Challenge tests to predict the dopaminergic response in untreated Parkinson's disease.

To clarify the predictive role of dopaminergic challenge tests, we compared the responses to subcutaneous apomorphine and oral levodopa with the therapeutic effect of ongoing levodopa treatment in 45 previously untreated patients with idiopathic Parkinson's disease. The response to long-term levodopa was accurately predicted by apomorphine in 67% (30) of patients and by levodopa in 80% (35) of patients. There were nine cases without a definite response to sustained levodopa, four in patients who developed atypical clinical features during the period of follow-up. These tests have a predictive value for subsequent dopaminergic responsiveness and may help in the early differential diagnosis of parkinsonian syndromes.