How can we judge the 'long term' outcomes of novel interventions in Parkinson's disease?

Despite all recent advances in symptomatic therapy of Parkinson's disease (PD), the underlying neurodegenerative process cannot yet be slowed and the long-term clinical picture is characterized by severe motor as well as nonmotor disability, loss of independent function and premature death. This has a major and increasing social and economic burden on ageing populations. There is a pressing unmet need for developing therapies capable of not only providing symptomatic relief, but that can also modify or slow down the progression of disability in PD. In this article we discuss our own experience of the long-term effects of fetal cell transplantation used as experimental restorative treatment in PD, and we consider these observations in relation to the long-term clinical course of PD as we currently comprehend it in the era of symptomatic treatment.