Importance: Mild cognitive impairment (MCI) is common in Parkinson disease (PD), but the prognostic value of MCI in early PD is unknown. Objective: To examine the course of MCI and its progression to dementia in an incident PD cohort.

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Epidemiological research aims to provide information on the development, prevalence and progression of diseases, and their associated risk factors. Epidemiological research is thus the basis of increasing our understanding on the aetiology of diseases and as a consequence the starting point for identifying at risk groups in the population, development for novel prevention and treatment strategies, and health care planning. This review provides an overview of the epidemiology of Parkinson's disease, the second most common neurodegenerative disorder, with special emphasis on population-based data on the clinical progression of motor and non-motor features of the disease.

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Early PD-MCI, regardless of persistence or reversion to normal cognition, has prognostic value for predicting dementia in patients with PD.

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These in vivo data support the role of Aβ pathology in the etiology and highlight the potential utility of CSF Aβ42 as an early prognostic biomarker of dementia associated with PD.

Apathy is a common behavioural problem in Parkinson's disease (PD), with important clinical consequences for patients and their families. However, little is known about apathy in early PD. We examined the frequency and clinical characteristics of apathy in 175 nondemented, drug-naïve patients with newly diagnosed PD and 165 control subjects matched for age, sex and education level in Western and Southern Norway. All participants underwent a comprehensive neurological, psychiatric and neuropsychological evaluation. Apathy was diagnosed based on Neuropsychiatric Inventory assessment and recently proposed consensus criteria. Apathy was found in 22.9% of the PD patients, of whom 37.5% had significant depressive symptoms, whereas none of the control subjects were apathetic. Apathy was significantly associated with male gender, higher depression scores and more severe motor symptoms, but was not associated with greater cognitive impairment. When excluding patients with significant depressive symptoms, apathy remained significantly associated with motor severity. Approximately 50% of the caregivers of patients with apathy reported the apathetic behaviour to be at least moderately distressing. The association between apathy and motor severity in our PD cohort suggests a common underlying pathophysiological mechanism. Future studies should explore the longitudinal effect of dopamine replacement therapy on apathetic behaviour in early PD. The relationship between apathy and male gender needs further study to be fully evaluated.