How valid is the clinical diagnosis of Parkinson's disease in the community?

BACKGROUND: Many patients diagnosed with Parkinson's disease are later found to have an erroneous diagnosis, often only when they come to necropsy; conversely, many patients with Parkinson's disease in the community remain undiagnosed.
OBJECTIVE: To assess the validity of a clinical diagnosis of parkinsonism in the general population according to strict published criteria.
METHODS: As part of a population based study on the prevalence of Parkinson's disease in London, all patients were identified with a diagnosis of parkinsonism, tremor with onset over age 50 years, or who had ever received antiparkinsonian drugs. All patients who agreed to participate were diagnosed according to strict clinical diagnostic criteria, after a detailed neurological interview and examination and discussion of the findings with examination of their video recordings. Follow up information was obtained over a period of at least one year, and atypical cases were reviewed at the end of the study.
RESULTS: A diagnosis of probable Parkinson's disease was confirmed in 83% of patients with this diagnosis, including three (2%) in whom atypical features were found that were insufficient to discard a diagnosis of Parkinson's disease. Two additional patients (2%) were found to have possible Parkinson's disease. However, in 15% of patients the diagnosis was unequivocally rejected. Conversely, 13 patients who had previously not been diagnosed with Parkinson's disease (19%) were found to have this disorder.
CONCLUSIONS: At least 15% of patients with a diagnosis of Parkinson's disease in the population do not fulfil strict clinical criteria for the disease, and approximately 20% of patients with Parkinson's disease who have already come to medical attention have not been diagnosed as such.