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The PARS pyramid: In this conceptual model there are 4 stages that precede clinically manifest PD: pre-physiological, pre-clinical, pre-motor and pre-diagnostic. There are a relatively large number of individuals that have a pre-disposition to develop PD, but only a fraction progress to each succeeding level. Ultimately, the number of individuals who develop clinically manifest PD is a small fraction of the at-risk pool.
