What cases PD?

Most people with Parkinson's disease are described as having idiopathic Parkinson's disease (having no specific cause). There are far less common causes of Parkinson's disease including genetic, toxins, head trauma, and drug-induced Parkinson's disease.

Genetic

In recent years, a number of specific genetic mutations causing Parkinson's disease have been discovered, including in certain populations (Contursi, Italy). These account for a small minority of cases of Parkinson's disease. Somebody who has Parkinson's disease is more likely to have relatives that also have Parkinson's disease. However, this does not mean that the disorder has been passed on genetically.

Toxins

One theory holds that the disease may result in many or even most cases from the combination of a genetically determined vulnerability to environmental toxins along with exposure to those toxins.

This hypothesis is consistent with the fact that Parkinson's disease is not distributed homogeneously throughout the population: rather, its incidence varies geographically. It would appear that incidence varies by time as well, for although the later stages of untreated PD are distinct and readily recognizable, the disease was not remarked upon until the beginnings of the Industrial Revolution, and not long thereafter become a common observation in clinical practice.

The toxins most strongly suspected at present are certain pesticides and transition-series metals such as manganese or iron, especially those that generate reactive oxygen species, and or bind to neuromelanin.

In the Cancer Prevention Study II Nutrition Cohort, a longitudinal investigation, individuals who were exposed to pesticides had a 70% higher incidence of PD than individuals who were not exposed.

Numerous studies have found an increase in PD in persons who consume rural well water; researchers theorize that water consumption is a proxy measure of pesticide exposure. In agreement with this hypothesis are studies which have found a dose-dependent an increase in PD in persons exposed to agricultural chemicals.

Head trauma

Past episodes of head trauma are reported more frequently by sufferers than by others in the population.

A strong recent study found that those who have experienced a head injury are four times more likely to develop Parkinson’s disease than those who have never suffered a head injury. The risk of developing Parkinson’s increases eightfold for patients who have had head trauma requiring hospitalization, and it increases 11-fold for patients who have experienced severe head injury. The authors comment that since head trauma is a rare event, the contribution to PD incidence is slight. They express further concern that their results may be biased by recall, i.e., the PD patients because they reflect upon the causes of their illness, may remember head trauma better than the non-ill control subjects. These limitations were overcome recently by Tanner and colleagues, who found a similar risk of 3.8, with increasing risk associated with more severe injury and hospitalization.

Drug-induced

Antipsychotics, which are used to treat schizophrenia and psychosis, can induce the symptoms of Parkinson's disease (or parkinsonism) by lowering dopaminergic activity. L-dopa can also eventually cause the symptoms of Parkinson's disease that it initially relieves. Dopamine agonists can also eventually contribute to Parkinson's disease symptoms by decreasing the sensitivity of dopamine receptors.