Subjective Cognitive Complaint in Parkinson's Disease Patients With Normal Cognition: Canary in the Coal Mine?

Objective: The objective of this study was to determine the frequency and impact of subjective cognitive complaint (SCC) in Parkinson's disease (PD) patients with normal cognition. Methods: Patients with PD with expert consensus-determined normal cognition at baseline were asked a single question regarding the presence of SCC. Baseline (N = 153) and longitudinal (up to 4 follow-up visits during a 5-year period; N = 121) between-group differences in patients with PD with (+SCC) and without (−SCC) cognitive complaint were examined, including cognitive test performance and self-rated and informant-rated functional abilities. Results: A total of 81 (53%) participants reported a cognitive complaint. There were no between-group differences in global cognition at baseline. Longitudinally, the +SCC group declined more than the −SCC group on global cognition (Mattis Dementia Rating Scale–2 total score, F_1,431 = 5.71, P = 0.02), processing speed (Symbol Digit Modalities Test, F_1,425 = 7.52, P = 0.006), and executive function (Trail Making Test Part B, F_1,419 = 4.48, P = 0.04), although the results were not significant after correction for multiple testing. In addition, the +SCC group was more likely to progress to a diagnosis of cognitive impairment over time (hazard ratio = 2.61, P = 0.02). The +SCC group also demonstrated significantly lower self-reported and knowledgeable informant–reported cognition-related functional abilities at baseline, and declined more on an assessment of global functional abilities longitudinally. Conclusions: Patients with PD with normal cognition, but with SCC, report poorer cognition-specific functional abilities, and are more likely to be diagnosed with cognitive impairment and experience global functional ability decline long term. These findings suggest that SCC and worse cognition-related functional abilities may be sensitive indicators of initial cognitive decline in PD.