Apathy and Reduced Voluntary Activity in Older Adults with Parkinson's Disease: Mechanisms, Clinical Assessment, and Rehabilitation Implications.

Apathy in Parkinson's disease is characterized by reduced self-initiated, goal-directed behaviour and may overlap clinically with bradykinesia, akinesia, freezing of gait, fatigue, depression, and executive dysfunction. This narrative Review argues that reduced voluntary activity in Parkinson's disease reflects two interacting but partially dissociable domains: motor-execution deficits mediated mainly by nigrostriatal cortico-basal ganglia circuits, and motivational-initiation deficits mediated by mesolimbic, prefrontal-striatal, limbic, and non-dopaminergic modulatory systems. We integrate clinical, neuroimaging, electrophysiological, computational, and preclinical evidence to explain how disrupted dopaminergic signalling, striatal D1/D2 pathway imbalance, prefrontal effort valuation, noradrenergic arousal, serotonergic and cholinergic dysfunction, and glutamatergic control may influence the transition from motor capacity to self-initiated behaviour. We also review validated clinical tools for assessing apathy and discuss how apathy should be distinguished from depression, fatigue, cognitive impairment, normal aging, and advanced motor disability. The proposed framework helps explain why dopaminergic therapy, behavioural rehabilitation, and conventional neuromodulation may improve motor performance without consistently restoring spontaneous initiative or rehabilitation engagement. Future work should prioritize validated motivational phenotyping, PD-specific rehabilitation trials, careful differentiation between established and emerging interventions, and clinically feasible strategies for improving self-initiated daily activity.