Apathy is a primary lack of motivation that is frequently reported in Parkinson’s disease (PD) and often misdiagnosed as depression. In PD, apathy worsens over time with motor symptom progression. Evidence over the past 15 years has documented that use of selective serotonin reuptake inhibitors (SSRIs) is associated with increased apathy in patients with depression, including individuals with PD. In PD, this appears to be related to downregulation of dopaminergic systems by serotonin. Despite increasing evidence, SSRIs continue to be heavily prescribed in individuals with PD— potentially worsening apathy and decreasing quality of life for these individuals. This study is an update, re-examining the relationship between apathy and the use of SSRIs and other antidepressants in a large cohort of individuals with PD.

Participants included a convenience sample of 387 nondemented individuals with idiopathic PD who were in their mid-60's (mean age=64.9+8.72 years), well-educated (mean=14.95+2.78 years), predominantly male (72.4%), non-Hispanic white (94.5%), and in mid-stage of disease severity (on medication Unified Parkinson Disease Rating Scale motor score=25.3+10.1). All scored above clinical cutoff for dementia on a cognitive screener (Dementia Rating Scale-2 (DRS) > 125). Medications, cognitive, mood, and clinical data were extracted from chart review. Depression and apathy were measured using the Beck Depression Inventory-II (BDI-Il) and the Apathy Scale (AS). Antidepressant medications were grouped into SSRIs, serotonin and norepinephrine reuptake inhibitors (SNRIs) and other. Analyses included bootstrapped Pearson’s correlations, Pearson’s chi-square, and linear regressions

Among 387 individuals with PD, 41.3% (N=160) were taking antidepressant medications. Of these 160, 61.3% were on SSRIs, 24.4% on SNRIs, and the remainder on other antidepressants. Approximately 36.9% of the 387 PD patients exceeded recommended clinical cutoffs for apathy (AS >14) and 23.5% for depression (BDI-II >14) (Starkstein et al., 1992; Beck et al., 1996). Individuals taking SSRIs (N=98, x2=5.14, p=0.023) or SNRIs (N=39; x2=5.43, p=0.020) were more likely to be clinically apathetic than those taking other depression medications (N=23; x2=1.28, p=0.26). Results of a multiple regression with age, education, disease duration, motor severity, DRS-2, BDI-II, and all psychotropic medications (anti-depressants, anti-anxiety, anti-psychotics) as independent variables explained 42.8% of the variance in total apathy scores (F[17,285]=12.550, p<0.001). SSRIs were the only medication to significantly predict greater AS scores (ß=0.110, p=0.020) in this model. Less education (ß=-0.119, p=0.017) worse cognition (ß=-0.128, p=0.009), and greater depressive symptoms (ß=0.561, p<0.001) were also significant predictors of apathy.

These findings suggest that use of SSRIs, but not other antidepressants, is associated with greater apathy in PD. Given the interactive relationship between serotonin and dopamine, the current findings highlight the importance of considering apathy as a potential adverse effect when determining which anti-depressants to prescribe to individuals with PD. Similarly, switching a SSRI for an alternative anti-depressant in individuals with PD who are apathetic may be a potential treatment for apathy that needs further study. Longitudinal studies are also needed to elucidate the relationship of apathy and anti-depressant use over time, specifically to determine potential causality of this observed association. Funding: T32-NS082168