Patients with Parkinson’s disease (PD) have a significant risk of developing dementia in the course of their illness. Cross sectional studies suggest a dementia prevalence rate of 30% to 40%.¹ Longitudinal studies indicate that the cumulative frequency of dementia in patients with PD is 60% to 80%.¹ The risk of dementia for individuals with PD is approximately six fold greater than that of age matched controls. The increased rate of dementia in PD reflects the substantial elevation of risk for dementia in patients with PD above the risk posed by Alzheimer’s disease in this same population. Recent studies demonstrate a strong correlation between the occurrence of Lewy body pathology and severity of dementia in patients with PD.² The sensitivity, specificity, positive predictive value, and negative predictive value of cortical Lewy bodies for dementia exceeds that of senile plaques or neurofibrillary tangles,² again suggesting that the dementia of PD is related to the unique pathology of PD and in most cases does not reflect co-occurring Alzheimer’s disease. There is a marked cholinergic deficit in patients with PD and dementia indicating that there is a cholinergic contribution to the cognitive decline.³

The presence of a cortical cholinergic …