Patient characteristics, MRI examination, and dopaminergic drug response in clinical possible Parkinson's disease

• Patients with parkinsonism can be classified as clinical definite, probable, possible, or unlikely Parkinson's disease (PD). Possible PD includes patients with PD according to conventional diagnostic criteria and with at least a moderate response to dopamine agonists. However, these patients have clinical features that may reduce the probability for idiopathic PD. The objective of this study was to clinically characterize patients with possible disease in a prevalence study of PD and to indicate the frequency of idiopathic PD in this group of patients. The diagnostic re-evaluation was based on detailed MRI examinations and investigations of dopaminergic drug response after several years of treatment. In a community-based prevalence study in Norway, comprising 245 PD patients, we found 36 patients (15%) with clinical possible PD. The patients with possible disease had significant and clinically important differences in demographic and disease characteristics compared to patients with definite and probable PD. Possible PD patients were older at disease onset, more disabled, and had more neurobehavioral disorders. MRI examinations of 14 of the 36 patients with possible PD in the prevalence study revealed significant group effects compared to an age-matched control group, with reduced pars compacta width and increased cortical atrophy. In individual patients, signal attenuation consistent with vascular lesions of the basal ganglia contributed to diagnostic reclassification. Dopaminergic drug withdrawal revealed no response in 4 of 12 examined patients. Two of the remaining eight patients had a clear short-duration drug response. Six patients had only a varying degree of long-duration response. The re-evaluation of diagnosis indicates that probably less than half of the patients with clinical possible PD have idiopathic disease. Patients with atypical features and diagnosed as possible PD should thus be excluded from studies with a presumed high specificity for idiopathic PD.

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