Parkinson's disease is associated with a loss of striatal dopaminergic neurons, particularly in the pars compacta region of the substantia nigra. Tremor is the best recognized symptom and is present in approximately half of individuals with PD (Martin et al., 1983). Often tremor begins unilaterally, increasing with stress and disappearing in sleep. Other early symptoms include aching, paresthesias, and numbness and tingling on one side of the body that ultimately spread to the other side. Other classic motor symptoms are rigidity, slowness of movement, and alterations in posture.
Not all individuals with PD develop dementia, and prevalence estimates vary. Marttila and Rinne (1976), in one of the most comprehensive studies of prevalence, reported it to be 29%. Other investigators have reported similar estimates of dementia prevalence (Rajput and Rozdilsky, 1975; Mindham, Ahmed, and Clough, 1982; Huber, Shuttleworth, and Christy, 1989). Widely debated is the cause of the dementia, with some attributing it to cortical degeneration and others to subcortical damage that impairs neurological control of attention (Brown and Marsden, 1988). Rinne and colleagues (2000) argue that reduced fluorodopa uptake in the caudate nucleus and frontal cortex produces impaired performance on neuropsychological tests that require executive function.
Individuals with PD, regardless of whether they develop dementia, have speech motor deficits because the disease damages the basal ganglia and striatal-cortical circuitry, which are involved in motor function. Those who develop dementia have problems communicating for other reasons, namely deficits in memory, attention, and executive functions. However, considerable evidence exists that language knowledge generally is preserved (Pirozzolo et al., 1982; Bayles and Tomoeda, 1983; Huber et al., 1986). Bayles (1997) argued that impaired performance on tests that manipulate language, such as confrontation naming and sentence comprehension, result more from nonlinguistic cognitive deficits than a loss of linguistic knowledge.
See also Alzheimer's disease.
—Kathryn A. Bayles References
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