoms are visual hallucinations (VH), cognitive fluctuations, severe neuroleptic sensitivity, and sleep disturbances such as excessive daytime sleepiness and rapid eye movement (REM) sleep behaviour disorder (RBD; a parasomnia manifested by vivid, often frightening dreams associated with simple or complex behaviour during REM sleep) .
The distinction between DLB and PD-D as operationally defined within the standardized clinical criteria for DLB depends entirely upon the duration of parkinsonism prior to dementia. An arbitrary cut-off of 1 year was chosen in the original consensus criteria for the clinical diagnosis of DLB . Thus, PD-D would be diagnosed if dementia occurred more than 1 year after the onset of parkinsonism, whereas dementia prior to, or within, 1 year after the onset of parkinsonism would be classified as DLB. The third report of the DLB Consortium  revised the operationalized diagnostic criteria for DLB. That report highlighted the unresolved issues in the relationship between DLB and PD-D by emphasizing the overall clinical and pathological similarities of the conditions, but at the same time maintaining the arbitrary 1-year rule for distinguishing the two syndromes for research studies, where a distinction is necessary. For clinical practice it was recommended that PD-D should be diagnosed if dementia emerges in the context of clinically diagnosed PD, independent of the time interval. The importance of further research to resolve ‘boundary issues’ was also highlighted, but several key conceptual questions still remain unresolved. For example: are these conditions distinct or part of the same spectrum? If they are distinct, is the arbitrary 1-year rule a meaningful distinction between clinical entities with different clinical presentations? Addressing these issues is critical if we are to take forward our understanding of this spectrum of conditions in establishing biological markers, determining prognostic indicators, and, most importantly, in designing appropriate intervention studies and developing treatment paradigms across the dementias associated with Lewy bodies (LBs).
Studies directly comparing DLB and PD-D patients probably represent the best approach for resolving ‘boundary issues’, but methodological limitations have precluded clear conclusions in many of the comparative studies. For example, critical issues have included the selection of participants (i.e. whether community-based or hospital-based), whether participants were matched for severity of dementia, sample size, sensitivity, and other psychometric properties of the tests used to characterize them, and diagnostic criteria and methods of diagnosis (e.g. diagnosis based on autopsy, prospective clinical assessment, or retrospective chart review). In addition, many studies comparing neuropathology or neurochemistry in PD and DLB have not specified whether PD patients had dementia or not, and, if so, separately described the PD-D patients.
The aim of the current chapter is to summarize similarities and differences between DLB and PD-D from the best available information.