The Parkinson chimera.

Although our concepts of what causes Parkinson disease (PD) are ever changing and the hunt for a reliable biomarker continues, the clinical picture remains as distinctive as when the malady was first described by James Parkinson and the neurologic Grand Masters of the nineteenth century. Hyposmia and visual hallucinations, however, can now be added as additional features of the clinical syndrome which may be helpful in distinguishing PD from atypical parkinsonism, as well as the growing list of causes of secondary parkinsonism. Selective vulnerability of catecholaminergic long axon projection neurons (part of the isodendritic core) in PD is an important, if recently somewhat neglected, fact and correlation of the severity of nigral loss with bradykinesia and rigidity is the only very reliable anatomo-clinical correlation. Although the Lewy body seems to be closely linked with our notion of PD as a clinicopathologic nosological entity, its role in the pathogenesis of the disorder is still obscure and hotly debated. Its presence in some of the long-surviving grafted neurons in fetal implants may provide important insights into its role in the disease process. Although Braak's hypothesis implicating the medulla oblongata as an obligate trigger for the subsequent spread of the pathologic process has generated much interest and encouraged more research, it seems unlikely as an explanation for the natural history of PD.