A proportion of DLB patients do not suffer from parkinsonism (29%), sometimes even over a prolonged dementia of 5 years or more. However, parkinsonism is more frequently present in DLB than among patients with AD or vascular dementia (VaD) [65]. In the most detailed comparative study of parkinsonism to date, Burn et al. [66] found that DLB patients had less severe parkinsonism than people with PD-D, but a similar severity of motor deficits compared with PD patients without dementia. The severity of parkinsonism, however, appears to progress similarly in both conditions [65]. The majority of patients with DLB will develop characteristics of parkinsonism, including generalized slowing and postural and gait disturbances [67]. There are some subtle but important differences between DLB, PD-D, and PD in the key parkinsonian signs and symptoms. Postural instability and gait disturbance (PIGD), predominantly mediated by non-dopaminergic lesions, is more pronounced in DLB and PD-D than in PD patients without dementia, whereas the opposite is true for tremor [66]. In addition, parkinsonian features tend to be more symmetrical in PD-D [68]. In a prospective study, those who had PIGD-dominant PD at baseline, or who developed this after having tremor-dominant PD initially, had a higher risk of dementia compared than those who maintained a tremor- dominant phenotype [69]. Importantly, parkinsonism is a key factor explaining the functional impairment in DLB [70] as well as PD-D.

Cognitive and functional decline

Only one study has directly compared the course of cognitive decline in PD-D and DLB. This study reported that DLB and PD-D patients had a similar rate of decline on cognition over 2 years [71]. There are some indications that the course of disease is more severe in PD-D and DLB than in AD, although few longitudinal comparative studies have been done. The cognitive decline in PD-D [61] and DLB [72] seems to be similar to that in AD, but, consistent with the complex clinical symptoms in DLB, functional decline and mortality [72] progress more rapidly in DLB than in AD.

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