Prevalence of functional (psychogenic) parkinsonism in two Swiss movement disorders clinics and review of the literature

Background: Functional parkinsonism (FP) is considered rare but no studies have looked at its frequency. Case series have described high rates of comorbidity with Parkinson's disease (PD), suggesting a possible association between these conditions. Objectives: To study the prevalence, epidemiology and clinical features of FP and its association with PD. Methods: We conducted a cross-sectional population-based prevalence study as well as a chart review of cases who received a diagnosis of FP over a 10-year-period in two movement disorder clinics in Switzerland. Epidemiological data regarding FP features were collected. The co-occurrence of PD, psychiatric disorders and other functional disorders were recorded. Clinical differences between FP and FP + PD groups are presented and discussed in light of a literature review. Results: The crude prevalence of FP was 0.64 per 100,000 in our population. FP represented 0.24% of patients with parkinsonism. Among 12 FP cases, female gender predominance (87%), mean age of onset of 45.5(+/-13.3 Standard deviation SD) years and prolonged diagnostic delay (mean 59 +/- 75 SD months) was found. Six patients had an additional diagnosis of PD, 83% of depression and 66% of other functional neurological disorder. In four patients with FP + PD, FP preceded PD by 6 to 56 months. Conclusions: These results suggest that FP should be considered in the differential diagnosis of patients presenting with parkinsonism. The high rate of co-occurrence with PD emphasizes the importance of long-term follow up of these patients. The observation that FP often precedes PD should be verified in prospective studies.