Incidence of Parkinson's disease in a large patient cohort with idiopathic smell and taste loss.

Patients with idiopathic smell loss constitute an at-risk population for the development of Parkinson's disease (PD). The study aimed to follow up a large number of patients with idiopathic smell and/or taste loss to define the incidence of PD in this population and, further, to assess characteristics of both olfactory and gustatory function and their possible association with PD development. In this prospective case-control study, 833 patients diagnosed with an idiopathic smell disorder at our Smell and Taste Center during the last 15 years were contacted for a telephone interview. In 474 patients, a complete data set containing of demographic data, clinical information, retrospective smell and taste testing results, and telephone assessment was obtained. Out of 474 patients with idiopathic smell loss 45 (9.8%) had been diagnosed with PD, since they received the diagnosis of idiopathic smell and/or taste loss (mean 10.9 years after olfactory loss onset). Thus, with respect to the classification into olfactory/gustatory disorders, 28.6% of the patients with a combined olfactory and gustatory disorder developed PD, whereas in 9.9% of those with a pure olfactory disorder and in 3.8% of those with a pure gustatory disorder, PD was diagnosed. No association emerged between qualitative smell or taste loss and PD development. This large patient cohort study extends the previous literature, indicating that risk stratification might be considerably improved by correct diagnostic allocation and emphasizes the need for an exhaustive olfactory and gustatory assessment in specialized centers.