Is apathy a valid and meaningful symptom or syndrome in Parkinson's disease? A critical review.

OBJECTIVE: To review the nearly 30 papers suggesting that apathy may occur frequently in Parkinson's disease (PD) and that it may be a symptom or syndrome that is separate from depression.
METHOD: Literature review.
RESULTS: The review revealed three possible explanations for the high rates of apathy found in PD. First, there is much interest in an endogenous explanation of apathy because the basal ganglia and dopamine are implicated in both PD and apathy. Researchers have suggested links between apathy, dopamine depletion, and basal ganglia dysfunction in PD. Second, apathy in PD may be exogenous, resulting from disability and activity restriction. Third, apathy findings are inflated due to conceptual problems and methodological confounds. Indeed, apathy may be consistently confounded with symptoms of PD, including expressive masking, depression, disability, and cognitive decline.
CONCLUSION: Because apathy has not yet been found to relate to meaningful patient outcomes, and it appears that other factors such as depression and cognition are more strongly related to quality of life than apathy, there is not enough evidence to conclude that apathy is a clinically meaningful syndrome in PD. The role of PD in motivation is of theoretical and practical interest and deserves further research.