About one-third of patients with Parkinson's disease (PD) are diagnosed with apathy in cross-sectional studies. However, once patients with concomitant depression and dementia are excluded, the frequency of apathy drops to 5% to 10%. Several scales have been recommended to rate apathy in PD, but specific psychiatric interviews have not been developed, and recently proposed standardized diagnostic criteria are still in the validation process. Most studies assessing the association between subthalamic deep brain stimulation (STN-DBS) and apathy have reported a relative increase in the frequency and severity of apathy, although discrepant findings have also been reported. Several mechanisms to explain apathy in PD have been proposed, from dopaminergic imbalances in frontal-basal ganglia circuits to dysfunction of nondopaminergic circuits and the cingulate gyrus. Future studies should provide reliable and valid instruments to diagnose apathy in PD, and should examine the mechanism of apathy accounting for relevant confounders, such as depression and cognitive deficits, and important contextual factors. Finally, treatment for apathy in PD should not be restricted to psychoactive drugs, but should also include nonpharmacological techniques such as psychotherapy and occupational therapy.
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