Table 2.

Studies investigating the association between depression and Parkinson’s disease.

Study	Study design	Main results
Shiba et al. (2000)	Prospective, case-control, USA	Frequency of depression is higher in PD than controls. Depression may precede PD motor symptoms.
Schuurman et al. (2002)	Retrospective, case-control, Netherlands	Strong positive association between depression and subsequent incidence of PD.
Leentjens et al. (2003b)	Prospective, case-control, Netherlands	Depression precedes PD. The average time-span between the first episode of depression and the diagnosis of PD was 10 years.
Fang et al. (2010)	Case-control, USA	Positive association between depression and a higher subsequent risk of PD. Depression was detected more than 15 years before the diagnosis of PD.
Jacob et al. (2010)	Retrospective, case-control, USA	Positive association between depression and subsequent risk of PD in men but not women.
Shen et al. (2013)	Retrospective, case-control	Patients with depression were 3.24 times more likely to develop PD.
Gustafsson et al. (2015)	Prospective, case-control	Positive association between depression and subsequent risk of PD. Depression may predate two decades before onset of PD.