Time to dementia in PD

The majority of studies report that the mean duration from onset of PD to the development of dementia is about 10 years [11, 18, 21]. There are, however, wide variations. In a study with two large community-based cohorts of patients with PD, a linear relationship was found between time from onset of PD to the diagnosis of dementia. Whereas some patients develop cognitive impairment and subsequent dementia within a few years of disease onset, others remain free from dementia for 20 or more years [11, 26]. The time from onset of PD to dementia is related to clinical risk factors and the type and extent of brain pathology [27]. As well as demonstrating a high risk for the development of PD-D, the Sydney and Stavanger studies also convincingly demonstrated that even after decades with PD there are some people who remain free of dementia. Thus, in addition to identifying the risk factors for developing PD-D, a key research question is to identify factors which protect against dementia in long-standing PD. Similarly, since the vast majority of PD patients will eventually develop dementia, another important question is to identify factors that are associated with the time to develop dementia.

Risk factors for dementia in PD

Many demographic and clinical features have been assessed as potential risk factors for dementia in PD. The most consistent risk factors in longitudinal studies are more severe parkinsonism, in particular non-tremor dominant parkinsonism, higher age, olfactory dysfunction, and mild cognitive impairment (MCI) at baseline, visual hallucinations (VH), and rapid eye movement sleep behaviour disorder (RBD) (Table 2.2).

Table 2.2 Risk factors for dementia in patients with PD

Risk factor

Risk rate


Increased risk [1]


Nine-fold increased risk in group over 80 years compared with group aged 50-59 years [15]. Four- to six-fold increased risk in group over 76 years of age compared with those <65 years [8, 28]

Mild cognitive impairment

Risk increased 3-3.9-fold [14, 30]

Olfactory dysfunction

Twenty-fold higher [31]

visual hallucinations

Four-fold increased risk [31]

Rapid eye movement sleep behaviour disorder

Two-fold increased risk [32]


1.7-fold increased risk for men [15]


Increased cumulative risk

Cerebrovascular factors

Increased risk [33-36]

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