cannot be made definitively before death (7,8). Distinction between different parkinsonism variants is difficult, especially during the early stages of disease. Even in a clinical setting where patients are repeatedly evaluated by experts, accurate clinical diagnosis may not be possible as the telltale features that distinguish other variants from PD may evolve late or never (8-10). Classification into possible, probable, and definite PD has limited value in epidemiological studies that are primarily aimed at measuring the magnitude of the disorder in the population. Response to levodopa, though valuable, does not always distinguish between different variants of parkin-sonism (12).
The classification of parkinsonism has been evolving with time. Following the first description in 1817 by James Parkinson (13) and the discovery of the substantia nigra neuronal loss and Lewy body inclusions, parkinsonism was regarded as a single clinicopathological entity. This concept changed in the 1920s and 1930s after von Economo encephalitis from which an estimated 60% of the patients developed postencephalitic parkinsonism (14). At one time, these patients constituted a large proportion of the parkinsonism cases in the general population, but no new posten-cephalitic parkinsonism cases have been reported since the mid-1950s. Arteriosclerosis was once reported as a common cause of parkinsonism (15,16), but this diagnosis is now rare (17,18). This change in diagnostic classification reflects changes in autopsy studies and the determination of underlying pathology rather than a dramatic decline in arteriosclerosis in the general population.
Neuroleptic induced parkinsonism was first recognized in the late 1950s and is now a common parkinsonism variant ranging between 7% (17) and 20% of all cases (18,19). Drug-induced parkinsonism is now second only to PD and is more common in women than men (18).
Large clinicopathological studies of Shy-Drager syndrome (SDS) (20), striaton-igral degeneration (SND) (21), and progressive supranuclear palsy (PSP) (22) were first reported in the 1960s. The current classification includes SND, SDS, and olivopontocerebellar atrophy under the common heading of multiple system atrophy (MSA). Prominent dysautonomia in SDS and akinetic rigid parkinsonian features in SND were not fully recognized until 1960 and 1964, respectively, and in all likelihood, such cases prior to that were classified as postencephalitic or atypical parkinsonism, as they occurred at a relatively young age and had widespread nervous system involvement.
In spite of the improved understanding of these uncommon parkinsonism variants, the diagnosis is not always possible clinically. Autopsy series may be biased as the families of those suffering from the unusual variants may have heightened interest in finding out the nature of the disease and therefore consent to autopsy. The accurate frequency of these variants in the general population is, therefore, not possible to determine. In one epidemiological study, 2.5% of all parkinsonian patients were classified as MSA and 4.3% as PSP (18). A previous study from the same community reported a diagnosis of PSP in 1.4% and MSA in 2.1% of parkinsonian cases (17). Thus MSA and PSP each represent less than 5% of the contemporary parkinsonian cases in North America.
The most common parkinsonism in epidemiological studies is idiopathic PD. The proportion of those with PD, however, varies widely in different studies from 42% (18) to 85% (17). Preponderance of PD is also noted in autopsy studies of unselected parkinsonian cases (12,23,24).
The clinical and pathological classification of different parkinsonism variants continues to evolve. Classification into different variants is valuable, but it should be recognized that this only provides approximate estimates. Autopsy studies to confirm the diagnosis are not possible in epidemiological surveys; therefore, for descriptive epidemiological studies, all parkinsonian variants should be considered. Further classification may then be made based on the best clinical evidence.
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