The two major considerations for inclusion in parkinsonism epidemiology are:
1. Does this individual have parkinsonism, normal aging, or another disorder?
2. Does this person have idiopathic Parkinson's disease (PD) or another variant of parkinsonism?
Primitive reflexes, slowed motor functions, flexed posture, and impaired postural reflexes characteristic of parkinsonism are also a part of normal aging (2). In general, age-related abnormalities are symmetrical while parkinsonism is often asymmetrical. Rest tremor, a common early feature of parkinsonism, is not part of normal aging and hence is the single most reliable feature of this disorder (3).
The most common tremor disorder misdiagnosed as PD is essential tremor (ET). Typically, ET is present on positioning a limb against gravity and during activity. ET is usually restricted to the upper limbs and/or head. By contrast, resting tremor is characteristic of parkinsonism and may involve the upper and lower limbs. Nearly one-third of ET patients may develop rest tremor after many years and may be mistaken as having parkinsonism; however, the risk of PD does not appear to be greater in patients with ET (4).
For epidemiological surveys, the diagnostic criteria should be simple, consistent through the study interval, and easy to apply. For example, after careful consideration of different diagnostic criteria utilized in epidemiological studies, de Rijk et al. (5) concluded that the most suitable for PD is the presence of two of at least three criteria—bradykinesia, rigidity, and resting tremor. In individuals with preexisting ET, the additional diagnosis of parkinsonism should be made only when all three criteria are present (6).
The second major consideration is to classify parkinsonism cases into different variants. Most neurologists use the term PD for Lewy body disease but this diagnosis
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