During the disease process there is progressive loss of dopaminergic cells from the substantia nigra in the basal ganglia (midbrain). This cell loss results in decreased dopamine (as well as serotonin and GABA) in the striatum.

The basal ganglia controls muscle tone and provides smooth muscle voluntary movements. PD symptoms present as impairment in carrying out learned voluntary actions (see table 1) when 80 per cent of dopaminergic cells are lost. Every person with PD has a different set of signs and symptoms (see table 2), so care and treatment needs to be individualised.

The exact cause of PD is unknown and the vast majority of cases are idiopathic (unknown). PD is thought to be a complex interaction between genetic and environmental risk factors. A number of medicines can also produce Parkinson-like symptoms and account for seven per cent of people presenting with suspected PD (see table 3).

These medication-induced effects respond poorly to levodopa therapy. Dependent on the particular agent, slow withdrawal is recommended, with 60 per cent recovering in two months, but others may take up to two years.