This progressive neurological disorder is named after James Parkinson who first described it in 1817 under the label of ‘shaking palsy’. The major symptoms are resting tremor, slowness of movement initiation and execution, muscular rigidity and postural abnormality. About 1 in 1000 of the population suffer from it. It is a disorder of old age, with the average age of onset in the sixties, although in hospital-based series 10 to 20% of cases started before the age of 40. The disorder is related to degeneration of dopamine-producing cells in the substantia nigra pars compacta resulting in depletion of striatal dopamine. Parkinson's disease provides a model of basal ganglia dysfunction, through which the contribution of the striatum to movement, cognition, affect and motivation can be studied. Dopamine replacement therapy is the main medical treatment, which initially controls the motor symptoms but after prolonged use is associated with side effects such as dyskinesias and on-off fluctuations. Since the mid 1990s, surgical techniques such as pallidotomy and high frequency deep brain stimulation of the internal segment of the globus pallidus or the subthalamic nucleus have been increasingly used and shown to be effective treatments of chronic Parkinson's disease.

Psychological features

Cognitive impairment

Dementia, that is a loss of intellectual abilities and memory impairment of sufficient severity to interfere with social or occupational functioning, when present develops late in the course of the illness (see also ‘Dementias’). The rate of dementia in Parkinson's disease has been estimated to be 15–20% (Brown & Marsden, 1984).