Levodopa peak response time reflects severity of dopamine neuron loss in Parkinson's disease.

We measured the time to peak antiparkinson response following injection of levodopa or apomorphine in 57 patients with Parkinson's disease. The peak response time for levodopa fell from 53 +/- 6.5 minutes in patients at Hoehn and Yahr stage I-II, and 28 +/- 4.8 minutes in those at stage IV (p < 0.0005). There was a significant correlation between levodopa peak response time and symptom duration (r = 0.65; p < 0.0001), but there was no relation between apomorphine peak response time and measures of disease severity. Peak response time to levodopa appears to reflect predominantly the status of compensatory presynaptic dopaminergic mechanisms and thus may provide an index to the degree of dopamine neuron degeneration in parkinsonian patients.