Dopa Responsiveness in Parkinson's Disease

Background Dopaminergic responsiveness is a defining feature of Parkinson's disease (PD). However, there is limited information on how this evolves over time. Objectives To examine serial dopaminergic responses, if there are distinct patterns, and which factors predict these. Methods We analyzed data from the Parkinson's Progression Markers Initiative on repeated dopaminergic challenge tests (≥24.5% defined as a definite response). Growth‐mixture modeling evaluated for different response patterns and multinomial logistic regression tested for predictors of these clusters. Results 1525 dopaminergic challenge tests were performed in 336 patients. At enrolment, mean age was 61.2 years (SD 9.6), 66.4% were male and disease duration was 0.5 years (SD 0.5). 1 to 2 years after diagnosis, 48.0% of tests showed a definite response, but this proportion increased with longer disease duration (51.1–74.3%). We identified 3 response groups: “Striking” ( n = 29, 8.7%); “Excellent” ( n = 110; 32.7%) and “Modest” ( n = 197, 58.6%). Significant differences were as follows: striking responders commenced treatment earlier ( P = 0.02), were less likely to be on dopamine agonist monotherapy ( P = 0.01), and had better cognition ( P < 0.01) and activities of daily living ( P = 0.01). Excellent responders had higher challenge doses ( P = 0.03) and were more likely to be on combination therapy ( P < 0.01). Conclusion Three distinct patterns of the dopaminergic response were observed. As the proportion of PD cases with definite dopa responsiveness increased over time, the initial treatment response may be an unreliable diagnostic aid.