Essay --

There is compelling evidence that cognitive dysfunction is inherent to Parkinson’s disease (PD) (Aarsland et al. 2003; Aarsland et al. 2007a; Aarsland et al. 2010a). Clinical manifestations of cognitive deterioration in PD range from slight deficits -only demonstrable by means of comprehensive neuropsychological testing - up to dementia (Muslimovic et al. 2005; Aarsland et al. 2009; Foltynie et al. 2004). From the earliest stages of the disease, including drug-naà ¯ve subjects, patients suffering from PD may show cognitive impairment (CI). This can be restricted to a single domain or affects multiple cognitive domains (Muslimovic et al. 2005; Foltynie et al. 2004). Based on recent longitudinal studies, there is some evidence suggesting that, along the evolution of the disease, a subgroup of patients presenting defects on distinct cognitive domains will eventually deteriorate to the point of dementia associated to PD (PDD) (Aarsland et al. 2003; Buter et al. 2008; Hely et al. 2008; Emre et al. 2007). Starting on a similar theoretical approach than those used for MCI in Alzheimer’s disease (AD) -where early cognitive deterioration linearly progress to dementia- (Petersen et al. 2001a; Petersen et al. 2001b) an operative redefinition of the construct of MCI in PD has been proposed to identify and diagnose these initial cognitive deficits as early indicators of PDD (Caviness et al. 2007). Historically, MCI has been considered as the transitional stage between normality to dementia, based on the measurable presence of cognitive dysfunctions in single or multiple cognitive domains without concurring disabilities on activities of daily living (Petersen 2004). MCI in AD usually follows a linear progression from subtle deficits to dementia (... ...mild cognitive impairment to dementia in PD patients is characterized by the addition of cortical-type cognitive deficits atop a prominent and progressive frontal-striatal dysfunction. Besides the search for biomarkers, a usefulness definition and development of diagnostic criteria for PD-MCI, should consider to: (i) delimitate the heterogeneous cognitive deficit of PD and how we can accurately assess it in large sample of PD subjects; (ii) establish with prospective studies whether the prognostic value of the severity and the nature of the cognitive deficits; (iii) find a consensus of the minimum of cognitive tasks and instruments to assess cognition in PD and finally, (iv) delimitate the role played by common PD-related neuropsychiatric features such apathy or visual hallucinations as early markers of dementia in absence of evident neuropsychological impairment.