excessive daytime sleepiness. 
Prevalence 
The prevalence of Parkinson’s disease in the United States steadily increases with age from
approximately 0.5% between ages 65 and 69 to  3% at age 85 years and older. Parkinson’s
disease is more common in males than in females. Among individuals with Parkinson’s
disease, as many as 75% will develop a major NC D sometime in the course of their disease.
The prevalence of mild NCD in Parkinso n’s disease has been estimated at 27%. 
Development and Course
Onset of Parkinson’s disease is typically betw een the sixth and ninth decades of life, with
most expression in the early 60s. Mild NCD ofte n develops relatively early in the course of
Parkinson’s disease, whereas major impairme nt typically does not occur until late. 
Risk and Prognostic Factors
Environmental. Risk factors for Parkinson’s disease include exposure to herbicides and
pesticides.
Genetic and physiological. Potential risk factors for NC D among individuals with Par-
kinson’s disease include older age at diseas e onset and increasing duration of disease. 
Diagnostic Markers
Neuropsychological testing, with a focus on test s that do not rely on motor function, is crit-
ical in detecting the core cogn itive deficits, particularly at the mild NCD phase. Structural
neuroimaging and dopamine transporter scans, such as DaT scans, may differentiate
Lewy body–related dementias (Parkinson’s and dementia wi th Lewy bodies) from non–