Prodromal Parkinson's Disease: The Decade Past, the Decade to Come.

The past decade has seen a dramatic expansion of the field of prodromal PD. Ten years ago, there were only six known prodromal markers of disease, none of which had more than two studies documenting diagnostic value. We now have at least 16 markers, with as many as 10 prospective studies for a single marker. This review summarizes the major advances over the last decade and speculates about the advances we will see in the decade to come. The most notable advances over the last decade came through the study of high-risk cohorts (REM sleep behavior disorder and later genetic and autonomic cohorts), the generation of more representative population-based cohorts for studying prodromal PD, major advances in neuroimaging of early disease stages, the emerging likelihood that tissue biopsy will be able to diagnose prodromal PD, and the coalescence of prodromal markers into discrete criteria. As the next decade dawns, we await increasing precision of sensitivity and specificity estimates of known markers, the discovery of new biomarkers of prodromal disease, improvements in diagnosis using combined methods/criteria (with increasing recognition of prodromal PD as one stage of the full PD spectrum), and ultimately the development of neuroprotective therapy that can be provided at the earliest stages of disease.