Intra-individual variability in cognitive performance predicts functional decline in Parkinson’s disease

Background: Cognitive deficits contribute to disability in Parkinson’s disease (PD). Cognitive intra-individual variability (IIV) is associated with cognitive decline in age-related disorders, but IIV has not been related to functional ability in PD. We examined IIV in predicting functional ability in participants with PD. Methods: De-identified National Alzheimer’s Coordinating Center data (N = 1,228) from baseline and follow-up visits included participants with PD propensity score matched to control participants at baseline on age (M = 72), education (M = 15), and gender (28% female). PD symptom duration averaged 6 years. Outcome measures included the Functional Ability Questionnaire (FAQ), overall test battery mean (OTBM) of ten cognitive variables, IIV calculated as the standard deviation of cognitive data for each participant, Geriatric Depression Scale (GDS), and Unified PD Rating Scale gait and posture items. Baseline FAQ status in the PD group was predicted using logistic regression with age, education, cognition, GDS, and motor function as predictors. We compared baseline characteristics of PD participants with and without functional impairment at follow up. Results: PD participants showed lower OTBM and greater IIV, GDS, and motor dysfunction than controls (p <.0001). Education, OTBM, IIV, GDS, and gait predicted functional status (77% overall classification; AUC =.84). PD participants with functional impairment at follow up showed significantly lower OTBM and greater IIV, GDS, and motor dysfunction at baseline (p <.001). Conclusion: IIV independently predicts functional status in participants with PD while controlling for other variables. PD participants with functional impairment at follow up showed greater IIV than those without functional impairment at follow up.