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The syndrome of parkinsonism must be understood before understanding what is Parkinson's disease (PD). Parkinsonism is defined as any combination of six specific, independent motoric features: tremor at rest, bradykinesia, rigidity, loss of postural reflexes, flexed posture, and the freezing phenomenon (where the feet are transiently “glued” to the ground). Not all six of these cardinal features need be present, but at least two should be before the diagnosis of parkinsonism is made, with at least one of them being tremor at rest or bradykinesia. Parkinsonism is divided into four categories (Table 66-1). PD or primary parkinsonism will be the principal focus of this chapter; not only as it is the one that is most commonly encountered by the general clinician, it is also the one on which much research has been expended and the one we know the most about. The great majority of cases of primary parkinsonism are sporadic, but in the last decade, several gene mutations have been discovered to cause PD (Table 66-2). Whether genetic or idiopathic in etiology, the common denominator is that this group of primary parkinsonism is not caused by known insults to the brain (the main feature of secondary parkinsonism) and is not associated with other motoric neurological features (the main feature of Parkinson-plus syndromes). The uncovering of genetic causes of primary parkinsonism has shed light on probable pathogenic mechanisms that may be a factor in even the more common idiopathic cases of PD. It may even turn out that many of the idiopathic cases will be linked to gene mutations, discoveries yet to be made. Although the term “idiopathic PD” has been applied to primary parkinsonism, the fact that there are known genetic causes should encourage us to adopt the term “primary parkinsonism” rather than “idiopathic parkinsonism.”

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Table 66-1 Classification of the Parkinsonian States
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Table 66-2 Genetic Forms of Primary Parkinsonism

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