Objective: Compare cognitive performance in participants with Parkinson Disease (PD) utilizing published normative data versus age-stratified norms from the Parkinson’s Progression Marker Initiative (PPMI) healthy control (HC) group.

Background: Many cognitive abilities change with age and raw scores from neuropsychological tests are often standardized using data from a demographically representative sample, typically based on age. Differences in the characteristics of the comparison group may result in variable findings in research.

Methods: 422 participants with PD were included [age = 61.7 (9.7) years, education = 15.6 (3.0) years]. Neuropsychological tests included Hopkins Verbal Memory Test- Revised, Judgment of Line Orientation, Letter Number Sequencing, Symbol Digit Modalities Test, and Category Fluency. Internal norms were calculated using the group mean and standard deviation of the PPMI HC group (n=196) baseline performances using the following age ranges: 30-45 (n=23), 46-60 (n=69), 61-75 (n=91), and 76-90 (n=13). Published norms were compared to the age-stratified norms from HC for each neuropsychological test. Based on the MDS MCI Task Force level I guidelines for PD–mild cognitive impairment (MCI), participants in the current study met criteria if they scored ≥1.5 SD below the normative mean on 2 or more neuropsychological tests (repeated with -1SD and -2SD).

Results: There were no significant differences in age or sex between the PD or HC groups; however, the HC group [16.04 (2.9) years] had a significantly higher level of education than the PD group [15.5 (3.0) years; p=.05]. The use of the PPMI HC group norms resulted in statistically lower standardized scores than published norms with the exception of memory tests. [Table1] Participants with PD were 1.5 times more likely to be diagnosed with MCI using the HC group norms than with published norms using a -1.5SD cutoff for impairment and at least 2.0 times more likely using -2SD cutoff [Table2].

Conclusions: Among participants with PD in PPMI, there are differences in standardized cognitive scores depending on the comparison group that is used. The selection of normative comparison groups requires careful consideration, as such decisions impact both research and clinical interpretations of cognitive data.

References: Litvan I, Goldman JG, Troster AI, et al. Diagnostic criteria for mild cognitive impairment in Parkinson’s disease: movement disorder society task force guidelines. Mov Disord. 2012;27: 349-356.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/effect-of-different-normative-samples-and-cut-off-values-on-determination-of-cognitive-impairment-in-the-parkinsons-progression-marker-initiative/