In the past decade, Parkinson's disease is increasingly suggested to be the clinical manifestation of a multisystem degenerative process with extensive involvement of both central and peripheral nervous systems. These systems comprise not only dopamine but also the noradrenergic, serotonergic, cholinergic and other central neurotransmitter systems.

Nearly all PD patients report one or more neuropsychiatric symptoms in the course of the disease. Initially depression and anxiety predominate and these may antedate motor symptoms by several years. Later, apathy, impulsive and compulsive behaviours and psychosis may emerge. Nearly all patients develop cognitive impairment and dementia as the disease progresses to involve mesocorticolimbic brain structures.

Many neuropsychiatric symptoms relate to disease related factors including depression and anxiety. Some symptoms may arise from an interaction between medications and the disease. Conversely, dopaminergic drug therapy used to treat motor symptoms of Parkinson's disease can underlie a wide spectrum of behaviours. These behaviours include pathologic gambling, hypersexuality, binge eating, compulsive shopping, hoarding, compulsive hobbyism or punding. The behaviours are generally characterized by impulsivity, compulsivity, and pre-occupation with a behaviour or mood state.

In addition, dopaminergic drug therapy itself has a small potential for abuse and a few individuals with PD appear to compulsively use dopaminergic medications well in excess of a dose that might be expected to achieve good motor control. Compulsive use may continue or recur despite the emergence of harmful drug-induced psychomotor effects that include disabling dyskinesias as well as a range of impulse control disorders.

Prompt recognition of neuropsychiatric disorders and utilisation of a range of nonpharmacological and pharmacological interventions are proposed to minimise their potential impact.