Parkinsonism , a group of chronic neurological disorders characterized by progressive loss of motor function resulting from the degeneration of neurons in the area of the brain that controls voluntary movement.

While the cause of deterioration of the substantia nigra in primary parkinsonism remains unknown, deterioration in secondary parkinsonism can result from trauma induced by certain drugs, exposure to viruses or toxins, or other factors. For example, a viral infection of the brain that caused a worldwide pandemic of encephalitis lethargica (sleeping sickness ) just after World War I resulted in the development of postencephalitic parkinsonism in some survivors. Toxin-induced parkinsonism is caused by carbon monoxide , manganese , or cyanide poisoning. A neurotoxin known as MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), previously found in contaminated heroin , also causes a form of toxin-induced parkinsonism. The ability of this substance to destroy neurons suggests that an environmental toxin similar to MPTP may be responsible for Parkinson disease. Pugilistic parkinsonism results from head trauma and has affected professional boxers such as Jack Dempsey and Muhammad Ali . The parkinsonism-dementia complex of Guam, which occurs among the Chamorro people of the Pacific Mariana Islands, is also thought to result from an unidentified environmental agent. In some individuals genetic defects are thought to incur susceptibility to the disease. Genetic factors appear to be particularly important in primary parkinsonism, although in most cases, genetic variations are not believed to be the sole factors giving rise to the disease. Parkinsonism-plus disease, or multiple-system degenerations, includes diseases in which the main features of parkinsonism are accompanied by other symptoms. Parkinsonism may appear in patients with other neurological disorders such as Huntington disease , Alzheimer disease , and Creutzfeldt-Jakob disease .