Symptoms of depression are commonly observed in patients with PD. Prevalence rates for depression in PD range from 7% to 90%, although 40% is the most frequently cited estimate. Approximately one-half of PD patients become depressed at some point during the disease course (94), with about half of these patients developing minor depression, whereas the other half develops major depression. Depression is a known risk factor for PD and PDD (66,95), and has been shown to adversely impact functional ability (96,97) and accelerate the progression of cognitive decline in PD (98,99).
Depression in PD is unique in that, unlike in other neurodegenerative conditions such as AD, it significantly affects cognition (100). Executive functions and memory are foremost among the neuropsychological abilities impaired by depression (101-103). The negative impact of depression on cognition is more readily evident in the later stages of PD, and depression must be of at least moderate severity before it markedly impacts cognition (104,105).
In light of depression's detrimental effect on cognition, an important clinical question with treatment implications is whether cognitive and/or functional decline in PD is a dementia due to neurodegeneration or due to depression. Little literature addresses the incidence and prevalence of dementia due to depression in PD, and whether dementia in patients with comorbid depression improves with treatment and resolution of depressive symptomatology (106). Etiologic inferences about an individual PD patient's dementia, when the dementia is accompanied by marked depression, should probably be deferred until such time as the depression has been adequately treated and neuropsychological reevaluation has been performed. Recent attention has also been drawn to the need to distinguish depression from apathy in PD (107). Apathy may occur in as many as 45% of patients with PD and, like depression, may be associated with executive deficits (108).
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