Mild cognitive impairment in Parkinson's disease: how much testing is needed for correct diagnosis?

Cognitive deficits are one of the most common non-motor symptoms of Parkinson's disease (PD). Mild cognitive impairment (MCI) affects about 27% of non-demented PD patients. The high prevalence of PD-MCI and PD with dementia (PD-D) as well as increasing life expectancy in PD create the need for valid serial cognitive assessment in every PD clinic. In this paper we discuss the PD-MCI criteria and testing recommendations (Movement Disorder Society Task Force Guidelines) in the context of referral for neuropsychological assessment in clinical practice. The methodology suggested in the PD-MCI diagnosis guidelines is compared against PD-D testing recommendations. The requirement for at least10 cognitive tests to allow PD-MCI subtype to be determined is questioned, as a direct correspondence between low scores on a particular test and a domain-specific deficit cannot be assumed. As a variety of factors may underlie an impaired test score, the same test score may be affected by different deficits in different patients (e.g. a low verbal fluency score may be due to executive or language decline). A pathway approach to PD-MCI diagnosis is presented, in line with PD-D diagnostic guidelines. In cases where thorough screening results and clinical history are consistent with each other and presentation seems typical for PD-MCI, level I diagnosis could be established without neuropsychological assessment. However, when screening results diverge from the clinical history or the presentation is atypical, neuropsychological assessment should precede the diagnostic formulation