The purpose of this review was to identify and recommend a brief, standard cognitive measure that could be used to screen PD subjects for dementia and cognitive impairment in research studies where cognitive performance is not the primary outcome measure. Of the 5 instruments reviewed, only the MoCA fulfilled the primary criteria established by the Task Force. Thus, the Task Force preliminarily recommends the MoCA as a minimum standard cognitive screening instrument in clinical trials of PD. It can be administered rapidly and has the potential to identify subtle executive dysfunction while also covering the major cognitive domains.
All available instruments that can be briefly administered have limitations. The MMSE has been used extensively in studies of patients with PD, can measure progression once patients develop dementia, and is sensitive to change in clinical trials. However, its deficiencies include a distinct ceiling effect and its inability to assess executive functions. Therefore, it is unlikely to detect MCI in PD, a primary concern if the instrument is to serve as a screening measure for mild cognitive dysfunction. While the MoCA, MMP, PANDA, and SCOPA-cog all appear suitable for detecting early cognitive dysfunction in PD, each has undergone limited cross-validation in PD samples. Some instruments (MoCA and MMSE) include language tasks, but do not assess this domain as adequately as longer and more comprehensive assessments. While all of the scales included in this review can be administered in less than 15 minutes, published administration times are likely based on non-demented populations and may take longer in demented PD participants.
Although preliminary data seems to suggest that the MoCA may be a poor screening measure for MCI in PD based on its reported low specificity (0.53) using a cutoff of 26/27,25
this study had several flaws that limit the strength of the conclusions that can be made regarding its utility to detect MCI. The diagnostic criteria for defining MCI has not been well established in PD and the requirement of self-reported cognitive decline in this study25
may require a degree of insight not commonly seen in patients with a neurodegenerative process affecting frontally-mediated functions such as meta-cognition. Additionally, the study had small numbers of subjects with cognitive impairment (only 12.9% PDD and 17.4% MCI out of 132 subjects), and a limited neuropsychological battery was used.25
A brief scale for screening dementia in PD (PDD-Short Screen) was recently published,30
which took 4.8–6.9 minutes to administer and had high sensitivity (0.898) and specificity (0.885) for diagnosing PDD. Unfortunately, it may not be sensitive enough to detect subtle executive dysfunction in PD and thus did not meet our Task Force’s objectives.
The aim of this Task Force is complementary yet distinguishable from a recent review by Kulisevsky and Pagonabarraga that evaluated different instruments on their appropriateness to assess cognition throughout the course of PD.2
That review identified both SCOPA-cog and PD-CRS as the most appropriate for capturing early PD cognitive changes and PD-CRS as the most suitable for monitoring cognitive progression in PD. However, SCOPA-cog does not measure orientation or language function, which would be useful in a screening measure to rule out alternative causes for dementia, such as AD. The lengthier administration time of the PD-CRS precludes its use as a screening instrument in clinical trials of PD not focusing primarily on cognition. Our review is also different from the recent efforts of a Movement Disorders Society (MDS) Task Force on Dementia in Parkinson’s Disease that established diagnostic criteria for dementia in PD31
as well as a practical approach to its diagnosis.32
Should the MoCA be utilized in clinical trials going forward, the dilemma of how to compare new data to existing trials that used the MMSE needs to be addressed. A potential strategy would be to transform MMSE and MoCA scores into equivalent z-scores,17
which would allow comparison of cognitive performance between the two groups. However, the absence of age and education normative data for the MoCA limits this practice. For now, the most parsimonious strategy would be to administer both the MoCA and the MMSE until further evidence is available to demonstrate whether it is an appropriate tool. For existing clinical trial databases that have extensive MMSE data only, using age and education adjusted MMSE cut-offs for cognitive impairment may be helpful. In a recent study on cognitive impairment in the DATATOP study cohort,33
Uc et al. found that subjects who developed cognitive impairment by age and education adjusted MMSE criteria showed significant decline on neuropsychological tests while the neuropsychological performance of non-impaired subjects remained stable.
This task force recommends consideration of the MoCA as a screening instrument for dementia and MCI in PD clinical studies where cognition is not the primary outcome measure. Widespread adoption of such an instrument in clinical trials will improve comparability among research studies on motor and non-motor aspects of PD. The MoCA still requires further study of its validity in PD populations to determine how well it detects MCI and dementia, and it could also benefit from the development of age and education normative data. Despite these flaws, the MoCA shows the most promise of the currently available brief cognitive assessments. Since the MoCA lacks data regarding sensitivity to change over time and to treatment, the Task Force does not recommend use of the MoCA as a stand-alone measure in PD trials investigating progression of cognitive impairment. If future evidence demonstrates that the MoCA is sensitive to longitudinal treatment effects and cognitive decline, then the MoCA may be considered for use as a primary outcome measure.