When do you evaluate patients with Parkinson's disease and cognitive decline for concurrent Alzheimer's disease? 

Parkinson’s disease (PD) often includes Alzheimer’s disease (AD) “co-pathology” in up to 80% of cases. This challenges the notion that PD is a synucleinopathy; it affects many proteins beyond α-synuclein. Thus, cognitive decline in PD may be associated with AD pathology, and one of the strongest predictors of dementia in PD is a lower CSF Aβ42 level, the functioning precursor of amyloid plaques.

Given this, I screen for cognitive function annually in patients with PD, especially if concerns have arisen. Currently, rivastigmine is the only FDA-approved treatment for Parkinson’s disease dementia (PDD) and appears particularly effective in those with orthostatic hypotension, likely due to its pyridostigmine-like effects (Espay et al., PMID 3301...

In general, I always ask myself and the patient, "What will we do with the results of a test?" Cognitive decline is part of Parkinson's disease (PD). Symptomatic treatment with rivastigmine (or other acetylcholinesterase inhibitors) and memantine can be helpful without the need for further testing for co-pathology. If anti-amyloid treatment is not an option for patients with PD, then testing for comorbid Alzheimer's would not impact management. Also, in general, I agree with Dr. @Dr. First Last.