General features, mode of onset, and course of dementia in Parkinson's disease
Dementia associated with Parkinson’s disease (PD-D) demonstrates characteristic features, making it a clinically recognizable entity that is frequently associated with typical pathological changes. These features include its mode of onset, the chronology and course of the symptoms, the profile of cognitive deficits and behavioural symptoms, the associated motor phenotype, and other accompanying features. The characteristic symptoms and signs in each of these domains are described in detail in subsequent chapters. The objective of this chapter is to describe the general features of PD-D including its mode of onset, its course, and its prognosis.
General features of PD-D
The prototypical form of PD-D can be described as a dysexecutive syndrome commonly associated with behavioural symptoms and a postural instability and gait disturbance (PIGD)-dominant motor phenotype . The defining neuropsychological deficits, described in detail in Chapter 4, include prominent and fluctuating impairment in attention, deficits in most aspects of executive function, early and prominent visuospatial deficits (often disproportionate to the overall severity of dementia), relatively mild and usually of retrieval type memory impairment, and largely preserved core language functions except for difficulties with word finding and impaired fluency. Behavioural symptoms include depression, hallucinations, apathy, delusions, and anxiety . Tremor is less frequent and a PIGD phenotype prevails; autonomic dysfunction, especially incontinence, is common. The cognitive, behavioural, and motor features of PD-D are similar to dementia with Lewy bodies (DLB) and different from those seen in Alzheimer’s disease (AD) .
Unlike the majority of patients with AD, who often deny that anything is wrong with their memory, insight into their mental deficits is usually preserved in people with PD-D; they tend to either complain themselves or admit when asked the presence of mental problems. Another defining feature is that patients who have initiation problems or frequent pauses during the performance of a task perform better if provided with guidance, or if external cues are given when they pause. In other words, internally driven performance is worse than that driven by external cues or help. This is similar to the ability of PD patients to walk better when visual guidance is provided. In this sense, cognitive deficits and dementia in PD (at least before the deficits become more advanced and severe) are more characterized by difficulties in the modulation of cognitive functions than losses in their contents.
Dysexecutive syndrome is the prototypical form of dementia seen in patients with PD. There are, however, some patients in whom deviations from this pattern are observed. For example, in the majority of patients the pattern of memory impairment is retrieval-type, with storage of new information being relatively spared, but some may develop a limbic-type amnesia where the new information is not stored, which is more typical of AD . Likewise some patients may show a more AD-like ‘cortical’ pattern of deficits . The extent and topography of Lewy body (LB)-type degeneration as well as the magnitude of the coexisting AD-type pathology are likely to determine the profile, severity, and time course of cognitive and behavioural symptoms. In a clinico- pathological study, patients with a pure LB pathology had a more dysexecutive syndrome, those with pure AD pathology an amnestic syndrome, and those with both pathologies a more mixed cognitive profile .