The unique and combined effects of apathy and depression on cognition in Parkinson's disease

Apathy and depression are associated with poor cognition in patients with Parkinson's disease (PD). However, the cognitive signature of each syndrome is not well understood. The cognitive consequences of having apathy or depression, versus apathy and depression, are also unclear. Objective: This study investigated the unique and combined effects of apathy and depression on cognition in PD patients. Methods: PD patients were identified from a clinical research database as having self-reported apathy (n = 21), depression (n = 11), or both (n = 43). PD patients without apathy or depression served as the Control group (n = 49). The groups were of similar age, education, disease severity, age of symptom onset, and medication status. A multivariate analysis of variance (MANOVA) compared the groups on 17 neuropsychological test scores in the domains of attention, motor and psychomotor speed, construction, language, episodic memory, and executive functioning. Results: There was a significant overall effect of group: F(18,276) = 2.12, p = 0.006, η²_p= 0.12. Univariate analyses and planned contrasts revealed medium-sized effects distinguishing only the control group and the group with both apathy and depression (η²_p = 0.06-0.12) on measures of verbal fluency and manual, processing, and psychomotor speed. Conclusions: PD patients with apathy alone or depression alone did not perform worse than PD controls on neuropsychological tests. Patients with apathy and depression performed worse than controls, but only on speed-based measures. This suggests that apathy and depression are associated with the same or similar circuits as those of cognitive and motor speed.